Posted on by Matt Williams

The Universe

Artists illustration of the expansion of the Universe (Credit: NASA, Goddard Space Flight Center)

What is the Universe? That is one immensely loaded question! No matter what angle one took to answer that question, one could spend years answering that question and still barely scratch the surface. In terms of time and space, it is unfathomably large (and possibly even infinite) and incredibly old by human standards. Describing it in detail is therefore a monumental task. But we here at Universe Today are determined to try!

So what is the Universe? Well, the short answer is that it is the sum total of all existence. It is the entirety of time, space, matter and energy that began expanding some 13.8 billion years ago and has continued to expand ever since. No one is entirely certain how extensive the Universe truly is, and no one is entirely sure how it will all end. But ongoing research and study has taught us a great deal in the course of human history.

Definition:

The term “the Universe” is derived from the Latin word “universum”, which was used by Roman statesman Cicero and later Roman authors to refer to the world and the cosmos as they knew it. This consisted of the Earth and all living creatures that dwelt therein, as well as the Moon, the Sun, the then-known planets (Mercury, Venus, Mars, Jupiter, Saturn) and the stars.

Illuminated illustration of the Ptolemaic geocentric conception of the Universe by Portuguese cosmographer and cartographer Bartolomeu Velho (?-1568) in his work Cosmographia (1568). Credit: Bibilotèque nationale de France, Paris

The term “cosmos” is often used interchangeably with the Universe. It is derived from the Greek word kosmos, which literally means “the world”. Other words commonly used to define the entirety of existence include “Nature” (derived from the Germanic word natur) and the English word “everything”, who’s use can be seen in scientific terminology – i.e. “Theory Of Everything” (TOE).

Today, this term is often to used to refer to all things that exist within the known Universe – the Solar System, the Milky Way, and all known galaxies and superstructures. In the context of modern science, astronomy and astrophysics, it also refers to all spacetime, all forms of energy (i.e. electromagnetic radiation and matter) and the physical laws that bind them.

Origin of the Universe:

The current scientific consensus is that the Universe expanded from a point of super high matter and energy density roughly 13.8 billion years ago. This theory, known as the Big Bang Theory, is not the only cosmological model for explaining the origins of the Universe and its evolution – for example, there is the Steady State Theory or the Oscillating Universe Theory.

It is, however, the most widely-accepted and popular. This is due to the fact that the Big Bang theory alone is able to explain the origin of all known matter, the laws of physics, and the large scale structure of the Universe. It also accounts for the expansion of the Universe, the existence of the Cosmic Microwave Background, and a broad range of other phenomena.

Illustration of the Big Bang Theory
The Big Bang Theory: A history of the Universe starting from a singularity and expanding ever since. Credit: grandunificationtheory.com

Working backwards from the current state of the Universe, scientists have theorized that it must have originated at a single point of infinite density and finite time that began to expand. After the initial expansion, the theory maintains that Universe cooled sufficiently to allow for the formation of subatomic particles, and later simple atoms. Giant clouds of these primordial elements later coalesced through gravity to form stars and galaxies.

This all began roughly 13.8 billion years ago, and is thus considered to be the age of the Universe. Through the testing of theoretical principles, experiments involving particle accelerators and high-energy states, and astronomical studies that have observed the deep Universe, scientists have constructed a timeline of events that began with the Big Bang and has led to the current state of cosmic evolution.

However, the earliest times of the Universe – lasting from approximately 10-43 to 10-11 seconds after the Big Bang –  are the subject of extensive speculation. Given that the laws of physics as we know them could not have existed at this time, it is difficult to fathom how the Universe could have been governed. What’s more, experiments that can create the kinds of energies involved are in their infancy.

Still, many theories prevail as to what took place in this initial instant in time, many of which are compatible. In accordance with many of these theories, the instant following the Big Bang can be broken down into the following time periods: the Singularity Epoch, the Inflation Epoch, and the Cooling Epoch.

Also known as the Planck Epoch (or Planck Era), the Singularity Epoch was the earliest known period of the Universe. At this time, all matter was condensed on a single point of infinite density and extreme heat. During this period, it is believed that the quantum effects of gravity dominated physical interactions and that no other physical forces were of equal strength to gravitation.

This Planck period of time extends from point 0 to approximately 10-43 seconds, and is so named because it can only be measured in Planck time. Due to the extreme heat and density of matter, the state of the Universe was highly unstable. It thus began to expand and cool, leading to the manifestation of the fundamental forces of physics. From approximately 10-43 second and 10-36, the Universe began to cross transition temperatures.

It is here that the fundamental forces that govern the Universe are believed to have began separating from each other. The first step in this was the force of gravitation separating from gauge forces, which account for strong and weak nuclear forces and electromagnetism. Then, from 10-36 to 10-32 seconds after the Big Bang, the temperature of the Universe was low enough (1028 K) that electromagnetism and weak nuclear force were able to separate as well.

With the creation of the first fundamental forces of the Universe, the Inflation Epoch began, lasting from 10-32 seconds in Planck time to an unknown point. Most cosmological models suggest that the Universe at this point was filled homogeneously with a high-energy density, and that the incredibly high temperatures and pressure gave rise to rapid expansion and cooling.

This began at 10-37 seconds, where the phase transition that caused for the separation of forces also led to a period where the Universe grew exponentially. It was also at this point in time that baryogenesis occurred, which refers to a hypothetical event where temperatures were so high that the random motions of particles occurred at relativistic speeds.

As a result of this, particle–antiparticle pairs of all kinds were being continuously created and destroyed in collisions, which is believed to have led to the predominance of matter over antimatter in the present Universe. After inflation stopped, the Universe consisted of a quark–gluon plasma, as well as all other elementary particles. From this point onward, the Universe began to cool and matter coalesced and formed.

As the Universe continued to decrease in density and temperature, the Cooling Epoch began. This was characterized by the energy of particles decreasing and phase transitions continuing until the fundamental forces of physics and elementary particles changed into their present form. Since particle energies would have dropped to values that can be obtained by particle physics experiments, this period onward is subject to less speculation.

For example, scientists believe that about 10-11 seconds after the Big Bang, particle energies dropped considerably. At about 10-6 seconds, quarks and gluons combined to form baryons such as protons and neutrons, and a small excess of quarks over antiquarks led to a small excess of baryons over antibaryons.

Since temperatures were not high enough to create new proton-antiproton pairs (or neutron-anitneutron pairs), mass annihilation immediately followed, leaving just one in 1010 of the original protons and neutrons and none of their antiparticles. A similar process happened at about 1 second after the Big Bang for electrons and positrons.

After these annihilations, the remaining protons, neutrons and electrons were no longer moving relativistically and the energy density of the Universe was dominated by photons – and to a lesser extent, neutrinos. A few minutes into the expansion, the period known as Big Bang nucleosynthesis also began.

Thanks to temperatures dropping to 1 billion kelvin and energy densities dropping to about the equivalent of air, neutrons and protons began to combine to form the Universe’s first deuterium (a stable isotope of hydrogen) and helium atoms. However, most of the Universe’s protons remained uncombined as hydrogen nuclei.

After about 379,000 years, electrons combined with these nuclei to form atoms (again, mostly hydrogen), while the radiation decoupled from matter and continued to expand through space, largely unimpeded. This radiation is now known to be what constitutes the Cosmic Microwave Background (CMB), which today is the oldest light in the Universe.

As the CMB expanded, it gradually lost density and energy, and is currently estimated to have a temperature of 2.7260 ± 0.0013 K (-270.424 °C/ -454.763 °F ) and an energy density of 0.25 eV/cm3 (or 4.005×10-14 J/m3; 400–500 photons/cm3). The CMB can be seen in all directions at a distance of roughly 13.8 billion light years, but estimates of its actual distance place it at about 46 billion light years from the center of the Universe.

Evolution of the Universe:

Over the course of the several billion years that followed, the slightly denser regions of the Universe’s matter (which was almost uniformly distributed) began to become gravitationally attracted to each other. They therefore grew even denser, forming gas clouds, stars, galaxies, and the other astronomical structures that we regularly observe today.

This is what is known as the Structure Epoch, since it was during this time that the modern Universe began to take shape. This consisted of visible matter distributed in structures of various sizes (i.e. stars and planets to galaxies, galaxy clusters, and super clusters) where matter is concentrated, and which are separated by enormous gulfs containing few galaxies.

The details of this process depend on the amount and type of matter in the Universe. Cold dark matter, warm dark matter, hot dark matter, and baryonic matter are the four suggested types. However, the Lambda-Cold Dark Matter model (Lambda-CDM), in which the dark matter particles moved slowly compared to the speed of light, is the considered to be the standard model of Big Bang cosmology, as it best fits the available data.

In this model, cold dark matter is estimated to make up about 23% of the matter/energy of the Universe, while baryonic matter makes up about 4.6%. The Lambda refers to the Cosmological Constant, a theory originally proposed by Albert Einstein that attempted to show that the balance of mass-energy in the Universe remains static.

In this case, it is associated with dark energy, which served to accelerate the expansion of the Universe and keep its large-scale structure largely uniform. The existence of dark energy is based on multiple lines of evidence, all of which indicate that the Universe is permeated by it. Based on observations, it is estimated that 73% of the Universe is made up of this energy.

During the earliest phases of the Universe, when all of the baryonic matter was more closely space together, gravity predominated. However, after billions of years of expansion, the growing abundance of dark energy led it to begin dominating interactions between galaxies. This triggered an acceleration, which is known as the Cosmic Acceleration Epoch.

When this period began is subject to debate, but it is estimated to have began roughly 8.8 billion years after the Big Bang (5 billion years ago). Cosmologists rely on both quantum mechanics and Einstein’s General Relativity to describe the process of cosmic evolution that took place during this period and any time after the Inflationary Epoch.

Through a rigorous process of observations and modeling, scientists have determined that this evolutionary period does accord with Einstein’s field equations, though the true nature of dark energy remains illusive. What’s more, there are no well-supported models that are capable of determining what took place in the Universe prior to the period predating 10-15 seconds after the Big Bang.

However, ongoing experiments using CERN’s Large Hadron Collider (LHC) seek to recreate the energy conditions that would have existed during the Big Bang, which is also expected to reveal physics that go beyond the realm of the Standard Model.

Any breakthroughs in this area will likely lead to a unified theory of quantum gravitation, where scientists will finally be able to understand how gravity interacts with the three other fundamental forces of the physics – electromagnetism, weak nuclear force and strong nuclear force. This, in turn, will also help us to understand what truly happened during the earliest epochs of the Universe.

Structure of the Universe:

The actual size, shape and large-scale structure of the Universe has been the subject of ongoing research. Whereas the oldest light in the Universe that can be observed is 13.8 billion light years away (the CMB), this is not the actual extent of the Universe. Given that the Universe has been in a state of expansion for billion of years, and at velocities that exceed the speed of light, the actual boundary extends far beyond what we can see.

Our current cosmological models indicate that the Universe measures some 91 billion light years (28 billion parsecs) in diameter. In other words, the observable Universe extends outwards from our Solar System to a distance of roughly 46 billion light years in all directions. However, given that the edge of the Universe is not observable, it is not yet clear whether the Universe actually has an edge. For all we know, it goes on forever!

Diagram showing the Lambda-CBR Universe, from the Big Bang to the the current era. Credit: Alex Mittelmann/Coldcreation

Within the observable Universe, matter is distributed in a highly structured fashion. Within galaxies, this consists of large concentrations – i.e. planets, stars, and nebulas – interspersed with large areas of empty space (i.e. interplanetary space and the interstellar medium).

Things are much the same at larger scales, with galaxies being separated by volumes of space filled with gas and dust. At the largest scale, where galaxy clusters and superclusters exist, you have a wispy network of large-scale structures consisting of dense filaments of matter and gigantic cosmic voids.

In terms of its shape, spacetime may exist in one of three possible configurations – positively-curved, negatively-curved and flat. These possibilities are based on the existence of at least four dimensions of space-time (an x-coordinate, a y-coordinate, a z-coordinate, and time), and depend upon the nature of cosmic expansion and whether or not the Universe is finite or infinite.

A positively-curved (or closed) Universe would resemble a four-dimensional sphere that would be finite in space and with no discernible edge. A negatively-curved (or open) Universe would look like a four-dimensional “saddle” and would have no boundaries in space or time.

Various possible shapes of the observable Universe – where mass/energy density is too high; too low – or just right, so that Euclidean geometry where the three anles of a triable do add up to 180 degrees. Credit: Wikipedia Commons

In the former scenario, the Universe would have to stop expanding due to an overabundance of energy. In the latter, it would contain too little energy to ever stop expanding. In the third and final scenario – a flat Universe – a critical amount of energy would exist and its expansion woudl only halt after an infinite amount of time.

Fate of the Universe:

Hypothesizing that the Universe had a starting point naturally gives rise to questions about a possible end point. If the Universe began as a tiny point of infinite density that started to expand, does that mean it will continue to expand indefinitely? Or will it one day run out of expansive force, and begin retreating inward until all matter crunches back into a tiny ball?

Answering this question has been a major focus of cosmologists ever since the debate about which model of the Universe was the correct one began. With the acceptance of the Big Bang Theory, but prior to the observation of dark energy in the 1990s, cosmologists had come to agree on two scenarios as being the most likely outcomes for our Universe.

In the first, commonly known as the “Big Crunch” scenario, the Universe will reach a maximum size and then begin to collapse in on itself. This will only be possible if the mass density of the Universe is greater than the critical density. In other words, as long as the density of matter remains at or above a certain value (1-3 ×10-26 kg of matter per m³), the Universe will eventually contract.

Alternatively, if the density in the Universe were equal to or below the critical density, the expansion would slow down but never stop. In this scenario, known as the “Big Freeze”, the Universe would go on until star formation eventually ceased with the consumption of all the interstellar gas in each galaxy. Meanwhile, all existing stars would burn out and become white dwarfs, neutron stars, and black holes.

Very gradually, collisions between these black holes would result in mass accumulating into larger and larger black holes. The average temperature of the Universe would approach absolute zero, and black holes would evaporate after emitting the last of their Hawking radiation. Finally, the entropy of the Universe would increase to the point where no organized form of energy could be extracted from it (a scenarios known as “heat death”).

Modern observations, which include the existence of dark energy and its influence on cosmic expansion, have led to the conclusion that more and more of the currently visible Universe will pass beyond our event horizon (i.e. the CMB, the edge of what we can see) and become invisible to us. The eventual result of this is not currently known, but “heat death” is considered a likely end point in this scenario too.

Other explanations of dark energy, called phantom energy theories, suggest that ultimately galaxy clusters, stars, planets, atoms, nuclei, and matter itself will be torn apart by the ever-increasing expansion. This scenario is known as the “Big Rip”, in which the expansion of the Universe itself will eventually be its undoing.

History of Study:

Strictly speaking, human beings have been contemplating and studying the nature of the Universe since prehistoric times. As such, the earliest accounts of how the Universe came to be were mythological in nature and passed down orally from one generation to the next. In these stories, the world, space, time, and all life began with a creation event, where a God or Gods were responsible for creating everything.

Astronomy also began to emerge as a field of study by the time of the Ancient Babylonians. Systems of constellations and astrological calendars prepared by Babylonian scholars as early as the 2nd millennium BCE would go on to inform the cosmological and astrological traditions of cultures for thousands of years to come.

By Classical Antiquity, the notion of a Universe that was dictated by physical laws began to emerge. Between Greek and Indian scholars, explanations for creation began to become philosophical in nature, emphasizing cause and effect rather than divine agency. The earliest examples include Thales and Anaximander, two pre-Socratic Greek scholars who argued that everything was born of a primordial form of matter.

By the 5th century BCE, pre-Socratic philosopher Empedocles became the first western scholar to propose a Universe composed of four elements – earth, air, water and fire. This philosophy became very popular in western circles, and was similar to the Chinese system of five elements – metal, wood, water, fire, and earth – that emerged around the same time.

Early atomic theory stated that different materials had differently shaped atoms. Credit: github.com

It was not until Democritus, the 5th/4th century BCE Greek philosopher, that a Universe composed of indivisible particles (atoms) was proposed. The Indian philosopher Kanada (who lived in the 6th or 2nd century BCE) took this philosophy further by proposing that light and heat were the same substance in different form. The 5th century CE Buddhist philosopher Dignana took this even further, proposing that all matter was made up of energy.

The notion of finite time was also a key feature of the Abrahamic religions – Judaism, Christianity and Islam. Perhaps inspired by the Zoroastrian concept of the Day of Judgement, the belief that the Universe had a beginning and end would go on to inform western concepts of cosmology even to the present day.

Between the 2nd millennium BCE and the 2nd century CE, astronomy and astrology continued to develop and evolve. In addition to monitoring the proper motions of the planets and the movement of the constellations through the Zodiac, Greek astronomers also articulated the geocentric model of the Universe, where the Sun, planets and stars revolve around the Earth.

These traditions are best described in the 2nd century CE mathematical and astronomical treatise, the Almagest, which was written by Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy). This treatise and the cosmological model it espoused would be considered canon by medieval European and Islamic scholars for over a thousand years to come.

A comparison of the geocentric and heliocentric models of the Universe. Credit: history.ucsb.edu

However, even before the Scientific Revolution (ca. 16th to 18th centuries), there were astronomers who proposed a heliocentric model of the Universe – where the Earth, planets and stars revolved around the Sun. These included Greek astronomer Aristarchus of Samos (ca. 310 – 230 BCE), and Hellenistic astronomer and philosopher Seleucus of Seleucia (190 – 150 BCE).

During the Middle Ages, Indian, Persian and Arabic philosophers and scholars maintained and expanded on Classical astronomy. In addition to keeping Ptolemaic and non-Aristotelian ideas alive, they also proposed revolutionary ideas like the rotation of the Earth. Some scholars – such as Indian astronomer Aryabhata and Persian astronomers Albumasar and Al-Sijzi – even advanced versions of a heliocentric Universe.

By the 16th century, Nicolaus Copernicus proposed the most complete concept of a heliocentric Universe by resolving lingering mathematical problems with the theory. His ideas were first expressed in the 40-page manuscript titled Commentariolus (“Little Commentary”), which described a heliocentric model based on seven general principles. These seven principles stated that:

  1. Celestial bodies do not all revolve around a single point
  2. The center of Earth is the center of the lunar sphere—the orbit of the moon around Earth; all the spheres rotate around the Sun, which is near the center of the Universe
  3. The distance between Earth and the Sun is an insignificant fraction of the distance from Earth and Sun to the stars, so parallax is not observed in the stars
  4. The stars are immovable – their apparent daily motion is caused by the daily rotation of Earth
  5. Earth is moved in a sphere around the Sun, causing the apparent annual migration of the Sun
  6. Earth has more than one motion
  7. Earth’s orbital motion around the Sun causes the seeming reverse in direction of the motions of the planets.
Frontispiece and title page of the Dialogue, 1632. Credit: moro.imss.fi.it

A more comprehensive treatment of his ideas was released in 1532, when Copernicus completed his magnum opus – De revolutionibus orbium coelestium (On the Revolutions of the Heavenly Spheres). In it, he advanced his seven major arguments, but in more detailed form and with detailed computations to back them up. Due to fears of persecution and backlash, this volume was not released until his death in 1542.

His ideas would be further refined by 16th/17th century mathematicians, astronomer and inventor Galileo Galilei. Using a telescope of his own creation, Galileo would make recorded observations of the Moon, the Sun, and Jupiter which demonstrated flaws in the geocentric model of the Universe while also showcasing the internal consistency of the Copernican model.

His observations were published in several different volumes throughout the early 17th century. His observations of the cratered surface of the Moon and his observations of Jupiter and its largest moons were detailed in 1610 with his Sidereus Nuncius (The Starry Messenger) while his observations were sunspots were described in On the Spots Observed in the Sun (1610).

Galileo also recorded his observations about the Milky Way in the Starry Messenger, which was previously believed to be nebulous. Instead, Galileo found that it was a multitude of stars packed so densely together that it appeared from a distance to look like clouds, but which were actually stars that were much farther away than previously thought.

In 1632, Galileo finally addressed the “Great Debate” in his treatise Dialogo sopra i due massimi sistemi del mondo (Dialogue Concerning the Two Chief World Systems), in which he advocated the heliocentric model over the geocentric. Using his own telescopic observations, modern physics and rigorous logic, Galileo’s arguments effectively undermined the basis of Aristotle’s and Ptolemy’s system for a growing and receptive audience.

Johannes Kepler advanced the model further with his theory of the elliptical orbits of the planets. Combined with accurate tables that predicted the positions of the planets, the Copernican model was effectively proven. From the middle of the seventeenth century onward, there were few astronomers who were not Copernicans.

The next great contribution came from Sir Isaac Newton (1642/43 – 1727), who’s work with Kepler’s Laws of Planetary Motion led him to develop his theory of Universal Gravitation. In 1687, he published his famous treatise Philosophiæ Naturalis Principia Mathematica (“Mathematical Principles of Natural Philosophy”), which detailed his Three Laws of Motion. These laws stated that:

  1. When viewed in an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by an external force.
  2. The vector sum of the external forces (F) on an object is equal to the mass (m) of that object multiplied by the acceleration vector (a) of the object. In mathematical form, this is expressed as: F=ma
  3. When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.
Animated diagram showing the spacing of the Solar Systems planet’s, the unusually closely spaced orbits of six of the most distant KBOs, and the possible “Planet 9”. Credit: Caltech/nagualdesign

Together, these laws described the relationship between any object, the forces acting upon it, and the resulting motion, thus laying the foundation for classical mechanics. The laws also allowed Newton to calculate the mass of each planet, calculate the flattening of the Earth at the poles and the bulge at the equator, and how the gravitational pull of the Sun and Moon create the Earth’s tides.

His calculus-like method of geometrical analysis was also able to account for the speed of sound in air (based on Boyle’s Law), the precession of the equinoxes – which he showed were a result of the Moon’s gravitational attraction to the Earth – and determine the orbits of the comets. This volume would have a profound effect on the sciences, with its principles remaining canon for the following 200 years.

Another major discovery took place in 1755, when Immanuel Kant proposed that the Milky Way was a large collection of stars held together by mutual gravity. Just like the Solar System, this collection of stars would be rotating and flattened out as a disk, with the Solar System embedded within it.

Astronomer William Herschel attempted to actually map out the shape of the Milky Way in 1785, but he didn’t realize that large portions of the galaxy are obscured by gas and dust, which hides its true shape. The next great leap in the study of the Universe and the laws that govern it did not come until the 20th century, with the development of Einstein’s theories of Special and General Relativity.

Einstein’s groundbreaking theories about space and time (summed up simply as E=mc²) were in part the result of his attempts to resolve Newton’s laws of mechanics with the laws of electromagnetism (as characterized by Maxwell’s equations and the Lorentz force law). Eventually, Einstein would resolve the inconsistency between these two fields by proposing Special Relativity in his 1905 paper, “On the Electrodynamics of Moving Bodies“.

Basically, this theory stated that the speed of light is the same in all inertial reference frames. This broke with the previously-held consensus that light traveling through a moving medium would be dragged along by that medium, which meant that the speed of the light is the sum of its speed through a medium plus the speed of that medium. This theory led to multiple issues that proved insurmountable prior to Einstein’s theory.

Special Relativity not only reconciled Maxwell’s equations for electricity and magnetism with the laws of mechanics, but also simplified the mathematical calculations by doing away with extraneous explanations used by other scientists. It also made the existence of a medium entirely superfluous, accorded with the directly observed speed of light, and accounted for the observed aberrations.

Between 1907 and 1911, Einstein began considering how Special Relativity could be applied to gravity fields – what would come to be known as the Theory of General Relativity. This culminated in 1911 with the publications of  “On the Influence of Gravitation on the Propagation of Light“, in which he predicted that time is relative to the observer  and dependent on their position within a gravity field.

He also advanced what is known as the Equivalence Principle, which states that gravitational mass is identical to inertial mass. Einstein also predicted the phenomenon of gravitational time dilation – where two observers situated at varying distances from a gravitating mass perceive a difference in the amount of time between two events. Another major outgrowth of his theories were the existence of Black Holes and an expanding Universe.

In 1915, a few months after Einstein had published his Theory of General Relativity, German physicist and astronomer Karl Schwarzschild found a solution to the Einstein field equations that described the gravitational field of a point and spherical mass. This solution, now called the Schwarzschild radius, describes a point where the mass of a sphere is so compressed that the escape velocity from the surface would equal the speed of light.

In 1931, Indian-American astrophysicist Subrahmanyan Chandrasekhar calculated, using Special Relativity, that a non-rotating body of electron-degenerate matter above a certain limiting mass would collapse in on itself. In 1939, Robert Oppenheimer and others concurred with Chandrasekhar’s analysis, claiming that neutron stars above a prescribed limit would collapse into black holes.

Another consequence of General Relativity was the prediction that the Universe was either in a state of expansion or contraction. In 1929, Edwin Hubble confirmed that the former was the case. At the time, this appeared to disprove Einstein’s theory of a Cosmological Constant, which was a force which “held back gravity” to ensure that the distribution of matter in the Universe remained uniform over time.

To this, Edwin Hubble demonstrated using redshift  measurements that galaxies were moving away from the Milky Way. What’s more, he showed that the galaxies that were farther from Earth appeared to be receding faster – a phenomena that would come to be known as Hubble’s Law. Hubble attempted to constrain the value of the expansion factor – which he estimated at 500 km/sec per Megaparsec of space (which has since been revised).

And then in 1931, Georges Lemaitre, a Belgian physicist and Roman Catholic priest, articulated an idea that would give rise to the Big Bang Theory. After confirming independently that the Universe was in a state of expansion, he suggested that the current expansion of the Universe meant that the father back in time one went, the smaller the Universe would be.

In other words, at some point in the past, the entire mass of the Universe would have been concentrated on a single point. These discoveries triggered a debate between physicists throughout the 1920s and 30s, with the majority advocating that the Universe was in a steady state (i.e. the Steady State Theory). In this model, new matter is continuously created as the Universe expands, thus preserving the uniformity and density of matter over time.

After World War II, the debate came to a head between proponents of the Steady State Model and proponents of the Big Bang Theory – which was growing in popularity. Eventually, the observational evidence began to favor the Big Bang over the Steady State, which included the discovery and confirmation of the CMB in 1965. Since that time, astronomers and cosmologists have sought to resolve theoretical problems arising from this model.

In the 1960s, for example, Dark Matter (originally proposed in 1932 by Jan Oort) was proposed as an explanation for the apparent “missing mass” of the Universe. In addition, papers submitted by Stephen Hawking and other physicists showed that singularities were an inevitable initial condition of general relativity and a Big Bang model of cosmology.

In 1981, physicist Alan Guth theorized a period of rapid cosmic expansion (aka. the “Inflation” Epoch) that resolved other theoretical problems. The 1990s also saw the rise of Dark Energy as an attempt to resolve outstanding issues in cosmology. In addition to providing an explanation as to the Universe’s missing mass (along with Dark Matter) it also provided an explanation as to why the Universe is still accelerating, and offered a resolution to Einstein’s Cosmological Constant.

Significant progress has been made in our study of the Universe thanks to advances in telescopes, satellites, and computer simulations. These have allowed astronomers and cosmologists to see farther into the Universe (and hence, farther back in time). This has in turn helped them to gain a better understanding of its true age, and make more precise calculations of its matter-energy density.

The introduction of space telescopes – such as the Cosmic Background Explorer (COBE), the Hubble Space Telescope, Wilkinson Microwave Anisotropy Probe (WMAP) and the Planck Observatory – has also been of immeasurable value. These have not only allowed for deeper views of the cosmos, but allowed astronomers to test theoretical models to observations.

Illustration of the depth by which Hubble imaged galaxies in prior Deep Field initiatives, in units of the Age of the Universe. Credit: NASA and A. Feild (STScI)

For example, in June of 2016, NASA announced findings that indicate that the Universe is expanding even faster than previously thought. Based on new data provided by the Hubble Space Telescope (which was then compared to data from the WMAP and the Planck Observatory) it appeared that the Hubble Constant was 5% to 9% greater than expected.

Next-generation telescopes like the James Webb Space Telescope (JWST) and ground-based telescopes like the Extremely Large Telescope (ELT) are also expected to allow for additional breakthroughs in our understanding of the Universe in the coming years and decades.

Without a doubt, the Universe is beyond the reckoning of our minds. Our best estimates say hat it is unfathomably vast, but for all we know, it could very well extend to infinity. What’s more, its age in almost impossible to contemplate in strictly human terms. In the end, our understanding of it is nothing less than the result of thousands of years of constant and progressive study.

And in spite of that, we’ve only really begun to scratch the surface of the grand enigma that it is the Universe. Perhaps some day we will be able to see to the edge of it (assuming it has one) and be able to resolve the most fundamental questions about how all things in the Universe interact. Until that time, all we can do is measure what we don’t know by what we do, and keep exploring!

To speed you on your way, here is a list of topics we hope you will enjoy and that will answer your questions. Good luck with your exploration!

Further Reading:

Sources:

Posted on by Fraser Cain

Do Stars Move? Tracking Their Movements Across the Sky

How Fast Are Stars Moving?
How Fast Are Stars Moving?

The night sky, is the night sky, is the night sky. The constellations you learned as a child are the same constellations that you see today. Ancient people recognized these same constellations. Oh sure, they might not have had the same name for it, but essentially, we see what they saw.

But when you see animations of galaxies, especially as they come together and collide, you see the stars buzzing around like angry bees. We know that the stars can have motions, and yet, we don’t see them moving?

How fast are they moving, and will we ever be able to tell?

Stars, of course, do move. It’s just that the distances are so great that it’s very difficult to tell. But astronomers have been studying their position for thousands of years. Tracking the position and movements of the stars is known as astrometry.

We trace the history of astrometry back to 190 BC, when the ancient Greek astronomer Hipparchus first created a catalog of the 850 brightest stars in the sky and their position. His student Ptolemy followed up with his own observations of the night sky, creating his important document: the Almagest.

Printed rendition of a geocentric cosmological model from Cosmographia, Antwerp, 1539. Credit: Wikipedia Commons/Fastfission

In the Almagest, Ptolemy laid out his theory for an Earth-centric Universe, with the Moon, Sun, planets and stars in concentric crystal spheres that rotated around the planet. He was wrong about the Universe, of course, but his charts and tables were incredibly accurate, measuring the brightness and location of more than 1,000 stars.

A thousand years later, the Arabic astronomer Abd al-Rahman al-Sufi completed an even more detailed measurement of the sky using an astrolabe.

One of the most famous astronomers in history was the Danish Tycho Brahe. He was renowned for his ability to measure the position of stars, and built incredibly precise instruments for the time to do the job. He measured the positions of stars to within 15 to 35 arcseconds of accuracy. Just for comparison, a human hair, held 10 meters away is an arcsecond wide.

Also, I’m required to inform you that Brahe had a fake nose. He lost his in a duel, but had a brass replacement made.

In 1807, Friedrich Bessel was the first astronomer to measure the distance to a nearby star 61 Cygni. He used the technique of parallax, by measuring the angle to the star when the Earth was on one side of the Sun, and then measuring it again 6 months later when the Earth was on the other side.

With parallax technique, astronomers observe object at opposite ends of Earth’s orbit around the Sun to precisely measure its distance. Credit: Alexandra Angelich, NRAO/AUI/NSF.

Over the course of this period, this relatively closer star moves slightly back and forth against the more distant background of the galaxy.

And over the next two centuries, other astronomers further refined this technique, getting better and better at figuring out the distance and motions of stars.

But to really track the positions and motions of stars, we needed to go to space. In 1989, the European Space Agency launched their Hipparcos mission, named after the Greek astronomer we talked about earlier. Its job was to measure the position and motion of the nearby stars in the Milky Way. Over the course of its mission, Hipparcos accurately measured 118,000 stars, and provided rough calculations for another 2 million stars.

That was useful, and astronomers have relied on it ever since, but something better has arrived, and its name is Gaia.

Credit: ESA/ATG medialab; Background Credit: ESO/S. Brunier

Launched in December 2013, the European Space Agency’s Gaia in is in the process of mapping out a billion stars in the Milky Way. That’s billion, with a B, and accounts for about 1% of the stars in the galaxy. The spacecraft will track the motion of 150 million stars, telling us where everything is going over time. It will be a mind bending accomplishment. Hipparchus would be proud.

With the most precise measurements, taken year after year, the motions of the stars can indeed be calculated. Although they’re not enough to see with the unaided eye, over thousands and tens of thousands of years, the positions of the stars change dramatically in the sky.

The familiar stars in the Big Dipper, for example, look how they do today. But if you go forward or backward in time, the positions of the stars look very different, and eventually completely unrecognizable.

When a star is moving sideways across the sky, astronomers call this “proper motion”. The speed a star moves is typically about 0.1 arc second per year. This is almost imperceptible, but over the course of 2000 years, for example, a typical star would have moved across the sky by about half a degree, or the width of the Moon in the sky.

A 20 year animation showing the proper motion of Barnard’s Star. Credit: Steve Quirk, images in the Public Domain.

The star with the fastest proper motion that we know of is Barnard’s star, zipping through the sky at 10.25 arcseconds a year. In that same 2000 year period, it would have moved 5.5 degrees, or about 11 times the width of your hand. Very fast.

When a star is moving toward or away from us, astronomers call that radial velocity. They measure this by calculating the doppler shift. The light from stars moving towards us is shifted towards the blue side of the spectrum, while stars moving away from us are red-shifted.

Between the proper motion and redshift, you can get a precise calculation for the exact path a star is moving in the sky.

Credit: ESA/ATG medialab

We know, for example, that the dwarf star Hipparcos 85605 is moving rapidly towards us. It’s 16 light-years away right now, but in the next few hundred thousand years, it’s going to get as close as .13 light-years away, or about 8,200 times the distance from the Earth to the Sun. This won’t cause us any direct effect, but the gravitational interaction from the star could kick a bunch of comets out of the Oort cloud and send them down towards the inner Solar System.

The motions of the stars is fairly gentle, jostling through gravitational interactions as they orbit around the center of the Milky Way. But there are other, more catastrophic events that can make stars move much more quickly through space.

When a binary pair of stars gets too close to the supermassive black hole at the center of the Milky Way, one can be consumed by the black hole. The other now has the velocity, without the added mass of its companion. This gives it a high-velocity kick. About once every 100,000 years, a star is kicked right out of the Milky Way from the galactic center.

A rogue star being kicked out of a galaxy. Credit: NASA, ESA, and G. Bacon (STScI)

Another situation can happen where a smaller star is orbiting around a supermassive companion. Over time, the massive star bloats up as supergiant and then detonates as a supernova. Like a stone released from a sling, the smaller star is no longer held in place by gravity, and it hurtles out into space at incredible speeds.

Astronomers have detected these hypervelocity stars moving at 1.1 million kilometers per hour relative to the center of the Milky Way.

All of the methods of stellar motion that I talked about so far are natural. But can you imagine a future civilization that becomes so powerful it could move the stars themselves?

In 1987, the Russian astrophysicist Leonid Shkadov presented a technique that could move a star over vast lengths of time. By building a huge mirror and positioning it on one side of a star, the star itself could act like a thruster.

An example of a stellar engine using a mirror and a Dyson Swarm. Credit: Vedexent at English Wikipedia (CC BY-SA 3.0)

Photons from the star would reflect off the mirror, imparting momentum like a solar sail. The mirror itself would be massive enough that its gravity would attract the star, but the light pressure from the star would keep it from falling in. This would create a slow but steady pressure on the other side of the star, accelerating it in whatever direction the civilization wanted.

Over the course of a few billion years, a star could be relocated pretty much anywhere a civilization wanted within its host galaxy.

This would be a true Type III Civilization. A vast empire with such power and capability that they can rearrange the stars in their entire galaxy into a configuration that they find more useful. Maybe they arrange all the stars into a vast sphere, or some kind of geometric object, to minimize transit and communication times. Or maybe it makes more sense to push them all into a clean flat disk.

Amazingly, astronomers have actually gone looking for galaxies like this. In theory, a galaxy under control by a Type III Civilization should be obvious by the wavelength of light they give off. But so far, none have turned up. It’s all normal, natural galaxies as far as we can see in all directions.

For our short lifetimes, it appears as if the sky is frozen. The stars remain in their exact positions forever, but if you could speed up time, you’d see that everything is in motion, all the time, with stars moving back and forth, like airplanes across the sky. You just need to be patient to see it.

Posted on by Tammy Plotner

Messier 41 – the NGC 2287 Open Star Cluster

Image of the open star cluster Messier 41, highlighting its combination of red dwarf, white dwarf and K3-type class stars. Credit: Wikisky

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the double star known as Messier 41. Enjoy!

During the 18th century, famed French astronomer Charles Messier noted the presence of several “nebulous objects” in the night sky. Having originally mistaken them for comets, he began compiling a list of them so that others would not make the same mistake he did. In time, this list (known as the Messier Catalog) would come to include 100 of the most fabulous objects in the night sky.

One of these objects is the open star cluster known as Messier 41 (aka. M41, NGC 2287). Located in the Canis Major constellation – approximately 4,300 light years from Earth – this cluster lies just four degrees south of Sirius, the brightest star in the night sky. Like most open clusters, it is relatively young – 190 million years old – and contains over 100 stars in a region measuring 25 to 26 light years in diameter.

Description:

Running away from us at a speed of about 34 kilometers per second, this field of about 100 stars measures about 25 light years across. Born about 240 million years ago, it resides in space approximately 2300 light years away from our solar system. Larger aperture telescopes will reveal the presence of many red (or orange) giant stars and the hottest star in this group is a spectral type A.

View of the night sky in North Carolina, showing the constellations of Orion, Hyades, Canis Major and Canis Minor. Credit: NASA

As G.L.H. Harris (et al) explained in a 1993 study:

“We have obtained photoelectric UBV photometry for 100 stars, uvbyb photometry for 39 stars and MK spectral types for 80 stars in the field of NGC 2287. After combination with data from other sources, several interesting cluster properties are apparent. Both the UBV and uvbyb photometry point to a small but nonzero reddening, while our spectral types confirm previous results indicating a high binary frequency for the cluster. Based on our spectral and photometric data for the cluster members, we find a minimum binary frequency of 40% and discuss the possibility that the results may imply a binary frequency closer to 80%. The cluster age is found to be based on both the main-sequence turnoff and the red giant distribution; the width of the turn up region can probably be explained by a combination of duplicity and a range in stellar rotation.”

But there’s more than just red giant stars and various spectral types to be found hiding in Messier 41. There’s at least two white dwarf stars, too. As P.D Dobbie explained in a 2009 study:

“[W]e use our estimates of their cooling times together with the cluster ages to constrain the lifetimes and masses of their progenitor stars. We examine the location of these objects in initial mass-final mass space and find that they now provide no evidence for substantial scatter in initial mass-final mass relation (IFMR) as suggested by previous investigations. This form is generally consistent with the predictions of stellar evolutionary models and can aid population synthesis models in reproducing the relatively sharp drop observed at the high mass end of the main peak in the mass distribution of white dwarfs.”

Messier 41 and Collinder 121. Image: Wikisky

As you view Messier 41, you’ll be impressed with its wide open appearance… and knowing it’s simply what happens to star clusters as they get passed around our galaxy. As Giles Bergond (et al.) stated in their 2001 study:

“Taking into account observational biases, namely the galaxy clustering and differential extinction in the Galaxy, we have associated these stellar overdensities with real open cluster structures stretched by the galactic gravitational field. As predicted by theory and simulations, and despite observational limitations, we detected a general elongated (prolate) shape in a direction parallel to the galactic Plane, combined with tidal tails extended perpendicularly to it. This geometry is due both to the static galactic tidal field and the heating up of the stellar system when crossing the Disk. The time varying tidal field will deeply affect the cluster dynamical evolution, and we emphasize the importance of adiabatic heating during the Disk-shocking. During the 10-20 Z-oscillations experienced by a cluster before its dissolution in the Galaxy, crossings through the galactic Disk contribute to at least 15% of the total mass loss. Using recent age estimations published for open clusters, we find a destruction time-scale of about 600 million years for clusters in the solar neighborhood.”

That means we’ve only got another 360 million years to observe it before it’s completely gone (though some estimates place it at about 500 million). Either way, this star cluster is destined to disappear, perhaps before we are!

History of Observation:

Messier 41 was “possibly” recorded by Aristotle about 325 B.C. as a patch in the Milky Way… quite understandable since it is very much within unaided eye visibility from a dark sky location. Said Aristotle:

“.. some of the fixed stars have tails. And for this we need not rely only on the evidence of the Egyptians who say they have observed it; we have observed it also ourselves. For one of the stars in the thigh of the Dog had a tail, though a dim one: if you looked hard at it the light used to become dim, but to less intent glance it was brighter.”

Messier 41 and Sirius. Image: Wikisky

However, Giovanni Batista Hodierna was the first to catalog it in 1654, and the star cluster became a bit more astronomically known when John Flamsteed independently found it again on February 16, 1702. Doing his duty, Charles Messier also logged it:

“In the night of January 16 to 17, 1765, I have observed below Sirius and near the star Rho of Canis Major a star cluster; when examining it with a night refractor, this cluster appeared nebulous; instead, there is nothing but a cluster of small stars. I have compared the middle with the nearest known star; and I found its right ascension of 98d 58′ 12″, and its declination 20d 33′ 50″ north.”

Following suit, other historical astronomers also observed M41 – including Sir John Herschel to include it in the NGC catalog. While none found it particularly thrilling… their notes range from a “coarse collection of stars” to “very large, bright, little compressed”, perhaps you will feel much differently about this easy, bright target!

Locating Messier 41:

Finding Messier 41 isn’t very difficult for binoculars and small telescopes – all you have to know is the brightest star in the northern hemisphere, Sirius, and south! Simply aim your optics at Sirius and move due south approximately four degrees. That’s about one standard field of view for binoculars, about one field of view for the average telescope finderscope and about 6 fields of view for the average wide field, low power eyepiece.

The location of Messier 41 in the Canis Major constellation. Credit: IAU and Sky & Telescope magazine/Roger Sinnott & Rick Fienberg

Because Messier 41 is a large star cluster, remember to use lowest magnification to get the best effect. Higher magnification can always be used once the star cluster is identified to study individual members. M41 is quite bright and easily resolved and makes a wonderful target for urban skies and moonlit nights!

Because you understand what’s there…

Object Name: Messier 41
Alternative Designations: M41, NGC 2287
Object Type: Open Galactic Star Cluster
Constellation: Canis Major
Right Ascension: 06 : 46.0 (h:m)
Declination: -20 : 44 (deg:m)
Distance: 2.3 (kly)
Visual Brightness: 4.5 (mag)
Apparent Dimension: 38.0 (arc min)

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, , M1 – The Crab Nebula, M8 – The Lagoon Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.

Be to sure to check out our complete Messier Catalog. And for more information, check out the SEDS Messier Database.

Sources:

Posted on by Matt Williams

Does Jupiter Have a Solid Core?

Damian Peach reprocessed one of the latest images taken by Juno's JunoCam during its 3rd close flyby of the planet on Dec. 11. The photo highlights two large 'pearls' or storms in Jupiter's atmosphere. Credit: NASA/JPL-Caltech/SwRI/MSSS

The gas giants have always been a mystery to us. Due to their dense and swirling clouds, it is impossible to get a good look inside them and determine their true structure. Given their distance from Earth, it is time-consuming and expensive to send spacecraft to them, making survey missions few and far between. And due to their intense radiation and strong gravity, any mission that attempts to study them has to do so carefully.

And yet, scientists have been of the opinion for decades that this massive gas giant has a solid core. This is consistent with our current theories of how the Solar System and its planets formed and migrated to their current positions. Whereas the outer layers of Jupiter are composed primarily of hydrogen and helium, increases in pressure and density suggest that closer to the core, things become solid.

Structure and Composition:

Jupiter is composed primarily of gaseous and liquid matter, with denser matter beneath. It’s upper atmosphere is composed of about 88–92% hydrogen and 8–12% helium by percent volume of gas molecules, and approx. 75% hydrogen and 24% helium by mass, with the remaining one percent consisting of other elements.

upiter's structure and composition. (Image Credit: Kelvinsong CC by S.A. 3.0)
Jupiter’s structure and composition. Credit: Kelvinsong CC by S.A. 3.0

The atmosphere contains trace amounts of methane, water vapor, ammonia, and silicon-based compounds, as well as trace amounts of benzene and other hydrocarbons. There are also traces of carbon, ethane, hydrogen sulfide, neon, oxygen, phosphine, and sulfur. Crystals of frozen ammonia have also been observed in the outermost layer of the atmosphere.

The interior contains denser materials, such that the distribution is roughly 71% hydrogen, 24% helium and 5% other elements by mass. It is believed that Jupiter’s core is a dense mix of elements – a surrounding layer of liquid metallic hydrogen with some helium, and an outer layer predominantly of molecular hydrogen. The core has also been described as rocky, but this remains unknown as well.

In 1997, the existence of the core was suggested by gravitational measurements, indicating a mass of 12 to 45 times the mass of Earth, or roughly 4%–14% of the total mass of Jupiter. The presence of a core is also supported by models of planetary formation that indicate how a rocky or icy core would have been necessary at some point in the planet’s history. Otherwise, it would not have been able to collect all of its hydrogen and helium from the protosolar nebula – at least in theory.

However, it is possible that this core has since shrunk due to convection currents of hot, liquid, metallic hydrogen mixing with the molten core. This core may even be absent now, but a detailed analysis is needed before this can be confirmed. The Juno mission, which launched in August 2011 (see below), is expected to provide some insight into these questions, and thereby make progress on the problem of the core.

Formation and Migration:

Our current theories regarding the formation of the Solar System claim that the planets formed about 4.5 billion years ago from a Solar Nebula (i.e. Nebular Hypothesis). Consistent with this theory, Jupiter is believed to have formed as a result of gravity pulling swirling clouds of gas and dust together.

Jupiter acquired most of its mass from material left over from the formation of the Sun, and ended up with more than twice the combined mass of the other planets. In fact, it has been conjectured that it Jupiter had accumulated more mass, it would have become a second star. This is based on the fact that its composition is similar to that of the Sun – being made predominantly of hydrogen.

Artist’s concept of a young star surrounded by a disk of gas and dust – called a protoplanetary disk. Credit: NASA/JPL-Caltech

In addition, current models of Solar System formation also indicate that Jupiter formed farther out from its current position. In what is known as the Grand Tack Hypothesis, Jupiter migrated towards the Sun and settled into its current position by roughly 4 billion years ago. This migration, it has been argued, could have resulted in the destruction of the earlier planets in our Solar System – which may have included Super-Earths closer to the Sun.

Exploration:

While it was not the first robotic spacecraft to visit Jupiter, or the first to study it from orbit (this was done by the Galileo probe between 1995 and 2003), the Juno mission was designed to investigate the deeper mysteries of the Jovian giant. These include Jupiter’s interior, atmosphere, magnetosphere, gravitational field, and the history of the planet’s formation.

The mission launched in August 2011 and achieved orbit around Jupiter on July 4th, 2016. The probe entered its polar elliptical orbit after completing a 35-minute-long firing of the main engine, known as Jupiter Orbital Insertion (or JOI). As the probe approached Jupiter from above its north pole, it was afforded a view of the Jovian system, which it took a final picture of before commencing JOI.

Since that time, the Juno spacecraft has been conducting perijove maneuvers – where it passes between the northern polar region and the southern polar region – with a period of about 53 days. It has completed 5 perijoves since it arrived in June of 2016, and it is scheduled to conduct a total of 12 before February of 2018. At this point, barring any mission extensions, the probe will de-orbit and burn up in Jupiter’s outer atmosphere.

As it makes its remaining passes, Juno will gather more information on Jupiter’s gravity, magnetic fields, atmosphere, and composition. It is hoped that this information will teach us much about how the interaction between Jupiter’s interior, its atmosphere and its magnetosphere drives the planet’s evolution. And of course, it is hoped to provide conclusive data on the interior structure of the planet.

Does Jupiter have a solid core? The short answer is, we don’t know… yet. In truth, it could very well have a solid core composed of iron and quartz, which is surrounded by a thick layer of metallic hydrogen. It is also possible that interaction between this metallic hydrogen and the solid core caused the the planet to lose it some time ago.

The South Pole of Jupiter, taken during the Juno mission’s third orbit (Perijove 3). Credits: NASA/JPL-Caltech/SwRI/MSSS/ Luca Fornaciari © cc nc sa

At this point, all we can do is hope that ongoing surveys and missions will yield more evidence. These are not only likely to help us refine our understanding of Jupiter’s internal structure and its formation, but also refine our understanding of the history of the Solar System and how it came to be.

We have written many articles about Jupiter for Universe Today. Here Ten Interesting Facts About Jupiter, How Big is Jupiter?, How Long Does it Take to get to Jupiter?, What is the Weather Like on Jupiter?, How Far is Jupiter from the Sun?, and The Orbit of Jupiter. How Long is a Year on Jupiter?

If you’d like more information on Jupiter, check out Hubblesite’s News Releases about Jupiter, and here’s a link to NASA’s Solar System Exploration Guide to Jupiter.

We’ve also recorded an episode of Astronomy Cast just about Jupiter. Listen here, Episode 56: Jupiter.

Sources:

Posted on by Tammy Plotner

The Cetus Constellation

The Cetus Constellation. Credit and Copyright ©: Torsten Bronger/Wikipedia Commons

Welcome back to Constellation Friday! Today, in honor of the late and great Tammy Plotner, we will be dealing with the sea monster – the Cetus constellation!

In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of all the then-known 48 constellations. This treatise, known as the Almagest, would be used by medieval European and Islamic scholars for over a thousand years to come, effectively becoming astrological and astronomical canon until the early Modern Age.

One of these constellations is Cetus, which was named in honor of the sea monster from Greek mythology.  Cetus is the fourth largest constellation in the sky, the majority of which resides just below the ecliptic plane. Here, it is bordered by many “watery” constellations – including Aquarius, Pices, Eridanus, Piscis Austrinus, Capricornus – as well as Aries, Sculptor, Fornax and Taurus. Today, it is one of the 88 modern constellations recognized by the IAU.

Name and Meaning:

In mythology, Cetus ties in with the legendary Cepheus,Cassiopeia, Andromeda, Perseus tale – for Cetus is the monster to which poor Andromeda was to be sacrificed. (This whole tale is quite wonderful when studied, for we can also tie in Pegasus as Perseus’ horse, Algol and the whom he slew to get to Andromeda and much, much more!)

Cetus, as represented by Sidney Hall in this card from Urania’s Mirror (1825). Credit: Library of Congress/Sidney Hall

As for poor, ugly Cetus. He also represents the gates to the underworld thanks to his position just under the ecliptic plane. Arab legend has it that Cetus carries two pearl necklaces – one broken and the other intact – which oddly enough, you can see among its faint stars in the circular patterns when nights are dark. No matter what the legends are, Cetus is an rather dim, but interesting constellation!

History of Observation:

Cetus was one of many Mesopotamian constellations that passed down to the Greeks. Originally, Cetus may have been associated with a whale, and is often referred to as the Whale. However, its most common representation is that of the sea monster that was slain by Perseus.

In the 17th century, Cetus was depicted variously as a “dragon fish” (by Johann Bayer), and as a whale-like creature by famed 17th-century cartographers Willem Blaeu and Andreas Cellarius. However, Cetus has also been variously depicted with animal heads attached to an aquatic animal body.

The constellation is also represented in many non-Western astrological systems.In Chinese astronomy, the stars of Cetus are found among the Black Tortoise of the North (B?i F?ng Xuán W?) and the White Tiger of the West (X? F?ng Bái H?).

Cetus, as depicted by famed 17th century cartographer Willem Blaeu, 1602. Credit: WIkipedia Commons/Erik Lernestål

Notable Features:

Cetus sprawls across 1231 square degrees of sky and contains 15 main stars, highlighted by 3 bright stars and 88 Bayer/Flamsteed designations. It’s brightest star is Beta Ceti, otherwise known as Deneb Kaitos (Diphda), a type K0III orange giant which is located approximately 96.3 light years away. This star has left its main sequence and is on its way to becoming a red giant.

The name Deneb Kaitos is derived from the Arabic “Al Dhanab al Kaitos al Janubiyy”, which translates as “the southern tail of Cetus”. The name Diphda comes “ad-dafda at-tani“, which is Arabic for “the second frog” – the star Fomalhaut in neighboring Piscis Austrinus is usually referred to as the first frog.)

Then there’s Alpha Ceti, a very old red giant star located approximately 249 light years from Earth. It’s traditional name (Menkar), is derived from the Arabic word for “nostril”. Then comes Omicron Ceti, also known as Mira, binary star consisting located approximately 420 light years away. This binary system consists of an oscillating variable red giant (Mira A).

After being recorded for the first time by David Fabricius (on August 3, 1596), Mira has since gone on to become the prototype for the Mira class of variables (of which there are six or seven thousand known examples). These stars are red giants whose surfaces oscillate in such a way as to cause variations in brightness over periods ranging from 80 to more than 1,000 days.

Composite image of Messier 77 (NGC 1068), showing it in the visible, X-ray, and radio spectrums. Credit: NASA/CXC/MIT/C.Canizares/D.Evans et al/STScI/NSF/NRAO/VLA

Cetus is also home to many Deep Sky Objects. A notable examples is the barred spiral galaxy known as Messier 77, which is located approximately 47 million light years away and is 170,000 light years in diameter, making it one of the largest galaxies listed in Messier’s catalogue. It has an Active Galactic Nucleus (AGN) which is obscured from view by intergalactic dust, but remains an active radio source.

Then there’s NGC 1055, a spiral galaxy that lies just 0.5 north by northeast of Messier 77. It is located approximately 52 million light years away and is seen edge-on from Earth. Next to Messier 77, NGC 1055 is a largest member of a galaxy group – measuring 115,800 light years in diameter – that also includes NGC 1073 and several smaller irregular galaxies. It has a diameter of about 115,800 light years. The galaxy is a known radio source.

Finding Cetus:

Cetus is the fourth largest constellation in the sky, is visible at latitudes between +70° and -90° and is best seen at culmination during the month of November. Of all the stars in Cetus, the very first you must look for in binoculars is Mira. Omicron Ceti was the very first variable star discovered and was perhaps known as far back as ancient China, Babylon or Greece. The variability was first recorded by the astronomer David Fabricius while observing Mercury.

Now aim your binoculars at Alpha Ceti. It’s name is Menkar and we do know something about it. Menkar is an old and dying star, long past the hydrogen and perhaps even past the helium stage of its stellar evolution. Right now it’s a red giant star but as it begins to burn its carbon core it will likely become highly unstable before finally shedding its outer layers and forming a planetary nebula, leaving a relatively large white dwarf remnant.

Location of Mira and Tau Ceti. Credit: Constellation Guide/Torsten Bronger

Hop down to Beta Ceti – Diphda. Oddly enough, Diphda is actually the brightest star in Cetus, despite its beta designation. It is a giant star with a stellar corona that’s brightening with age – exerting about 2000 times more x-ray power than our Sun! For some reason, it has gone into an advanced stage if stellar evolution called core helium burning – where it is converting helium directly to carbon.

Are you ready to get out your telescope now? Then aim at Diphda and drop south a couple of degrees for NGC 247. This is a very definite spiral galaxy with an intense “stellar” nucleus! Sitting right up in the eyepiece as a delightful oval, the NGC 247 is has a very proper galaxy structure with a defined core area and a concentration that slowly disperses toward its boundaries with one well-defined dark dust lane helping to enhance a spiral arm. Most entertaining! Continuing “down” we move on to the NGC 253. Talk about bright!

Very few galactic studies come in this magnitude (small telescopes will pick it up very well, but it requires large aperture to study structure.) Very elongated and hazy, it reminds me sharply of the “Andromeda Galaxy”. The center is very concentrated and the spiral arms wrap their way around it beautifully! Dust lanes and bright hints of concentration are most evident. and its most endearing feature is that it seems to be set within a mini “Trapezium” of stars. A very worthy study…

Now, let’s hop off to Delta Ceti, shall we? I want to rock your world – because spiral galaxy M77 rocked mine! Once again, easily achieved in the small telescope, Messier 77 comes “alive” with aperture. This one has an incredible nucleus and very pronounced spiral arms – three big, fat ones! Underscored by dark dust lanes, the arms swirl away from the center in a galactic display that takes your breath away!

The location of the Cetus Constellation. Credit: IAU/Sky&Telescope magazine

The “mottling” inside the structure is not just a hint in this ovalish galaxy. I guarantee you won’t find this one “ho hum”… how could you when you know you’re looking at something that’s 47.0 million light-years away! Messier 77 is an active galaxy with an Active Galactic Nucleus (AGN) and one of the brightest Seyfert galaxies known.

Now, return to Delta and the “fall line” runs west to east on the north side. First up is galaxy NGC 1073, a very pretty little spiral galaxy with a very “stretched” appearing nucleus that seems to be “ringed” by its arms! Continuing along the same trajectory, we find the NGC 1055. Oh, yes… Edge-on, lenticular galaxy! This soft streak of light is accompanied by a trio of stars. The galaxy itself is cut through by a dark dust lane, but what appears so unusual is the core is to one side!

Now we’ve made it to back to the incredible M77, but let’s keep on the path and pick up the NGC 1087 – a nice, even-looking spiral galaxy with a bright nucleus and one curved arm. Ready to head for the beautiful variable Mira again? Then let her be the guide star, because halfway between there and Delta is the NGC 936 – a soft spiral galaxy with a “saturn” shaped nucleus. Nice starhoppin’!

We have written many interesting articles about the constellation here at Universe Today. Here is What Are The Constellations?, What Is The Zodiac?, and Zodiac Signs And Their Dates.

Be sure to check out The Messier Catalog while you’re at it!

For more information, check out the IAUs list of Constellations, and the Students for the Exploration and Development of Space page on Canes Venatici and Constellation Families.

Sources:

Posted on by Fraser Cain

Why Do Rockets Need Stages? The Quest to Build a Single Stage to Orbit (SSTO)

Single Stage To Orbit!
Single Stage To Orbit!


Now, don’t get me wrong, Science Fiction is awesome. Like almost everyone working in the field of space and astronomy, I was deeply influenced by science fiction. For me, it was Star Trek and Star Wars. I had a toy phaser that made this awesome really loud phaser sound, and I played with it non-stop until it disappeared one day. And I was sure I’d left it in the middle of my floor, like I did with all my toys, but I found it a few years later, hidden up in a closet that I couldn’t reach. And I always wondered how it got there.

Anyway, back to science fiction. For all of its inspiration, science fiction has put a few ideas into our brains which aren’t entirely helpful. You know, warp drives, artificial gravity, teleportation, and rockets that take off, fly to space, visit other planets orbiting stars, land again.

The Millennium Falcon, Firefly, and Enterprise Shuttles are all examples of single stage to orbit to orbit spacecraft, or SSTOs.

Consider the rockets that exist in reality, you know, the Atlases, Falcons and Deltas. They take off from a launch pad, fly for a bit until the fuel is used up in a stage of the rocket, then they jettison that stage and thrust with the next stage. The mighty Saturn V was so powerful that it had three stages, as it made it’s way to orbit.

Diagram of Saturn V Launch Vehicle. Credit: NASA/MSFC

As we discussed in a previous article, SpaceX is working to make the first stage, and maybe even the second stage reusable, which is a vast improvement over just letting everything burn up, but there are no rockets that actually fly to orbit and back in a single stage. In fact, using the technology we have today, it’s probably not a good idea.

Has anyone ever worked on a single stage to orbit? What technological advances will need to happen to make this work?

As I said earlier, a single stage to orbit rocket would be something like the Millennium Falcon. It carries fuel, and then uses that fuel to fly into orbit, and from world to world. Once it runs out of fuel, it gets filled up again, and then it’s off again, making the Kessel Run and avoiding Imperial Blockades.

This concept of a rocket matches our personal experience with every other vehicle we’ve ever been in. You drive your car around and refuel it, same with boats, airplanes and every other form of Earth-based transportation.

But flying into space requires the expenditure of energy that defies comprehension. Let me give you an example. A Falcon 9 rocket can lift about 22,800 kilograms into low-Earth orbit. That’s about the same as a fully loaded cement truck – which is a lot.

SpaceX Falcon 9 poised for Jan. 14, 2017, Return to Flight launch from Vandenberg Air Force Base in California carrying ten Iridium NEXT comsats to orbit. Credit: SpaceX

The entire fueled Falcon 9 weighs just over 540,000 kg, of which more than 510,000 kgs of it are fuel, with a little extra mass for the engines, fuel tanks, etc. Imagine if you drove a car that was essentially 95% fuel.

The problem is specific impulse; the maximum amount of thrust that a specific kind of engine and fuel type can achieve. I’m not going to go into all the details, but the most efficient chemical rockets we have, fueled by liquid hydrogen and oxygen, can just barely deliver enough thrust to get you to orbit. They have a maximum specific impulse of about 450 seconds.

Because the amount of fuel it takes to launch a rocket is so high, modern rockets use a staging system. Once a stage has emptied out all its fuel, it detaches and returns to Earth so that the second stage can keep going without having to drag along the extra weight of the empty fuel tanks.

After stage separation of the Falcon 9 rocket, flames are barely visible around nozzle as the second stage engine ignites and the first stage falls back to the Earth below. Credit: SpaceX

You might be surprised to know that many modern rockets are actually capable of reaching orbit with a single stage. The problem is that they wouldn’t be able to carry any significant payload.

At the end of the day, considering the chemical rockets we have today, the multi-staged profile is the most efficient and cost-effective strategy for carrying the most payload to space for the lowest cost possible.

Has anyone tried developing SSTOs in the past? Definitely. Probably the most widely publicized was NASA’s X-33/VentureStar program, developed by Lockheed Martin in the 1990s.

The proposed X-33 spacecraft. Credit: NASA

The purpose of the X-33 was to test out a range of new technologies for NASA, including composite fuel tanks, autonomous flight, and a new lifting body design.

In order to make this work, they developed a new kind of rocket engine called the “aerospike”. Unlike a regular rocket engine which provide a fixed amount of thrust, an aerospike could be throttled back like a jet engine, using less fuel at lower altitudes, where the atmosphere is thickest.

The test of twin Linear Aerospike XRS-2200 engines, originally built for the X-33 program, was performed on August 6, 2001 at NASA’s Sternis Space Center, Mississippi. The engines were fired for the planned 90 seconds and reached a planned maximum power of 85 percent. Credit: NASA’s Marshall Space Flight Center

Lockheed Martin was working on a 1/3rd scale prototype, but they struggled with many of the new technologies. In the end, their failure to be able to build a composite fuel tank that could contain the liquid oxygen and hydrogen forced them to abandon the project.

Even if they could get the technology working, so the X-33 was fully reusable, its ability to carry a payload would have been dramatically lower than a traditional multi-staged rocket.

In order to really achieve the dream of single stage to orbit, we need to step away from chemical rockets and move to a type of engine that can deliver thrust more efficiently.

We know that jets work more efficiently than rockets, because they only need to carry fuel. They pull oxygen in from the atmosphere, to burn the fuel. So one intriguing idea is to make a rocket that acts like a jet engine while in the atmosphere, and then acts like a rocket once it’s out in space.

And that’s the plan with the British Skylon rocket. It would take off from a regular runway, accelerate to about 6,600 km/h reaching an altitude of 26 kilometers. All this time, its SABRE engine would be pulling in oxygen from the atmosphere, combining it with hydrogen fuel.

An artist’s conception of Reaction Engines’ Skylon spacecraft. Credit: Reaction Engines

From this point, it would switch over to an internal liquid oxygen tank to provide oxidizer, and complete the flight to orbit. All the while using the same flexible SABRE engine. Once in orbit, it would release its 15-tonne payload and then return to Earth, landing on a runway like the space shuttle orbiter did. It’s a really creative idea.

Unfortunately, the development of the Skylon has taken a long time, with shrinking budgets limiting the amount of tests they’ve been able to do. If everything goes well, the first prototype might fly within a few years, so stay tuned to this story.

Another idea which has had some testing is the idea of a nuclear rocket. Unlike a chemical rocket, which burns fuel, and blasts it out the back for thrust, a nuclear rocket would carry a reactor on board. It would heat up some kind of working fuel, like liquid hydrogen, and then blast it out the back for propulsion.

The key elements of a NERVA solid-core nuclear-thermal engine. Credit: NASA

NASA did some tests a few decades ago with a nuclear thermal rocket called NERVA, and found that they could sustain high levels of thrust for very long periods of time. Their final prototype, provided continuous thrust for over 2 hours, including 28 minutes at full power.

NASA calculated that a nuclear-powered rocket would be roughly twice as efficient as a traditional chemical rocket. It would have a specific impulse of more than 950 seconds. But flying a nuclear rocket into space comes with a significant downside. Rockets explode. It’s bad when a chemical rocket explodes, but if a nuclear reactor detonated while making its way up through the atmosphere, it would rain down radioactive debris. For now, that’s considered too much of a risk; however, future interplanetary missions may very well use nuclear rockets.

There’s one more exotic fuel system that’s really exciting – metallic hydrogen. This solid form appears naturally at the heart of Jupiter, under the incredible pressure of the planet’s gravity. But earlier this year, researchers at Harvard finally created some in the lab. They used a tiny vice to squeeze hydrogen atoms with more force than the pressures at the center of the Earth.

Microscopic images of the stages in the creation of atomic molecular hydrogen: Transparent molecular hydrogen (left) at about 200 GPa, which is converted into black molecular hydrogen, and finally reflective atomic metallic hydrogen at 495 GPa. Credit: Isaac Silvera

It took an enormous amount of energy to squeeze hydrogen together that tightly, but in theory, once crafted, it should be relatively stable. And here’s the best part. When you ignite it, you get that energy back.

If used as a rocket fuel, it would provide a specific impulse of 1700 seconds. Compare that to the mere 450 from chemical rockets. A rocket powered by metallic hydrogen would easily get to orbit with a single stage, and travel efficiently to other planets.

Single Stage to Orbit rockets would be awesome. Science fiction has foretold it. That said, at the end of the day, whatever gets the most amount of payload into orbit for the lowest price is the most interesting rocket system. And right now, that’s staged rockets.

However, a bigger issue might be reliability and reusability. If you can get a single vehicle that takes off, travels to orbit and then returns to its launch pad, you can’t get anything simpler than that. No rockets to restack, no barges to navigate. You just use and reuse the same system again and again, and that’s a really exciting idea.

Right this moment, reusable staged rockets like SpaceX has the edge, but if and when the Skylon gets flying, I think we’ll have some serious competition.

Once we master metallic hydrogen, spaceflight will look very very different. Science reality will nearly match science fiction, and I’ll finally be able to fly my own personal Millennium Falcon.

Posted on by Fraser Cain

What Are Fast Radio Bursts?

298 What Are Fast Radio Bursts?
298 What Are Fast Radio Bursts?


You might think you’re reading an educational website, where I explain fascinating concepts in space and astronomy, but that’s not really what’s going on here.

What’s actually happening is that you’re tagging along as I learn more and more about new and cool things happening in the Universe. I dig into them like a badger hiding a cow carcass, and we all get to enjoy the cache of knowledge I uncover.

Okay, that analogy got a little weird. Anyway, my point is. Squirrel!

Fast radio bursts are the new cosmic whatzits confusing and baffling astronomers, and now we get to take a front seat and watch them move through all stages of process of discovery.

Stage 1: A strange new anomaly is discovered that doesn’t fit any current model of the cosmos. For example, strange Boyajian’s Star. You know, that star that probably doesn’t have an alien megastructure orbiting around it, but astronomers can’t rule that out just yet?

Stage 2: Astronomers struggle to find other examples of this thing. They pitch ideas for new missions and scientific instruments. No idea is too crazy, until it’s proven to be too crazy. Examples include dark matter, dark energy, and that idea that we’re living in a

Stage 3: Astronomers develop a model for the thing, find evidence that matches their predictions, and vast majority of the astronomical community comes to a consensus on what this thing is. Like quasars and gamma ray bursts. YouTuber’s make their videos. Textbooks are updated. Balance is restored.

Today we’re going to talk about Fast Radio Bursts. They just moved from Stage 1 to Stage 2. Let’s dig in.

Fast radio bursts, or FRBs, or “Furbys” were first detected in 2007 by the astronomer Duncan Lorimer from West Virginia University.

He was looking through an archive of pulsar observations. Pulsars, of course, are newly formed neutron stars, the remnants left over from supernova explosions. They spin rapidly, blasting out twin beams of radiation. Some can spin hundreds of times a second, so precisely you could set your watch to them.

Parkes radio dish
Lorimer’s archive of pulsar observations was captured at the Parkes radio dish in Australia. Credit: CSIRO (CC BY 3.0)

In this data, Lorimer made a “that’s funny” observation, when he noticed one blast of radio waves that squealed for 5 milliseconds and then it was gone. It didn’t match any other observation or prediction of what should be out there, so astronomers set out to find more of them.

Over the last 10 years, astronomers have found about 25 more examples of Fast Radio Bursts. Each one only lasts a few milliseconds, and then fades away forever. A one time event that can appear anywhere in the sky and only last for a couple milliseconds and never repeats is not an astronomer’s favorite target of study.

Actually, one FRB has been found to repeat, maybe.

The question, of course, is “what are they?”. And the answer, right now is, “astronomers have no idea.”

In fact, until very recently, astronomers weren’t ever certain they were coming from space at all. We’re surrounded by radio signals all the time, so a terrestrial source of fast radio bursts seems totally logical.

About a week ago, astronomers from Australia announced that FRBs are definitely coming from outside the Earth. They used the Molonglo Observatory Synthesis Telescope (or MOST) in Canberra to gather data on a large patch of sky.

Then they sifted through 1,000 terabytes of data and found just 3 fast radio bursts. Three.

Since MOST is farsighted and can’t perceive any radio signals closer than 10,000 km away, the signals had to be coming outside planet Earth. They were “extraterrestrial” in origin.

Right now, fast radio bursts are infuriating to astronomers. They don’t seem to match up with any other events we can see. They’re not the afterglow of a supernova, or tied in some way to gamma ray bursts.

In order to really figure out what’s going on, astronomers need new tools, and there’s a perfect instrument coming. Astronomers are building a new telescope called the Canadian Hydrogen Intensity Mapping Experiment (or CHIME), which is under construction near the town of Penticton in my own British Columbia.

CHIME under construction in Penticton, British Columbia. Credit: Mateus A. Fandiño (CC BY-SA 4.0)

It looks like a bunch of snowboard halfpipes, and its job will be to search for hydrogen emission from distant galaxies. It’ll help us understand how the Universe was expanding between 7 and 11 billion years ago, and create a 3-dimensional map of the early cosmos.

In addition to this, it’s going to be able to detect hundreds of fast radio bursts, maybe even a dozen a day, finally giving astronomers vast pools of signals to study.

What are they? Astronomers have no idea. Seriously, if you’ve got a good suggestion, they’d be glad to hear it.

In these kinds of situations, astronomers generally assume they’re caused by exploding stars in some way. Young stars or old stars, or maybe stars colliding. But so far, none of the theoretical models match the observations.

This artist’s conception illustrates one of the most primitive supermassive black holes known (central black dot) at the core of a young, star-rich galaxy. Image credit: NASA/JPL-Caltech

Another idea is black holes, of course. Specifically, supermassive black holes at the hearts of distant galaxies. From time to time, a random star, planet, or blob of gas falls into the black hole. This matter piles upon the black hole’s event horizon, heats up, screams for a moment, and disappears without a trace. Not a full on quasar that shines for thousands of years, but a quick snack.

The next idea comes with the only repeating fast radio burst that’s ever been found. Astronomers looked through the data archive of the Arecibo Observatory in Puerto Rico and found a signal that had repeated at least 10 times in a year, sometimes less than a minute apart.

Since the quick blast of radiation is repeating, this rules out a one-time collision between exotic objects like neutron stars. Instead, there could be a new class of magnetars (which are already a new class of neutron stars), that can release these occasional shrieks of radio.

An artist’s impression of a magnetar. Credit: ESO/L. Calçada

Or maybe this repeating object is totally different from the single events that have been discovered so far.

Here’s my favorite idea. And honestly, the one that’s the least realistic. What I’m about to say is almost certainly not what’s going on. And yet, it can’t be ruled out, and that’s good enough for my fertile imagination.

Avi Loeb and Manasvi Lingam at Harvard University said the following about FRBs:

“Fast radio bursts are exceedingly bright given their short duration and origin at distances, and we haven’t identified a possible natural source with any confidence. An artificial origin is worth contemplating and checking.”

Artificial origin. So. Aliens. Nice.

Loeb and Lingam calculated how difficult it would be to send a signal that strong, that far across the Universe. They found that you’d need to build a solar array with twice the surface area of Earth to power the radio wave transmitter.

And what would you do with a transmission of radio or microwaves that strong? You’d use it to power a spacecraft, of course. What we’re seeing here on Earth is just the momentary flash as a propulsion beam sweeps past the Solar System like a lighthouse.

But in reality, this huge solar array would be firing out a constant beam of radiation that would propel a massive starship to tremendous speeds. Like the Breakthrough Starshot spacecraft, but for million tonne spaceships.

Credit: NASA/Pat Rawlings (SAIC)

In other words, we could be witnessing alien transportation systems, pushing spacecraft with beams of energy to other worlds.

And I know that’s probably not what’s happening. It’s not aliens. It’s never aliens. But in my mind, that’s what I’m imagining.

So, kick back and enjoy the ride. Join us as we watch astronomers struggle to understand what fast radio bursts are. As they invalidate theories, and slowly unlock one of the most thrilling mysteries in modern astronomy. And as soon as they figure it out, I’ll let you know all about it.

What do you think? Which explanation for fast radio bursts seems the most logical to you? I’d love to hear your thoughts and wild speculation in the comments.

Posted on by Tammy Plotner

Messier 40 – the Winnecke 4 Double Star

The double star Messier 40 (Winnecke 4), along with PGC 39934, NGC 4290 and NGC 4284. Credit: Wikisky

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the double star known as Messier 40. Enjoy!

During the 18th century, famed French astronomer Charles Messier noted the presence of several “nebulous objects” in the night sky. Having originally mistaken them for comets, he began compiling a list of them so that others would not make the same mistake he did. In time, this list (known as the Messier Catalog) would come to include 100 of the most fabulous objects in the night sky.

One of these objects is Messier 40, this double star is now known to be an optical double star (i.e. two independent stars at different distances that appear aligned based on our perspective). It is also included in the Winnecke Catalogue of Double Stars as number 4, and is located in the constellation of Ursa Major (aka. the Big Dipper).

Description:

At roughly 500 light years away from us, no one is quite sure if this pair of stars is truly a binary system or an optical double star. According to Richard Nugent’s 2002 data, “The observed relative proper motion, as measured in separation and position angle, is consistent with a straight, independent motion of the two stars, one crossing between us and the other.”

The double star Messier 40 (Winnecke 4), along with PGC 39934, NGC 4290 and NGC 4284. Credit: Wikisky

The two stars are nearly the same brightness as each other, with the primary star being magnitude 9 and the secondary being magnitude 9.3 and they are separated by about 49 arc seconds – a wide gap. At one time, the angular separation of the pair was measured at 49.2″, but has gradually changed to about 52.8″ in more recent years.

History of Observation:

Messier 40 was discovered by Charles Messier in 1764 while he was searching for a nebula that had been reported in the area by Johann Hevelius. As he wrote at the time:

“The same night on October 24-25, [1764], I searched for the nebula above the tail of the Great Bear [Ursa Major], which is indicated in the book Figure of the Stars, second edition: it should have, in 1660, the right ascension 183d 32′ 41″, and the northern declination 60d 20′ 33″. I have found, by means of this position, two stars very near to each other and of equal brightness, about the 9th magnitude, placed at the beginning of the tail of Ursa Major: one has difficulty to distinguish them with an ordinary refractor of 6 feet. Here are their position: right ascension, 182 deg 45′ 30″, and 59 deg 23′ 50″ northern declination. There is reason to presume that Hevelius mistook these two stars for a nebula.”

History often credits Messier for being a little bit crazy for cataloging a double star, but upon having read Messier’s report, I feel like he was an astronomer doing his job. If Hevelius reported a nebula here – then he was bound to look and write down what he saw. He didn’t just stumble on a double star and catalog it for no reason!

Close-up of the double star Messier 40. Credit: Wikisky

Later astronomers would also search for M40 and report a double star, and it was cataloged by such as by Friedrich August Theodor Winnecke at Pulkovo Observatory in 1863 as WNC 4. However, to give the good Hevelius credit, John Mallas reports, “the Hevelius object is the 5th-magnitude star 74 Ursae Majoris, more than one degree away, as reference to his star catalogue will show.”

In 1991, the separation between the stars was measured at 52.8 arcseconds, which represented an increase since 1966, when it was measured at 51.7. In 2001 and 2002, studies conducted by Brian Skiff and Richard L. Nugent suggested that the stars comprising the double star (HD 238107 and HD 238108) were in fact an optical double star, rather than a double star system.

In 2016, by using parallax measurements from the Gaia satellite, this theory was proven for the first time. Distance estimates were also produced, indicating that the two components are 350±30 and 140±5 parsecs (~1141±98 and 456±16 light years).

Locating Messier 40:

Finding Messier 40 isn’t very difficult for fairly large binoculars and small telescopes – but you need to remember that it’s a double star. First locate the easily recognized constellation of Ursa Major and focus on the ‘Big Dipper’ and look for the two stars that form the edge that connect to the handle – Gamma and Delta.

The location of Messier 40 in Ursa Major, above and to the left of MegrezCredit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

Aim your telescope’s finderscope at Delta – the point where the ‘handle’ would connect. In the finder, you will see a fainter star to the northeast. Hop there. Now, using a low power eyepiece, scan slightly further northeast and you will locate M40. Once located, you may go to higher magnification to more closely examine this Messier catalog curiosity.

While this pair of stars will show easily in binoculars, you must remember that binoculars give such a wide field that it will be difficult to distinguish them from surrounding stars. However, this is a great object for light-polluted skies and moonlit nights!

Enjoy the controversy… and this pair! And here are the quick facts on M40 to help you get started:

Object Name: Messier 40
Alternative Designations: M40, WNC 4
Object Type: Double Star
Constellation: Ursa Major
Right Ascension: 12 : 22.4 (h:m)
Declination: +58 : 05 (deg:m)
Distance: 0.51 (kly)
Visual Brightness: 8.4 (mag)
Apparent Dimension: 0.8 (arc min)

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, , M1 – The Crab Nebula, M8 – The Lagoon Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.

Be to sure to check out our complete Messier Catalog. And for more information, check out the SEDS Messier Database.

Sources:

Posted on by Matt Williams

What is the Average Surface Temperature of Mercury?

MESSENGER image of Mercury from its third flyby (NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

Of all the planets in the Solar System, Mercury is the closest to our Sun. As such, you would think it is the hottest of all the Solar planets. But strangely enough, it is not. That honor goes to Venus, which experiences an average surface temperature of 750 K (477 °C; 890 °F). Not only that, but Mercury is also cold enough in some regions to maintain water in ice form.

Overall, Mercury experiences considerable variations in temperatures, ranging from the extremely hot to the extremely cold. All of this arises from the fact that Mercury has an extremely thin atmosphere, as well as the nature of its orbit. Whereas the side facing the Sun experiences temperatures hot enough to melt lead, the darkened areas are cold enough to freeze water.

Orbital Characteristics:

Mercury has the most eccentric orbit of any planet in the Solar System (0.205). Because of this, its distance from the Sun varies between 46 million km (29 million mi) at its closest (perihelion) to 70 million km (43 million mi) at its farthest (aphelion). And with an average orbital velocity of 47.362 km/s (29.429 mi/s), it takes Mercury a total 87.969 Earth days to complete a single orbit around the Sun.

With an average rotational speed of 10.892 km/h (6.768 mph), Mercury also takes 58.646 days to complete a single rotation. This means that Mercury has a spin-orbit resonance of 3:2, which means that it completes three rotations on its axis for every two orbits around the Sun. This does not, however, mean that three days last the same as two years on Mercury.

In fact, its high eccentricity and slow rotation mean that it takes 176 Earth days for the Sun to return to the same place in the sky (aka. a solar day), which means that one day is twice as long as a single year on Mercury. The planet also has the lowest axial tilt of any planet in the Solar System – approximately 0.027° compared to Jupiter’s 3.1°, (the second smallest). This means that there is virtually no seasonal variation in surface temperature.

Exosphere:

Another factor that affects Mercury’s surface temperatures is its extremely thin atmosphere. Mercury is essentially too hot and too small to retain anything more than a variable “exosphere”, one which is made up of hydrogen, helium, oxygen, sodium, calcium, potassium and water vapor.

The Fast Imaging Plasma Spectrometer on board MESSENGER has found that the solar wind is able to bear down on Mercury enough to blast particles from its surface into its wispy atmosphere. Credit: Carolyn Nowak/Media Academica, LLC

These trace gases have a combined atmospheric pressure of about 10-14 bar (one-quadrillionth of Earth’s atmospheric pressure). It is believed this exosphere was formed from particles captured from the Sun, volcanic outgassing and debris kicked into orbit by micrometeorite impacts.

Surface Temperatures:

Because it lacks a viable atmosphere, Mercury has no way to retain the heat from the Sun. As a result of this and its high eccentricity, the planet experiences considerable variations in temperature between its light side and dark side. Whereas the side that faces the Sun can reach temperatures of up to 700 K (427° C; 800 °F), the side in shadow dips down to 100 K (-173° C: -279 °F).

Despite its extreme highs in temperature, the existence of water ice and even organic molecules has been confirmed on Mercury’s surface, specifically in the cratered northern polar region. Since the floors of these deep craters are never exposed to direct sunlight, temperatures there remain below the planetary average.

View of Mercury’s north pole. based on MESSENGER probe data, showing polar deposits of water ice. Credit: NASA/JHUAPL/Carnegie/National Astronomy and Ionosphere Center, Arecibo Observatory.

These icy regions are believed to contain about 1014–1015 kg of frozen water, and may be covered by a layer of regolith that inhibits sublimation. The origin of the ice on Mercury is not yet known, but the two most likely sources are from outgassing of water from the planet’s interior or deposition by the impacts of comets. There are thought to be craters at the south pole as well, where temperatures are similarly cold enough to sustain water in ice form.

Mercury is a planet of extremes. It has an extremely eccentric orbit, an extremely thin-atmosphere, and experiences extremely hot and cold surface temperatures. Little wonder then why there is no life on the planet (at least, that we know about!) But perhaps someday, human beings may live there, sheltered in the cratered regions and using the water ice to create a habitat.

We have written many interesting articles about the average surface temperatures of the planets. Here’s What is the Average Surface Temperature of the Planets in our Solar System?, What is the Average Surface Temperature of Venus?, What is the Average Surface Temperature of Earth?, What is the Average Surface Temperature of Mars?, What is the Average Surface Temperature of Jupiter?, What is the Average Surface Temperature of Saturn?, What is the Average Surface Temperature of Uranus?, What is the Average Surface Temperature of Neptune?, and What is the Average Surface Temperature of Pluto?

If you’d like more information on Mercury, check out NASA’s Solar System Exploration Guide, and here’s a link to NASA’s MESSENGER Misson Page.

We have also recorded a whole episode of Astronomy Cast that’s just about planet Mercury. Listen to it here, Episode 49: Mercury.

Sources:

Posted on by Matt Williams

The Orbit of Neptune. How Long is a Year on Neptune?

Neptune from Voyager 2. Image credit: NASA/JPL

Here on Earth, a year lasts roughly 365.25 days, each of which lasts 24 hours long. During the course of a single year, our planet goes through some rather pronounced seasonal changes. This is the product of our orbital period, our rotational period, and our axial tilt. And when it comes to the other planets in our Solar System, much the same is true.

Consider Neptune. As the eight and farthest planet from the Sun, Neptune has an extremely wide orbit and a comparatively slow orbital velocity. As a result, a year on Neptune is very long, lasting the equivalent of almost 165 Earth years. Combined with its extreme axial tilt, this also means that Neptune experiences some rather extreme seasonal changes.

Orbital Period:

Neptune orbits our Sun at an average distance (semi-major axis) of 4,504.45 million km (2,798.656 million mi; 30.11 AU). Because of its orbital eccentricity (0.009456), this distance varies somewhat, ranging from 4,460 million km (2,771 million mi; 29.81 AU) at its closest (perihelion) to 4,540 million km (2,821 million mi; 30.33 AU) at its farthest (aphelion).

The orbit of Neptune and the other outer Solar planets, as well as the ice-rich Kuiper Belt that lies just beyond it. Credit: NASA

With an average orbital speed of 5.43 km/s, it takes Neptune 164.8 Earth years (60,182 Earth days) to complete a single orbital period. This means, in effect, that a year on Neptune lasts as long as about 165 years here on Earth. However, given its rotational period of 0.6713 Earth days (16 hours 6 minutes 36 seconds), a year on Neptune works out to 89,666 Neptunian solar days.

Given that Neptune was discovered in 1846, humanity has only known about its existence for 171 years (at the time of this article’s writing). That means that since its discovery, the planet has only completed a single orbital period (which ended in 2010) and is only seven years into its second. This orbital period will be complete by 2179.

Orbital Resonance:

Because of its location in the outer Solar System, Neptune’s orbit has a profound impact on the neighboring Kuiper Belt. This region, which is similar (but significantly larger) than the Main Asteroid Belt, consists of many small icy worlds and objects that extends from Neptune’s orbit (at 30 AU) to a distance of about 55 AU from the Sun.

Animated diagram showing the spacing of the Solar Systems planet’s, the unusually closely spaced orbits of six of the most distant KBOs, and the possible “Planet 9”. Credit: Caltech/nagualdesign

So much as Jupiter’s gravity has dominated the Asteroid Belt, affecting its structure and occasionally kicking asteroids and planetoids into the inner Solar System, Neptune’s gravity dominates the Kuiper Belt. This has led to the creation of gaps in the belt, empty regions where objects have achieved an orbital resonance with Neptune.

Within these gaps, objects have a 1:2, 2:3 or 3:4 resonance with Neptune, meaning they complete one orbit of the Sun for every two completed by Neptune, two for every three, or three for every four. The over 200 known objects that exist in the 2:3 resonance (the most populous) are known as plutinos, since Pluto is the largest of them.

Although Pluto crosses Neptune’s orbit on a regular basis, their 2:3 orbital resonance ensures they can never collide. On occasion, Neptune’s gravity also causes icy bodies to be kicked out of the Kuiper Belt. Many of these then travel to the Inner Solar System, where they become comets with extremely long orbital periods.

Neptune’s largest satellite, Triton, is believed to have once been a Kuiper Belt Object (KBO) – and Trans-Neptunian Object (TNO) – that was captured by Neptune’s gravity. This is evidenced by its retrograde motion, meaning it orbits the planet in the opposite direction as its other satellites. It also has a number of Trojan Objects occupying its L4 and L5 Lagrange points. These “Neptune Trojans” can be said to be in a stable 1:1 orbital resonance with Neptune.

Seasonal Change:

Much like the other planets of the Solar System, Neptune’s axis is tilted towards the Sun’s ecliptic. In Neptune’s case, it is tilted 28.32° relative to its orbit (whereas Earth is tilted at 23.5°). Because of this, Neptune undergoes seasonal change during the course of a year because one of its hemispheres will be receiving more sunlight than the other. But in Neptune’s case, a single season lasts a whopping 40 years, making it very hard to witness a full cycle.

While much of the heat that powers Neptune’s atmosphere comes from an internal source (which is currently unknown), a study conducted by researchers from Wisconsin-Madison University and NASA’s Jet Propulsion Laboratory revealed that seasonal change is also driven by solar radiation. This consisted of examining images of Neptune taken by the Hubble Space Telescope between 1996 and 2002.

These images revealed that Neptune’s massive southern cloud bands were becoming steadily wider and brighter over the six year period – which coincided with the southern hemisphere beginning its 40-year summer. This growing cloud cover was attributed to increased solar heating, as it appeared to be concentrated in the southern hemisphere and was rather limited at the equator.

Images taken by Hubble, showing seasonal change in its southern hemisphere. Credit: NASA, L. Sromovsky, and P. Fry (University of Wisconsin-Madison)

Neptune remains a planet of mystery in many ways. And yet, ongoing observations of the planet have revealed some familiar and comforting patterns. For instance, while it’s composition is vastly different and its orbit puts it much farther away from the Sun than Earth, its axial tilt and orbital period still result in its hemispheres experiencing seasonal changes.

It’s good to know that no matter how far we venture out into the Solar System, and no matter how different things may seem, there are still some things that stay the same!

We have written many articles about how long year is on the Solar planets here at Universe Today. Here’s The Orbit of the Planets. How Long Is A Year On The Other Planets?, The Orbit of Earth. How Long is a Year on Earth?, The Orbit of Mercury. How Long is a Year on Mercury?, The Orbit of Venus. How Long is a Year on Venus?,  The Orbit of Mars. How Long is a Year on Mars?, The Orbit of Jupiter. How Long is a Year on Jupiter?, The Orbit of Saturn. How Long is a Year on Saturn?, The Orbit of Uranus. How Long is a Year on Uranus?, The Orbit of Pluto. How Long is a Year on Pluto?

If you’d like more information on Neptune, take a look at Hubblesite’s News Releases about Neptune, and here’s a link to NASA’s Solar System Exploration Guide to Neptune.

We have recorded an entire episode of Astronomy Cast just about Neptune. You can listen to it here, Episode 63: Neptune.

Sources: