Hi! When I was only six (or so), I went out one clear but windy night with my uncle and peered through the eyepiece of his home-made 6" Newtonian reflector. The dazzling, shimmering, perfect globe-and-ring of Saturn entranced me, and I was hooked on astronomy, for life. Today I'm a freelance writer, and began writing for Universe Today in late 2009. Like Tammy, I do like my coffee, European strength please.
Contact me: [email protected]
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Velocity, in physics, is a vector quantity (it has both magnitude and direction), and is the time rate of change of position (of an object). However, quite often when you read ‘velocity’, what is meant is speed, the magnitude of the velocity vector (speed is a scalar quantity, it has only magnitude). For example: escape velocity (the minimum speed an object needs to escape from a planet, say); note that this can be easily turned into a velocity, by adding ‘in the direction radially out from the center of the planet’, and that this direction is sometimes implied (if not actually stated).
In astronomy, it is often quite straight-forward to measure the component of velocity of a distant object along the line of sight to it, by measuring its redshift. This is a one-dimensional velocity (it has both magnitude and direction – either towards the observer, or away), but only one component of the object’s space motion. In most cases, it is clear from the context what is meant by ‘velocity’; for example, a ‘galaxy rotation curve’ often has ‘velocity’ on the vertical axis, meaning something like the estimated magnitude of the orbital velocity of the stars/gas/dust/plasma in the galaxy, assuming circular orbits. However, if you are not clued in to this context, it is all too easy to misunderstand what ‘velocity’ means!
Perhaps the most common form of Newton’s first law of motion is “In the absence of net force, a body is either at rest or moves at a constant speed in a straight line”. It is easy to re-write this using the textbook physics definition of velocity: “In the absence of net force, a body’s velocity is constant”.
Some further reading: Terminal Velocity (NASA), Velocity (Scienceworld), and Group Velocity and Phase Velocity (University of Virginia).
The gravity formula that most people remember, or think of, is the equation which captures Newton’s law of universal gravitation, which says that the gravitational force between two objects is proportional to the mass of each, and inversely proportional to the distance between them. It is usually written like this (G is the gravitational constant):
F = Gm1m2/r2
Another, common, gravity formula is the one you learned in school: the acceleration due to the gravity of the Earth, on a test mass. This is, by convention, written as g, and is easily derived from the gravity formula above (M is the mass of the Earth, and r its radius):
g = GM/r2
In 1915, Einstein published his general theory of relativity, which not only solved a many-decades-long mystery concerning the observed motion of the planet Mercury (the mystery of why Uranus’ orbit did not match that predicted from applying Newton’s law was solved by the discovery of Neptune, but no hypothetical planet could explain why Mercury’s orbit didn’t), but also made a prediction that was tested just a few years’ later (deflection of light near the Sun). Einstein’s theory contains many gravity formulae, most of which are difficult to write down using only simple HTML scripts (so I’m not going to try).
The Earth is not a perfect sphere – the distance from surface to center is smaller at the poles than the equator, for example – and it is rotating (which means that the force on an object includes the centripetal acceleration due to this rotation). For people who need accurate formulae for gravity, both on the Earth’s surface and above it, there is a set of international gravity formulae which define what is called theoretical gravity, or normal gravity, g0. This corrects for the variation in g due to latitude (and so both the force due to the Earth’s rotation, and its non-spherical shape).
Astronomy Cast’s episode Gravity gives you much more on not just one gravity formula, but several; and Gravitational Waves is good too. Be sure to check them out!
Cuneiform clay tablet. Image Credit: Bristol University
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Ephemerides is the plural form of ephemeris; one ephemeris, two (three, four, …) ephemerides. Why such a strange word? Why not ‘ephemerises’? Because of its Greek-via-Latin origin, and because it’s a word rarely used outside an academic/technical setting (there are only a few other words in English which form plurals in this way, one is iris -> irides). An ephemeris is a table (or similar) giving the position in the sky of astronomical objects, at a particular time (or set of times).
The most common ephemerides are those of solar system bodies, such as the planets, asteroids, and comets; ephemerides of greatest public interest are those of solar eclipses. For amateur astronomers, ephemerides giving eclipses, occulations, and transits – of asteroids, the Galilean moons, eclipsing binaries, exoplanets, etc – are of particular interest.
Waaay back when, ephemerides took a very long time to calculate, because all calculation was done by hand; today there are free apps and software packages you can download that will generate ephemerides for you, even ones of entirely fictional things such as settings for a sci-fi novel.
And here’s a fun fact: you can generate half-way decent ephemerides of the classical planets (Mercury, Venus, Mars, Jupiter, Saturn), the Moon, and of solar eclipses, using a heliocentric, Ptolemaic model! Today, for high accuracy – especially over centuries and millennia – ephemerides of Mercury, the Moon, etc require gravity to be modeled using General Relativity (GR), rather than Newtonian mechanics. And accurate ephemerides for Pioneer 10 and 11? Well, even GR is not enough … you have to model the Pioneer anomaly!
First, some simple answers: space is everything in the universe beyond the top of the Earth’s atmosphere – the Moon, where the GPS satellites orbit, Mars, other stars, the Milky Way, black holes, and distant quasars. Space also means what’s between planets, moons, stars, etc – it’s the near-vacuum otherwise known as the interplanetary medium, the interstellar medium, the inter-galactic medium, the intra-cluster medium, etc; in other words, it’s very low density gas or plasma (‘space physics’ is, in fact, just a branch of plasma physics!).
But you really want to know what space is, don’t you? You’re asking about the thing that’s like time, or mass.
And one simple, but profound, answer to the question “What is space?” is “that which you measure with a ruler”. And why is this a profound answer? Because thinking about it lead Einstein to develop first the theory of special relativity, and then the theory of general relativity. And those theories overthrew an idea that was built into physics since before the time of Newton (and built into philosophy too); namely, the idea of absolute space (and time). It turns out that space isn’t something absolute, something you could, in principle, measure with lots of rulers (and lots of time), and which everyone else who did the same thing would agree with you on.
Space, in the best theory of physics on this topic we have today – Einstein’s theory of general relativity (GR) – is a component of space-time, which can be described very well using the math in GR, but which is difficult to envision with our naïve intuitions. In other words, “What is space?” is a question I can’t really answer, in the short space I have in this Guide to Space article.
It goes by the super-catchy (not!) title “A Catalog of MIPSGAL Disk and Ring Sources”. I chose it, over 213 competitors, because it’s pure astronomy, and because it’s something you don’t need a PhD to be able to do, or even a BSc.
Oh, and also because Don Mizuno and co-authors may have found two, quite local, spiral galaxies that no one has ever seen before!
Some quick background: arXiv has been going for several years now, and provides preprints, on the web, of papers “in the fields of physics, mathematics, non-linear science, computer science, quantitative biology and statistics”. It’s owned, operated and funded by Cornell University. astro-ph is the collection of preprints classified as astro physics; the “recent” category in astro-ph is the new preprints submitted in the last week.
When I have any, one of my favorite spare-time activities is browsing astro-ph (Hey, I did say, in my profile, that I am hooked on astronomy!)
Briefly, what Mizuno and his co-authors did was get hold of some of the images from Spitzer (something that anyone can do, provided their internet connection has enough bandwidth), and eyeball them, looking for things which look like disks and rings. Having found over 400 of them, they did what the human brain does superbly well: they grouped them by similarity of appearance, and gave the groups names. They then checked out other images – from different parts of Spitzer’s archive, and from IRAS – and checked to see how many had already been cataloged.
And what did they find? Well, first, that most of the objects they found had not been cataloged before, and certainly not given definite classifications! Many, perhaps most, of the new objects are planetary nebulae, and their findings may help address a long-standing puzzle in this part of astronomy. MGE314.2378+00.9793 (Mizuno et al./Spitzer)MGE351.2381-00.0145 (Mizuno et al./Spitzer)
But they also may have found two local spiral galaxies, which had not been noticed before because they are obscured by the gas-and-dust clouds in the Milky Way plane. How cool is that!
Here’s the ‘credits’ section of the preprint: “This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Support for this work was provided by NASA in part through an award issued by JPL/Caltech. This research made use of the SIMBAD database and the Vizier catalog access tool, operated by the Centre de Donnees Astronomique de Strasbourg. This research has also made use of NASA’s Astrophysics Data System Bibliographic Services.”
And here’s the preprint itself: arXiv:1002.4221 A Catalog of MIPSGAL Disk and Ring Sources; D.R. Mizuno(1), K. E. Kraemer(2), N. Flagey(3), N. Billot(4), S. Shenoy(5), R. Paladini(3), E. Ryan(6), A. Noriega-Crespo(3), S. J. Carey(3). ((1) Institute for Scientific Research, (2) Air Force Research Laboratory, (3) Spitzer Science Center, (4) NASA Herschel Science Center, (5) Ames Research Center, (6) University of Minnesota)
PS, going over the Astronomy Cast episode How to be Taken Seriously by Scientists is what motivated me to pick this preprint (however, I must tell you, in all honesty, that there are at least ten other preprints that are equally pickable).
Illustration of WASP-12b in orbit about its host star (Credit: ESA/C Carreau)
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WASP-12b, discovered in 2008, is a real outlier among the 400 or so exoplanets discovered to date. Not that it’s particularly massive (it’s a gas giant, not unlike Jupiter), nor that its homesun (host star) is particularly unusual (it’s rather similar to our own Sun), but it orbits very close to its homesun, and is considerably larger than any other gas giant discovered to date.
Results from recent research explain why WASP-12b is so unusual; we’re watching it die a painful death at the hands of its homesun, which is snacking on it.
“This is the first time that astronomers are witnessing the ongoing disruption and death march of a planet,” says UC Santa Cruz professor Douglas N.C. Lin. Lin is a co-author of the new study and the founding director of the Kavli Institute for Astronomy and Astrophysics (KIAA) at Peking University, which was deeply involved with the research.
The research was led by Shu-lin Li of the National Astronomical Observatories of China. A graduate of KIAA, Li and a research team analyzed observational data on the planet to show how the gravity of its parent star is both inflating its size and spurring its rapid dissolution.
WASP-12b, like most known exoplanets discovered to date, is large and gaseous, resembling Jupiter and Saturn; however, unlike Jupiter, Saturn, or most other exoplanets, it orbits its homesun at extremely close range – 75 times closer than the Earth is to the Sun, or just over 1.5 million km. It is also larger than astrophysical models predict. Its mass is estimated to be almost 50% larger than Jupiter’s and it is 80% larger, giving it six times Jupiter’s volume. It is also unusually toasty, with a daytime temperature of more than 2500° C.
Some mechanism must be responsible for expanding this planet to such an unexpected size, say the researchers. They have focused their analysis on tidal forces, which they say are strong enough to produce the effects observed on WASP-12b.
On Earth, tidal forces between the Earth and the Moon cause local sea levels rise and fall, modestly, twice a day. WASP-12b, however, is so close to its homesun that the gravitational forces are enormous. The tremendous tidal forces acting on the planet completely change the shape of the planet into something similar to that of a rugby or American football.
These tides not only distort the shape of WASP-12b. By continuously deforming the planet, they also create friction in its interior. The friction produces heat, which causes the planet to expand. “This is the first time that there is direct evidence that internal heating (or ‘tidal heating’) is responsible for puffing up the planet to its current size,” says Lin.
Huge as it is, WASP-12b faces an early demise, say the researchers. In fact, its size is part of its problem. It has ballooned to such a point that it cannot retain its mass against the pull of its homesun’s gravity. As the study’s lead author Li explains, “WASP-12b is losing its mass to the host star at a tremendous rate of six billion metric tons each second. At this rate, the planet will be completely destroyed by its host star in about ten million years. This may sound like a long time, but for astronomers it’s nothing. This planet will live less than 500 times less than the current age of the Earth.” The WASP-12 system (Courtesy: KIAA/Graphic: Neil Miller) About this image: The massive gas giant WASP-12b is shown in purple with the transparent region representing its atmosphere. The gas giant planet’s orbit is somewhat non-circular. This indicates that there is probably an unseen lower mass planet in the system, shown in brown, that is perturbing the larger planet’s orbit. Mass from the gas giant’s atmosphere is pulled off and forms a disk around the star, shown in red.
The material that is stripped off WASP-12b does not fall directly onto the parent star; instead it forms a disk around the star and slowly spirals inwards. A careful analysis of the orbital motion of WASP-12b suggests circumstantial evidence of the gravitational force of a second, lower-mass planet in the disk. This planet is most likely a massive version of the Earth – a so-called “super-Earth.”
The disk of planetary material and the embedded super-Earth should be detectable with currently available telescope facilities. Their properties can be used to further constrain the history and fate of the mysterious planet WASP-12b.
In addition to KIAA, support for the WASP-12b research came from NASA, the Jet Propulsion Laboratory, and the National Science Foundation. Along with Li and Lin, co-authors include UC Santa Cruz professor Jonathan Fortney and Neil Miller, a graduate student at the university.
For Goldilocks, the porridge had to be not too hot, and not too cold … the right temperature was all she needed.
For an Earth-like planet to harbor life, or multicellular life, certainly temperature is important, but what else is important? And what makes the temperature of an exo-Earth “just right”?
Some recent studies have concluded that answering these questions can be surprisingly difficult, and that some of the answers are surprisingly curious.
Consider the tilt of an exo-Earth’s axis, its obliquity.
In the “Rare Earth” hypothesis, this is a Goldilocks criterion; unless the tilt is kept stable (by a moon like our Moon), and at a “just right” angle, the climates will swing too wildly for multicellular life to form: too many snowball Earths (the whole globe covered in snow and ice with an enhanced albedo effect), or too much risk of a runaway greenhouse.
“We find that planets with small ocean fractions or polar continents can experience very severe seasonal climatic variations,” Columbia University’s David Spiegel writes*, summing up the results of an extensive series of models investigating the effects of obliquity, land/ocean coverage, and rotation on Earth-like planets, “but that these planets also might maintain seasonally and regionally habitable conditions over a larger range of orbital radii than more Earth-like planets.” And the real surprise? “Our results provide indications that the modeled climates are somewhat less prone to dynamical snowball transitions at high obliquity.” In other words, an exo-Earth tilted nearly right over (much like Uranus) may be less likely to suffer snowball Earth events than our, Goldilocks, Earth! Ultraviolet view of the Sun. Image credit: SOHO
Consider ultra-violet radiation.
“Ultraviolet radiation is a double-edged sword to life. If it is too strong, the terrestrial biological systems will be damaged. And if it is too weak, the synthesis of many biochemical compounds cannot go along,” says Jianpo Guo of China’s Yunnan Observatory** “For the host stars with effective temperatures lower than 4,600 K, the ultraviolet habitable zones are closer than the habitable zones. For the host stars with effective temperatures higher than 7,137 K, the ultraviolet habitable zones are farther than the habitable zones.” This result doesn’t change what we already knew about habitability zones around main sequence stars, but it effectively rules out the possibility of life on planets around post-red giant stars (assuming any could survive their homesun going red giant!) (Credit: NASA)
Consider the effects of clouds.
Calculations of the habitability zones – the radii of the orbits of an exo-Earth, around its homesun – for main sequence stars usually assume an astronomers’ heaven – permanent clear skies (i.e. no clouds). But Earth has clouds, and clouds most definitely have an effect on average global temperatures! “The albedo effect is only weakly dependent on the incident stellar spectra because the optical properties (especially the scattering albedo) remain almost constant in the wavelength range of the maximum of the incident stellar radiation,” a German team’s recent study*** on the effects of clouds on habitability concludes (they looked at main sequence homesuns of spectral classes F, G, K, and M). This sounds like Gaia is Goldilocks’ friend; however, “The greenhouse effect of the high-level cloud on the other hand depends on the temperatures of the lower atmosphere, which in turn are an indirect consequence of the different types of central stars,” the team concludes (remember that an exo-Earth’s global temperature depends upon both the albedo and greenhouse effects). So, the take-home message? “Planets with Earth-like clouds in their atmospheres can be located closer to the central star or farther away compared to planets with clear sky atmospheres. The change in distance depends on the type of cloud. In general, low-level clouds result in a decrease of distance because of their albedo effect, while the high-level clouds lead to an increase in distance.”
“Just right” is tricky to pin down.
* lead author; Princeton University’s Kristen Manou and Colombia University’s Caleb Scharf are the co-authors (“Habitable Climates: The Influence of Obliquity”, The Astrophysical Journal, Volume 691, Issue 1, pp. 596-610 (2009); arXiv:0807.4180 is the preprint)
** lead author; Fenghui Zhang, Xianfei Zhang, and Zhanwen Han, all also at the Yunnan Observatory, are the co-authors (“Habitable zones and UV habitable zones around host stars”, Astrophysics and Space Science, Volume 325, Number 1, pp. 25-30 (2010))
*** “Clouds in the atmospheres of extrasolar planets. I. Climatic effects of multi-layered clouds for Earth-like planets and implications for habitable zones”, Kitzmann et al., accepted for publication in Astronomy & Astrophysics (2010); arXiv:1002.2927 is the preprint.
The Parkes Radio Antenna. Credit: R. Hollow, CSIRO
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Radio waves are electromagnetic waves, or electromagnetic radiation, with wavelengths of about a centimeter or longer (the boundary is rather fuzzy; microwaves and terahertz radiation are sometimes considered to be radio waves; these have wavelengths as short as a tenth of a millimeter or so). In other words, radio waves are electromagnetic radiation at the lowest energy end of the electromagnetic spectrum.
Radio waves were predicted two decades or so before they were generated and detected; in fact, the historical story is one of the great triumphs of modern science.
Many years – centuries even – of work on electrical and magnetic phenomena, by many scientists, culminated in the work of James Clerk Maxwell. In 1865 he published a set of equations which describe everything known about electricity and magnetism (electromagnetism) up till that time (the next major advance was the work of Planck and Einstein – among others – some four decades or so later, involving the discovery of photons, or quantized electromagnetic radiation). Maxwell’s equations, as they are now called, predicted that there should be a kind of wave of interacting electrical and magnetic fields, which is self-propagating, and which travels at the speed of light.
In 1887, Heinrich Hertz created radio waves in his lab, and detected them after they’d travelled a short distance … exactly as Maxwell had predicted! It wasn’t long before practical applications of this discovery were developed, leading to satellite TV, cell phones, GPS, radar, wireless home networks, and much, much, more.
For Universe Today readers, the discovery of radio waves lead to radio astronomy. Interestingly, theory again preceded observation … several scientists – Planck among them – predicted that the Sun should emit radio waves (be a source of radio waves), but the Sun’s radio emission was not detected until 1942 (by Hey, in England), nearly a decade after celestial radio waves were detected and studied, by Jansky (and Reber, among others).
The mass of the proton, proton mass, is 1.672 621 637(83) x 10 -27 kg, or 938.272013(23) MeV/c2, or 1.007 276 466 77(10) u (that’s unified atomic mass units).
The most accurate measurements of the mass of the proton come from experiments involving Penning traps, which are used to study the properties of stable charged particles. Basically, the particle under study is confined by a combination of magnetic and electric fields in an evacuated chamber, and its velocity reduced by a variety of techniques, such as laser cooling. Once trapped, the mass-to-charge ratio of a proton, deuteron (nucleus of a deuterium atom), singly charged hydrogen molecule, etc can be measured to high precision, and from these the mass of the proton estimated.
It would be nice if the experimentally observed mass of a proton were the same as that derived from theory. But how to work out what the mass of a proton should be, from theory?
The theory is quantum chromodynamics, or QCD for short, and is the strong force counterpart to quantum electrodynamics (QED). As the proton is made up of three quarks – two up and one down – its mass is the mass of those quarks and the mass of binding energy. This is a very difficult calculation to perform, in part because there are so many ways the quarks and gluons in a proton interact, but published results agree with experiment to within a percent or two.
More fundamentally, the proton has mass because of the Higgs boson … at least, it does according to the highly successful Standard Model of particle physics. Only trouble is, the Higgs boson has yet to be detected (the Large Hadron Collider was built with finding the Higgs boson as a key objective!).
Want to know the “official” value? Check out CODATA. And how does the proton mass compare with the mass of the anti-proton? Click here to find out! And how to determine the proton mass from first (theoretical) principles? This article from CNRS explains how.
Hickson 31 (Credit: NASA, ESA, and S. Gallagher (The University of Western Ontario), and J. English (University of Manitoba))
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Have you heard of ‘living fossils’? The coelacanth, the ginko tree, the platypus, and several others are species alive today which seem to be the same as those found as fossils, in rocks up to hundreds of millions of years old.
Now combined results from the Hubble Space Telescope, Spitzer, Galaxy Evolution Explorer (GALEX), and Swift show that there are ‘living galaxy fossils’ in our own backyard!
Hubble: red, yellow-green, and blue; Spitzer: orange; GALEX: purple
Hickson Compact Group 31 is one of 100 compact galaxy groups catalogued by Canadian astronomer Paul Hickson; the recent study of them – led by Sarah Gallagher of The University of Western Ontario in London, Ontario – shows that the four dwarf galaxies in it are in the process of coming together (or ‘merging’ as astronomers say).
Such encounters between dwarf galaxies are normally seen billions of light-years away and therefore occurred billions of years ago. But these galaxies are relatively nearby, only 166 million light-years away.
New images of this foursome by NASA’s Hubble Space Telescope offer a window into the universe’s formative years when the buildup of large galaxies from smaller building blocks was common.
Astronomers have known for decades that these dwarf galaxies are gravitationally tugging on each other. Their classical spiral shapes have been stretched like taffy, pulling out long streamers of gas and dust. The brightest object in the Hubble image is actually two colliding galaxies. The entire system is aglow with a firestorm of star birth, triggered when hydrogen gas is compressed by the close encounters between the galaxies and collapses to form stars.
The Hubble observations have added important clues to the story of this interacting group, allowing astronomers to determine when the encounter began and to predict a future merger.
“We found the oldest stars in a few ancient globular star clusters that date back to about 10 billion years ago. Therefore, we know the system has been around for a while,” says Gallagher; “most other dwarf galaxies like these interacted billions of years ago, but these galaxies are just coming together for the first time. This encounter has been going on for at most a few hundred million years, the blink of an eye in cosmic history. It is an extremely rare local example of what we think was a quite common event in the distant universe.”
In other words, a living fossil.
Everywhere the astronomers looked in this group they found batches of infant star clusters and regions brimming with star birth. The entire system is rich in hydrogen gas, the stuff of which stars are made. Gallagher and her team used Hubble’s Advanced Camera for Surveys to resolve the youngest and brightest of those clusters, which allowed them to calculate the clusters’ ages, trace the star-formation history, and determine that the galaxies are undergoing the final stages of galaxy assembly.
The analysis was bolstered by infrared data from NASA’s Spitzer Space Telescope and ultraviolet observations from the Galaxy Evolution Explorer (GALEX) and NASA’s Swift satellite. Those data helped the astronomers measure the total amount of star formation in the system. “Hubble has the sharpness to resolve individual star clusters, which allowed us to age-date the clusters,” Gallagher adds.
Hubble reveals that the brightest clusters, hefty groups each holding at least 100,000 stars, are less than 10 million years old. The stars are feeding off of plenty of gas. A measurement of the gas content shows that very little has been used up – further proof that the “galactic fireworks” seen in the images are a recent event. The group has about five times as much hydrogen gas as our Milky Way Galaxy.
“This is a clear example of a group of galaxies on their way toward a merger because there is so much gas that is going to mix everything up,” Gallagher says. “The galaxies are relatively small, comparable in size to the Large Magellanic Cloud, a satellite galaxy of our Milky Way. Their velocities, measured from previous studies, show that they are moving very slowly relative to each other, just 134,000 miles an hour (60 kilometers a second). So it’s hard to imagine how this system wouldn’t wind up as a single elliptical galaxy in another billion years.”
Adds team member Pat Durrell of Youngstown State University: “The four small galaxies are extremely close together, within 75,000 light-years of each other – we could fit them all within our Milky Way.”
Why did the galaxies wait so long to interact? Perhaps, says Gallagher, because the system resides in a lower-density region of the universe, the equivalent of a rural village. Getting together took billions of years longer than it did for galaxies in denser areas.
