A global view of Mercury, as seen by MESSENGER. Credit: NASA
Mercury is a planet of extremes. As the closest planet to our Sun, it experiences extremely high surface temperatures. But since it has virtually no atmosphere to speak of, and rotates very slowly on its axis, it gravitates between extremes of hot and cold. It also means that it’s Sun-facing side experiences prolonged periods of day while its dark side experiences extremely long periods of night.
It’s proximity to the Sun also means that it orbits the planet quite rapidly. To break it down, Mercury takes roughly 88 Earth days to complete a single orbit around the Sun. Between this rapid orbital period and its slow rotational period, a single year on Mercury is actually shorter than a single day!
Orbital Period:
Mercury orbits the Sun at a distance of 57,909,050 km (35,983,015 mi), which works out to o.387 AU – or slightly more than one-third the distance between the Sun and the Earth. It’s orbit is also highly eccentric, ranging from a distance of 46 million km/28.58 million mi at its closest (perihelion) to 70 million km/43.49 million mi at its most distant (aphelion).
Illustration of the orbit of Kepler-432b (inner, red) in comparison to the orbit of Mercury around the Sun (outer, orange). Credit: Dr. Sabine Reffert.
Like all the planets, Mercury moves fastest when it is at its closest point to the Sun, and slowest when it is at its farthest. However, it’s proximity to the Sun means that its average orbital velocity is a speedy 47.362 kilometers a second or 29.429 miles per second – approximately 170,500 km/h; 105,945 mph.
At this rate, it takes Mercury 87.969 days, or the equivalent of 0.24 Earth years, to complete a single orbit of the Sun. Thus, it can be said that a year on Mercury lasts almost as long as 3 months here on Earth.
Sidereal and Solar Day:
Astronomers used to think that Mercury was tidally locked to the Sun, where its rotational period matched its orbital period. This would mean that the same side it always pointed towards the Sun, thus ensuring that one side was perennially sunny (and extremely hot) while the other experienced constant night (and freezing cold).
However, improved observations and studies of the planet have led scientists to conclude that in fact, the planet has a slow rotational period of 58.646 days. Compared to its orbital period of 88 days, this means that Mercury has a spin-orbit resonance of 3:2, which means that the planet makes three completes rotations on its axis for every two orbits it makes around the Sun.
Another consequences of its spin-orbit resonance is that there is a significance difference between the time it takes the planet to rotate once on its axis (a sidereal day) and the time it takes for the Sun to reappear in the same place in the sky (a solar day). On Mercury, it takes a 176 days for the Sun to rise, set, and return to the same place in the sky. This means, effectively, that a single day on Mercury lasts as long as two years!
Yes, Mercury is a pretty extreme place. Not only do temperatures on its surface range from molten hot to freezing cold, but a single day lasts as long as six months here on Earth. Add to that the fact that it has virtually no atmosphere, and is exposed to extreme amounts of radiation, and you can begin to understand why life cannot exist there.
Artistic impression of a star going supernova, casting its chemically enriched contents into the universe. Credit: NASA/Swift/Skyworks Digital/Dana Berry
Supernovae are extremely energetic and dynamic events in the universe. The brightest one we’ve ever observed was discovered in 2015 and was as bright as 570 billion Suns. Their luminosity signifies their significance in the cosmos. They produce the heavy elements that make up people and planets, and their shockwaves trigger the formation of the next generation of stars.
There are about 3 supernovae every 100 hundred years in the Milky Way galaxy. Throughout human history, only a handful of supernovae have been observed. The earliest recorded supernova was observed by Chinese astronomers in 185 AD. The most famous supernova is probably SN 1054 (historic supernovae are named for the year they were observed) which created the Crab Nebula. Now, thanks to all of our telescopes and observatories, observing supernovae is fairly routine.
The supernova that produced the Crab Nebula was detected by naked-eye observers around the world in 1054 A.D. This composite image uses data from NASA’s Great Observatories, Chandra, Hubble, and Spitzer.
But one thing astronomers have never observed is the very early stages of a supernova. That changed in 2013 when, by chance, the automated Intermediate Palomar Transient Factory (IPTF) caught sight of a supernova only 3 hours old.
Spotting a supernovae in its first few hours is extremely important, because we can quickly point other ‘scopes at it and gather data about the SN’s progenitor star. In this case, according to a paper published at Nature Physics, follow-up observations revealed a surprise: SN 2013fs was surrounded by circumstellar material (CSM) that it ejected in the year prior to the supernova event. The CSM was ejected at a high rate of approximately 10 -³ solar masses per year. According to the paper, this kind of instability might be common among supernovae.
SN 2013fs was a red super-giant. Astronomers didn’t think that those types of stars ejected material prior to going supernova. But follow up observations with other telescopes showed the supernova explosion moving through a cloud of material previously ejected by a star. What this means for our understanding of supernovae isn’t clear yet, but it’s probably a game changer.
Catching the 3-hour-old SN 2013fs was an extremely lucky event. The IPTF is a fully-automated wide-field survey of the sky. It’s a system of 11 CCD’s installed on a telescope at the Palomar Observatory in California. It takes 60 second exposures at frequencies from 5 days apart to 90 seconds apart. This is what allowed it to capture SN 2013fs in its early stages.
The 48 inch telescope at the Palomar Observatory. The IPTF is installed on this telescope. Image: IPTF/Palomar Observatory
Our understanding of supernovae is a mixture of theory and observed data. We know a lot about how they collapse, why they collapse, and what types of supernovae there are. But this is our first data point of a SN in its early hours.
SN 2013fs is 160 million light years away in a spiral-arm galaxy called NGC7610. It’s a type II supernova, meaning that it’s at least 8 times as massive as our Sun, but not more than 50 times as massive. Type II supernovae are mostly observed in the spiral arms of galaxies.
A supernova is the end state of some of the stars in the universe. But not all stars. Only massive stars can become supernova. Our own Sun is much too small.
Stars are like dynamic balancing acts between two forces: fusion and gravity.
As hydrogen is fused into helium in the center of a star, it causes enormous outward pressure in the form of photons. That is what lights and warms our planet. But stars are, of course, enormously massive. And all that mass is subject to gravity, which pulls the star’s mass inward. So the fusion and the gravity more or less balance each other out. This is called stellar equilibrium, which is the state our Sun is in, and will be in for several billion more years.
But stars don’t last forever, or rather, their hydrogen doesn’t. And once the hydrogen runs out, the star begins to change. In the case of a massive star, it begins to fuse heavier and heavier elements, until it fuses iron and nickel in its core. The fusion of iron and nickel is a natural fusion limit in a star, and once it reaches the iron and nickel fusion stage, fusion stops. We now have a star with an inert core of iron and nickel.
Now that fusion has stopped, stellar equilibrium is broken, and the enormous gravitational pressure of the star’s mass causes a collapse. This rapid collapse causes the core to heat again, which halts the collapse and causes a massive outwards shockwave. The shockwave hits the outer stellar material and blasts it out into space. Voila, a supernova.
The extremely high temperatures of the shockwave have one more important effect. It heats the stellar material outside the core, though very briefly, which allows the fusion of elements heavier than iron. This explains why the extremely heavy elements like uranium are much rarer than lighter elements. Only large enough stars that go supernova can forge the heaviest elements.
In a nutshell, that is a type II supernova, the same type found in 2013 when it was only 3 hours old. How the discovery of the CSM ejected by SN 2013fs will grow our understanding of supernovae is not fully understood.
Supernovae are fairly well-understood events, but their are still many questions surrounding them. Whether these new observations of the very earliest stages of a supernovae will answer some of our questions, or just create more unanswered questions, remains to be seen.
Artist’s conception of the Gaia telescope backdropped by a photograph of the Milky Way taken at the European Southern Observatory. Credit: ESA/ATG medialab; background: ESO/S. Brunier
In 2013, the European Space Agency deployed the long-awaited Gaia space observatory. As one of a handful of next-generation space observatories that will be going up before the end of the decade, this mission has spent the past few years cataloging over a billion astronomical objects. Using this data, astronomers and astrophysicists hope to create the largest and most precise 3D map of the Milky Way to date.
Though it is almost to the end of its mission, much of its earliest information is still bearing fruit. For example, using the mission’s initial data release, a team of astrophysicists from the University of Toronto managed to calculate the speed at which the Sun orbits the Milky Way. From this, they were able to obtain a precise distance estimate between our Sun and the center of the galaxy for the first time.
For some time, astronomers have been unsure as to exactly how far our Solar System is from the center of our galaxy. Much of this has to do with the fact that it is impossible to view it directly, due to a combination of factors (i.e. perspective, the size of our galaxy, and visibility barriers). As a result, since the year 2000, official estimates have varied between 7.2 and 8.8 kiloparsecs (~23,483 to 28,700 light years).
Infrared image from Spitzer Space Telescope, showing the stars at the center of the Milky Way Galaxy. Credit: NASA/JPL-Caltech/S. Stolovy (SSC/Caltech)
For the sake of their study, the team – which was led by Jason Hunt, a Dunlap Fellow at the Dunlap Institute for Astronomy & Astrophysics at the University of Toronto – combined Gaia’s initial release with data from the RAdial Velocity Experiment (RAVE). This survey, which was conducted between 2003 and 2013 by the Australian Astronomical Observatory (AAO), measured the positions, distances, radial velocities and spectra of 500,000 stars.
Over 200,000 of these stars were also observed by Gaia and information on them was included in its initial data release. As they explain in their study, which was published in the Journal of Astrophysical Letters in November 2016, they used this to examined the speeds at which these stars orbit the center of the galaxy (relative to the Sun), and in the process discovered that there was an apparent distribution in their relative velocities.
In short, our Sun moves around the center of the Milky Way at a speed of 240 km/s (149 mi/s), or 864,000 km/h (536,865 mph). Naturally, some of the more than 200,000 candidates were moving faster or slower. But for some, there was no apparent angular momentum, which they attributed to these stars being scattering onto “chaotic, halo-type orbits when they pass through the Galactic nucleus”.
As Hunt explained in Dunlap Institute press release:
“Stars with very close to zero angular momentum would have plunged towards the Galactic center where they would be strongly affected by the extreme gravitational forces present there. This would scatter them into chaotic orbits taking them far above the Galactic plane and away from the Solar neighbourhood… By measuring the velocity with which nearby stars rotate around our Galaxy with respect to the Sun, we can observe a lack of stars with a specific negative relative velocity. And because we know this dip corresponds to 0 km/sec, it tells us, in turn, how fast we are moving.”
Detection of an unusually bright X-Ray flare from Sagittarius A*, a supermassive black hole in the center of the Milky Way galaxy. Credit: NASA/CXC/Stanford/I. Zhuravleva et al.
The next step was to combine this information with proper motion calculations of Sagittarius A* – the supermassive black hole believed to be at the center of our galaxy. After correcting for its motion relative to background objects, they were able to effectively triangulate the Earth’s distance from the center of the galaxy. From this, they derived a refined distance of estimate of 7.6 to 8.2 kpc – which works out to about 24,788 to 26,745 light years.
This study builds upon previous work conducted by the study’s co-authors – Prof. Ray Calberg, the current chair of the Department of Astronomy & Astrophysics at the University of Toronto. Years ago, he and Prof. Kimmo Innanen of the Department of Physics and Astronomy at York University conducted a similar study using radial velocity measurement from 400 of the Milky Way’s stars.
But by incorporating data from the Gaia observatory, the UofT team was able to obtain a much more comprehensive data set and narrow the distance to galactic center by a significant amount. And this was based on only the initial data released by the Gaia mission. Looking ahead, Hunt anticipates that further data releases will allow his team and other astronomers to refine their calculations even more.
“Gaia’s final release in late 2017 should enable us to increase the precision of our measurement of the Sun’s velocity to within approximately one km/sec,” he said, “which in turn will significantly increase the accuracy of our measurement of our distance from the Galactic center.”
As more next-generation space telescopes and observatories are deployed, we can expect them to provide us with a wealth of new information about our Universe. And from this, we can expect that astronomers and astrophysicists will begin to shine the light on a number of unresolved cosmological questions.
An impact between a Mars-sized protoplanet and early Earth is the most widely-accepted origin of the Moon. Did smaller impacts seed the formation of continents? (NASA/JPL-Caltech)
For centuries, scientists have been attempting to explain how the Moon formed. Whereas some have argued that it formed from material lost by Earth due to centrifugal force, others asserted that a preformed Moon was captured by Earth’s gravity. In recent decades, the most widely-accepted theory has been the Giant-impact hypothesis, which states that the Moon formed after the Earth was struck by a Mars-sized object (named Theia) 4.5 billion years ago.
According to a new study by an international team of researchers, the key to proving which theory is correct may come from the first nuclear tests conducted here on Earth, some 70 years ago. After examining samples of radioactive glass obtained from the Trinity test site in New Mexico (where the first atomic bomb was detonated), they determined that samples of Moon rocks showed a similar depletion of volatile elements.
This composite of 25 images of asteroid 2017 BQ6 was generated with radar data collected using NASA’s Goldstone Solar System Radar in California’s Mojave Desert. It sped by Earth on Feb. 7 at a speed of around 25,560 mph (7.1 km/s) relative to the planet. The images have resolutions as fine as 12 feet (3.75 meters) per pixel. Credit: NASA/JPL-Caltech/GSSR
To radar imager Lance Benner at JPL in Pasadena, asteroid 2017 BQ6 resembles the polygonal dice used in Dungeons and Dragons. But my eyes see something closer to a stepping stone or paver you’d use to build a walkway. However you picture it, this asteroid is more angular than most imaged by radar.
It flew harmlessly by Earth on Feb. 7 at 1:36 a.m. EST (6:36 UT) at about 6.6 times the distance between Earth and the moon or some about 1.6 million miles. Based on 2017 BQ6’s brightness, astronomers estimate the hurtling boulder about 660 feet (200 meters) across. The recent flyby made for a perfect opportunity to bounce radio waves off the object, harvest their echoes and build an image of giant space boulder no one had ever seen close up before.
NASA’s 70-meter antennas are the largest and most sensitive Deep Sky Network antennas, capable of tracking a spacecraft traveling tens of billions of miles from Earth. This one at Goldstone not only tracked Voyager 2’s Neptune encounter, it also received Neil Armstrong’s famous communication from Apollo 11: “That’s one small step for a man. One giant leap for mankind.” Credit: JPL-Caltech/GSSR
The images of the asteroid were obtained on Feb. 6 and 7 with NASA’s 230-foot (70-meter) antenna at the Goldstone Deep Space Communications Complex in California and reveal an irregular, angular-appearing asteroid:
Animation of 2017 BQ6. The near-Earth asteroid has a rotation period of about 3 hours. Credit: NASA/JPL-Caltech/GSSR
“The radar images show relatively sharp corners, flat regions, concavities, and small bright spots that may be boulders,” said Lance Benner of NASA’s Jet Propulsion Laboratory in Pasadena, California, who leads the agency’s asteroid radar research program. “Asteroid 2017 BQ6 reminds me of the dice used when playing Dungeons and Dragons.”
Radar has been used to observe hundreds of asteroids. Even through very large telescopes, 2017 BQ6 would have appeared exactly like a star, but the radar technique reveals shape, size, rotation, roughness and even surface features.
This chart shows how data from NASA’s Wide-field Infrared Survey Explorer, or WISE, has led to revisions in the estimated population of near-Earth asteroids. Credit: NASA/JPL-Caltech
To create the images, Benner conducted a controlled experiment on the asteroid, transmitting a signal with well-known characteristics to the object and then, by comparing the echo to the transmission, deduced its properties. According to NASA’s Asteroid Radar Research site, measuring how the echo power spreads out over time along with changes in its frequency caused by the Doppler Effect (object approaching or receding from Earth), provide the data to construct two-dimensional images with resolutions finer than 33 feet (10 meters) if the echoes are strong enough.
This orbital diagram shows the close approach of 2017 BQ6 to Earth on Feb. 7, 2017. Credit: NASA/JPL Horizons
In late October 2016, the number of known near-Earth asteroids topped 15,000 with new discoveries averaging about 30 a week. A near-Earth asteroid is defined as a rocky body that approaches within approximately 1.3 times Earth’s average distance to the Sun. This distance then brings the asteroid within roughly 30 million miles (50 million km) of Earth’s orbit. To date, astronomers have already discovered more than 90% of the estimated number of the large near-Earth objects or those larger than 0.6 miles (1 km). It’s estimated that more than a million NEAs smaller than 330 feet (100 meters) lurk in the void. Time to get crackin’.
Using 1,000 images taken by 91 amateurs from around the world, Peter Rosen has created a high-resolution film of Jupiter's dynamic atmosphere. Credit: Peter Rosén et al. via YouTube
A renewed era of space exploration is underway. Compared to the Space Race of the 20th century, which was characterized by two superpowers locked in a game of “getting there first”, the new era is defined predominantly by cooperation and open participation. One way in which this is evident is the role played by “citizen scientists” and amateur astronomers in exploration missions.
Consider the recently-released short film titled “A Journey to Jupiter” by Peter Rosen – a photographer and digital artist in Stockholm, Sweden. Using over 1000 images taken by amateur planetary photographers from around the world, this film takes viewers on a virtual journey to the Jovian planet, showcasing its weather patterns and dynamic nature in a way that is truly inspiring.
The images that went into making this video were collected by over 91 amateur astronomers over the course of three and a half months (between December 19th, 2014 and March 31st, 2015). After Rosen collected them, he and his associates (Christoffer Svenske and Johan Warell) then spent a year remapping them into cylindrical projections. Rosen then added color corrections, and stitched all the images into a total of 107 maps.
Much like fast-motion videos that illustrate weather patterns on Earth, or the passage of the stars across the night sky, the end result of was a film that shows the motions of Jupiter’s cloud belts and its Great Red Spot in high-resolution. Some 250 revolutions of the planet are illustrated, including from the equatorial band, the south pole, and the north pole.
As Rosen told Universe Today via email, this project was the latest in a lifelong pursuit of making astronomy accessible to the public:
“I have been into Astronomy since I was a teenager in the early 1970’s and immediately I got a passion for astrophotography, and more specifically, photographing the planets. I see astronomy as a life-long passion, so it is quite normal to strive for an evolution in what you do. I had an idea growing slowly for some years that it should be possible to animate the cloud belts of Jupiter and reveal the intricate dynamics of its flows, not just taking still pictures that might point to the changes in the structures but without the obvious visual dynamics of an animation.”
“A Journey to Jupiter” was also Rosen’s contribution to the Mission Juno Pro-Amateur Collaboration Project, of which he is part. Established by Glenn Orton of NASA’s Jet Propulsion Laboratory, this effort is one of several that seeks to connect amateurs and professionals in support of space exploration. Back in May of 2016, this group met in Nice, France, for a workshop dedicated to projects and techniques related to Jupiter observations.
Still-pic from Rosen’s “A Journey to Jupiter” video. Credit: Peter Rosen et al via Youtube.
Among other items discussed was the limitations that missions like Juno have to deal with. While it is capable of taking very-high resolution images of Jupiter, these images are highly specific in nature. And before a team of mission scientists are able to color-correct them and stitch them together to create panoramas, etc., they are not always what you might call “visually stunning”.
However, Earth-based observatories are not hampered by this restriction, and can take multiple images of a planet over time that capture it as a whole. And thanks to the availability of sophisticated telescopes and imaging software, amateur astronomers are capable of making important contributions in this regard. And far from these being strictly for scientific purposes, there is also the added benefit of public engagement.
“This has been a very technical and scientifically correct project,” said Rosen, “but as a photographer and digital artist I also wanted to create a work of art that would inspire and appeal to people who are fascinated by the universe but who are not necessarily into astronomy.”
Of course, this does not detract from the scientific value that this film has. For example, it showcases the turbulent nature of Jupiter’s atmosphere in a way that is scientifically accurate. Hence why Ricardo Hueso Alonso – a physicist at the University of Basque Country and a member of the Planetary Virtual Observatory and Laboratory (PVOL) – plans to use the maps to measure Jupiter’s wind speeds at different latitudes.
Reprocessed image taken by the 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
On top of its artistic and scientific merit, “A Journey to Jupiter” also serves as a testament to the skill and capability of the today’s amateur astronomers and planetary photographers. And of course, it draws attention to the efforts of space missions such as Juno, which is currently skimming the clouds of Jupiter to obtain the most comprehensive information about the planet’s atmosphere and magnetic field to date.
Not surprisingly, this is not the first film by Rosen that combines scientific accuracy and fast-motion visuals. The short film Voyager3, released back in June of 2014, was an homage by Rosen and six other Swedish amateur astronomers to the Voyager 1 mission. As the probe made its 28-day final approach to Jupiter in 1979, it snapped what were the most detailed images of Jupiter at the time.
These images helped to improve our understanding of the gas giant, its atmosphere, and its moons. Among other things, hey revealed the turbulent nature of Jupiter’s atmosphere, and that the Great Red Spot had changed color since the Pioneer 10 and 11 missions had flown by in 1973 and 74. Produced 35 years later, Voyager 3 was an attempt to recreate this historic event using images taken by Swedish amateur astronomers using their own ground-based telescopes.
Over the course of 90 days, Rosen and his colleagues captured one million frames of Jupiter, which resulted in 560 still images of the planet. These were then stitched together using a series of software programs (Winjupos, Photoshop CS6, Fantamorph, and StarryNightPro+) to create a simulation that gives the impression of a probe approaching the planet – i.e. like a third Voyager mission, hence the name of the film.
“As Jupiter was ideally positioned high in the sky in 2013-2014 for us living far up in the northern hemisphere, I decided that it was the right moment to give it a try, so I contacted 6 other amateurs on our local forum that shared my passion for the planets,” Rosen said. “We photographed Jupiter as often as we could during a 3-month period and I took care of the processing of the images which took me a total of 6 months.”
It is an exciting time to be alive. Not only are a greater number of national space agencies taking part in the exploration of the Solar System; but more than ever, citizen scientists, amateurs and members of the general public are able to participate in a way that was never before possible.
To view more work by Peter Rosen, be sure to check out his page at Vimeo.
Earth, seen from space, above the Pacific Ocean. Credit: NASA
Whoever coined the phrase “it’s a small world” obviously never tried to travel around it! In truth, the planet’s dimensions are quite impressive, and determining just how big it is took many thousands of years. From astronomers determining that Earth was in fact round (and not a flat disc, cube or ziggurat), to the first successful attempts at circumnavigation, our estimates have changed over time.
And in the era of modern astronomy, improvements in instrumentation, methodology, and the ability to see Earth from space have certainly helped. According to modern estimates, the surface area of the Earth is approximately 510 million square km (5.1 x 108 km2) or 196,900,000 square miles. Determining this was not only a matter of ascertaining Earth’s dimensions, but also its proper shape.
Shape of the Earth:
For starters, and contrary to what scientists have believed since classical antiquity, Earth is not a perfect sphere. Since the 17th and 18th centuries – thanks to improvements made in the field of astronomy and geodesy (a branch of mathematics dealing with the measurement of the Earth) – scientists have understood that the Earth is actually a flattened sphere.
This is what is known as an “oblate spheroid”, which is a sphere that is wider at its horizontal axis than it is at its vertical axis. According to the 2004 Working Group of the International Earth Rotation and Reference Systems Service (IERS), Earth experiences a flattening of 0.0033528 at the poles. This flattening is due to Earth’s rotational velocity – a rapid 1,674.4 km/h (1,040.4 mph) – which causes the planet to bulge at the equator.
Because of this, the diameter of the Earth at the equator is about 43 kilometers (27 mi) larger than the pole-to-pole diameter. The latest measurements indicate that the Earth has an equatorial diameter of 12,756 km (7926 mi), and a polar diameter of 12713.6 km (7899.86 mi). This is true for other planets in the Solar System that have rapid rotations (like Jupiter and Saturn), and even stars like the rapidly-spinning Altair.
Calculation:
Given its particular shape, calculating the Earth’s surface area requires a specific equation. Whereas determining the surface area of a sphere is a simple matter of multiplying pi by four, and these by the square of its radius (4 x 3.14159… x r²), to calculate the surface area of an oblate spheroid – where the distance from the center to a pole (c) is less than its semi-axis (a) – the following equation has to come into play:
Whereas S equals the surface area, c represents the distance from the center to a pole, and a represents the semi-axis, e represents the eccentricity. Naturally, Earth’s surface area can also be subdivided between water and land segments (aka. oceans or continental crust).
And since 70% of the Earth’s surface is covered by water, that works out to 361 million km² (139.4 million mi²). Earth’s continents, on the other hand, cover the remaining 149 million km² (57.5 or million mi²). This is a phenomena unique to Earth (at least in our Solar System) since no other Solar planet has liquid water covering a significant amount of its surface.
Other Solar Planets:
Compared to the other planets of the Solar System, Earth ranks somewhere in the middle. Of the terrestrial planets (i.e. Mercury, Venus, Earth and Mars) it is the largest. However, when compared to the gas giants (Jupiter, Saturn, Uranus and Neptune) it comes in dead last! Let’s see just how Earth stacks up against these other worlds…
Mercury is the smallest planet in our Solar System (ever since the 2006 IAU decision that designated Pluto as “dwarf planet”). It has a surface area of 7.48 x 107 km2, which is only about 15% of Earth’s surface area. Venus is similar in size to Earth, hence why it has earned the title of “Earth’s Sister Planet”. Consistently, Venus has a surface area of 4.6 x 108 km2, which is roughly 90% of Earth’s.
Mars is also a small planet, the second smallest in our Solar System. This is evident in Mars’ diminutive surface area of 1.45 x 108 km2, which is roughly 28% that of Earth’s. Moving to the outer Solar System, it is quickly made apparent that all of the gas giants have the four planets of the inner Solar System beat (at least in a size contest)!
An illustration showing the 8 planets of the Solar System to scale. Credit: NASA
Jupiter is the largest planet in our Solar System, with a surface area of 6.14 x 1010 km2 – which is about 122 times greater than the surface area of Earth! Saturn is the second largest planet in our Solar System and has a surface area of 4.27 x 1010 km2 – which is roughly 83.7 times that of Earth.
As for the “ice giants”, Uranus has a surface area of 8.1156 x 109 km2 (15.91 times that of Earth) while Neptune has a slightly smaller surface area of 7.618 x 109 km2, which is close to 15 times that of Earth.
All told, Earth is relatively spacious place, as terrestrial bodies go. But the amount of surface that we humans can actually live on is rather limited. Once you subtract all the space that’s occupied by water, you begin to see that the world may be a little on the smallish side after all.
The Centaurus A galaxy (NGC 5128), a luminous galaxy located in the Centaurus constellation. Credit: ESO
Welcome back to Constellation Friday! Today, in honor of the late and great Tammy Plotner, we will be dealing with the “Centaur”, the Centaurus 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 is the famous Centaur of classical antiquity, otherwise known as the constellation Centaurus. As one of the 48 constellation included in the Almagest, it is now one of the 88 modern constellations recognized by the IAU. Located in the southern sky, this constellation is bordered by the Antlia, Carina, Circinus,Crux,Hydra,Libra,Lupus,Musca, and Vela constellations.
Name and Meaning:
In classic Greco-Roman mythology, Centaurus is often associated with Chiron the Centaur – the wise half-man, half-horse who was a teacher to both Hercules and Jason and the son of the Titan king Cronus and the sea nymph Philyra. According to legend, Cronus seduced the nymph, but they were interrupted by Cronus’ wife Rhea. To evade being caught in the act, Cronus turned himself into a horse.
Centaurus, as depicted on a globe created by Gullielmus Janssonius Blaeu (1602), photographed at Skokloster Castle in Stockholm, Sweden. Credit: Wikipedia Commons/Erik Lernestål
As a result, Philyra gave birth to a hybrid son. He died a tragic death in the end, having been accidentally struck by one of Heracles’ poisoned arrows. As an immortal god, he suffered terrible pains but could not die. Zeus eventually took pity on the centaur and released him from immortality and suffering, allowing him to die, and placed him among the stars.
It is believed that the constellation of Sagitta is the arrow which Chiron fired towards Aquila the Eagle to release the tortured Prometheus. The nearby constellation of Lupus the Wolf may also signify an offering of Hercules to Chiron – whom he accidentally poisoned. Just as Virgo above represents the maid placed in the sky as a sign of pity for the Centaur’s plight.
History of Observation:
The first recorded examples of Centaurus date back to ancient Sumeria, where the constellation was depicted as the Bison-man (MUL.GUD.ALIM). This being was depicted in one of two ways – either as a four-legged bison with a human head, or as a creature with a human head and torso attached to the rear legs of a bison or bull. In the Babylonian pantheon, he was closely associated with the Sun god Utu-Shamash.
The Greek depiction of the constellation as a centaur is where its current name comes from. Centaurus is usually depicted as sacrificing an animal, represented by the constellation Lupus, to the gods on the altar represented by the Ara constellation. The centaur’s front legs are marked by two of the brightest stars in the sky, Alpha and Beta Centauri (aka. Rigil Kentaurus and Hadar), which also serve as pointers to the Southern Cross.
Johannes Hevelius’ depiction of Centaurus, taken from Uranographia (1690). Credit: NASA/Chandra
In the 2nd century AD, Ptolemy catalogued 37 stars in the constellation and included it as one of the 48 constellations listed in the Almagest. In 1922, it was included in the 88 modern constellations recognized by the International Astronomical Union (IAU).
Notable Features:
Centaurus contains 11 main stars, 9 bright stars and 69 stars with Bayer/Flamsteed designations. Its brightest star – Alpha Centauri (Rigel Kentaurus) – is the Solar System’s closest neighbor. Located just 4.365 light years from Earth, this multiple star system consists of a yellow-white main sequence star that belongs to the spectral type G2V (Alpha Centauri A), and a spectral type K1V star (Alpha Centauri B).
Alpha Centauri A, the brightest component in the system, is the fourth brightest individual star (behind Arcturus) in the night sky, B is the 21st individual brightest star in the sky. Taken together, however, they are brighter than Arcturus, and rank third among the brightest star system (behind Sirius and Canopus). The two stars are believed to be roughly the same age – ~4.85 billion years old – and are close in mass to our Sun.
Proxima Centauri, a red dwarf system (spectral class M5Ve or M5Vie), if often considered to be a third member of this star system. Located about 0.24 light years from the binary pair (and 4.2 light years from Earth), this star system was confirmed in 2016 to be home to the closest exoplanet to Earth (Proxima b).
The two brightest stars of the Centaurus constellation – (left) Alpha Centauri and (right) Beta Centauri. The faint red star in the center of the red circle is Proxima Centauri. Credit: Wikipedia Commons/Skatebiker
Then there’s Beta Centauri, a blue-white giant star (spectral class B1III) located 348.83 light years from Earth that is the tenth brightest star in the sky. The star’s traditional names (Hadar or Agena), are derived from the Arabic words for “ground” and “the knee”, respectively. This multiple star system consists of Hadar A, a spectroscopic binary of two identical stars, while Hadar B orbits the primary pair with a period of at least 250 days.
Next up is Theta Centauri (aka. Menkent), an orange K-type giant (spectral class K0IIIb) that is located approximately 60.9 light years from Earth. Its traditional name, which comes from its location in the constellation, translates to “shoulder of the Centaur” in Arabic.
And then there’s Gamma Centauri (Muhlifain), a binary star system located 130 light years from Earth which is composed of two stars belonging to the spectral type A0. It’s name is translated from Arabic and means “two things”, or the “swearing of an oath”, which appears to be a case of name-transfer from Muliphein, a star located in the Canis Majoris constellation.
The constellation is also home to many Deep Sky Objects. For instance, there is the Centaurus A galaxy, the fifth brightest galaxy in the sky and one of the closest radio galaxies to the Solar System (between 10 and 16 million light years distant). The galaxy has an apparent visual magnitude of 6.84 and is believed to contain a supermassive black hole at its center.
Image of the Centaurus A galaxy, combining optical, x-ray and infrared data. Credit: X-ray: NASA/CXC/SAO/Rolf Olsen/JPL-Caltech
Centaurus A’s brightness is attributed to the intense burst of star formation going on inside it, which is believed to be the result of it undergoing a collision with a spiral galaxy. Centaurus A is located at the center of the Centaurus A subgroup of the Centaurus A/M83 Group of galaxies, which includes the Southern Pinwheel Galaxy (aka. Messier 83, M83).
Then there’s the famous Omega Centauri globular cluster, one of the brightest globular clusters in the Milky Way. Located approximately 15,800 light years distant, this cluster is bright enough to be visible to the naked eye. Originally listed as a star by Ptolemy in the Almagest, the cluster’s true nature was not discovered until John Herschel studied it in the early 19th century.
Next up is NGC 4945, one of the brightest galaxies in the Centaurus A/M83 group, and the second brightest galaxy in the Centaurus A subgroup. The spiral galaxy is approximately 11.7 million light years distant and has an active Seyfert II nucleus, which could be due to the presence of a supermassive black hole at its center.
The galaxy NGC 4650A is also located in Centaurus, some 130 million light years from Earth. This galaxy is one of only 100 polar-ring galaxies known to exist, which are so-named because their outer ring of stars and gas rotate over the poles of the galaxy. These rings are believed to have formed from the gravitational interaction of two galaxies, or from a collision with a smaller galaxy in the past.
The Blue Planetary (NGC 3918), as imaged by the Hubble telescope. Credit: ESA/Hubbl/e NASA
The Blue Planetary nebula (aka. the Southerner), is a bright planetary nebula in Centauru, approximately 4,900 light years distant. With an apparent visual magnitude of 8.5, it is the brightest planetary nebula in the far southern region of the sky and and can be observed in a small telescope.
Finding Centaurus:
Centaurus is one of the largest constellations in the night sky – covering over 1000 square degrees – and the brightest in the southern hemisphere. For observers located at latitudes between +30° and -90°, the entire constellation is visible and the northern portion of the constellation can be spotted easily from the northern hemisphere during the month of May.
For the unaided southern skies observer, the constellation of Centaurus holds a gem within its grasp – Omega Centauri (NGC 5139). But of course, this object isn’t a star – despite being listed on the catalogs as its Omega star. It’s a globular cluster, and the biggest and brightest of its kind known to the Milky Way Galaxy. Though visible to the naked eye, it is best observed through a telescope or with binoculars.
This 18,300 light-year beauty contains literally millions of stars with a density so great at its center the stars are less than 0.1 light year apart. It is possible Omega Centauri may be the remains of a galaxy cannibalized by our own. Even to this present day, something continues to pull at NGC 5139’s stars… tidal force? Or an unseen black hole?
Omega Centauri (NGC 5139), a massive globular cluster that is part of the Centaurus constellation. Credit: Jose Mtanous
Now, hop down to Alpha. Known as Rigil Kentaurus, Rigil Kent, or Toliman, is the third brightest star in the entire night sky and the closest star system to our own solar system. To the unaided eye it appears a single star, but it’s actually a binary star system. Alpha Centauri A and Alpha Centauri B are the individual stars and a distant, fainter companion is called Proxima Centauri – a red dwarf that is the nearest known star to the Sun.
Oddly enough, Proxima Centauri is also a visual double, which is assumed to be associated with Centaurus AB pair. Resolution of the binary star Alpha Cen AB is too close to be seen by the naked eye, as the angular separation varies between 2 and 22 arc seconds, but during most of the orbital period, both are easily resolved in binoculars or small telescopes.
Then stop for a moment to take a look at Beta Centauri. Beta Centauri is well-known in the Southern Hemisphere as the inner of the two “Pointers” to the Southern Cross. A line made from the other pointer, Alpha Centauri, through Beta Centauri leads to within a few degrees of Gacrux, the star at the top of the cross. Using Gacrux, a navigator can draw a line with Acrux to effectively determine south.
But, that’s not all! Hadar is also a very nice double star, too. The blue-white giant star primary is also a spectroscopic binary, accompanied by a widely spaced companion separated from the primary by 1.3″. Or try Gamma Centauri! Muhlifain has an optical companion nearby, but check it out in the telescope… it’s really two spectral type A0 stars each of apparent magnitude +2.9!
The location of the Centaurus constellation in the southern sky. Credit: IAU/Sky & Telescope magazine/Roger Sinnott & Rick Fienberg
For binoculars or telescopes, hop on over to Centaurus A. This incredible radio source galaxy is one of the closest to Earth and also the fifth brightest in the sky. When seen through an average telescope, this galaxy looks like a lenticular or elliptical galaxy with a superimposed dust lane, and oddity first noted in 1847 by John Herschel.
The galaxy’s strange morphology is generally recognized as the result of a merger between two smaller galaxies and photographs reveal a jet of material streaming from the galactic core. Although we cannot see it, there may be a supermassive black hole at the center of the galaxy is responsible for emissions in the X-ray and radio wavelengths!
For binoculars and rich field telescopes, head towards the Crux border and center on Lambda Centauri for open cluster, IC2944. Also known on some observing lists as Caldwell 100, this scattered star cluster contains about 30 stellar members and some faint nebulosity. About 2 degrees southwest of Beta you’ll find another pair of open clusters, NGCs 5281 and 5316. Or try your hand just about a degree west of Alpha for open cluster, NGC5617. These last three are far more rich in stars and photon satisfying!
Centaurus has been known to human astronomers since the Bronze Age and has gone through some changes since that time. But even after thousands of years’ time, the Centaur is still hunting in the night sky! And for those who love viewing classic constellations and bright objects, it still provides viewing opportunities that are bound to dazzle the eyes and inspire the mind!
This is the highest resolution image taken by Galileo at Europa — Jupiter’s 4th largest moon — until our next mission to the planet. It was obtained at an original image scale of 19 feet (6 meters) per pixel. The gray line down the middle resulted from missing data that was not transmitted by Galileo. Credit: NASA/JPL-Caltech
In the movie 2010: The Year We Make Contact, the sequel to Stanley’s Kubrick’s 2001: A Space Odyssey, black Monoliths multiply, converge and transform Jupiter into a new star. We next hear astronaut David Bowman’s disembodied voice with this message: “All these worlds are yours except Europa. Attempt no landing there.” The newborn sun warms Europa, transforming the icy landscape into a primeval jungle. At the end, a single Monolith appears in the swamp, waiting once again to direct the evolution of intelligent life forms.
Europa’s cracked, icy surface imaged by NASA’s Galileo spacecraft in 1998. Credit: NASA/JPL-Caltech/SETI Institute
Stay away from Europa? No way. It’s just too fascinating a place with its jigsaw-puzzle ice sheets, crisscross valleys, miles of ice on top and a warm, salty ocean below. The movie was prescient — if you’re going to search for life elsewhere in the solar system, Europa’s one of the best candidates.
While we’ve sent spacecraft to photograph and study the icy moon during orbital flybys, no lander has yet to touch the surface. That may change soon. In early 2016, in response to a congressional directive, NASA’s Planetary Science Division began a pre-Phase A study to assess the science value and engineering design of a future Europa lander mission. In June 2016, NASA convened a 21-member team of scientists for the Science Definition Team (SDT). The team put together set of science objectives and measurements for the mission concept and submitted the report to NASA on Feb. 7.
This artist’s rendering illustrates a conceptual design for a potential future mission to land a robotic probe on the surface of Jupiter’s moon Europa. The lander is shown with a sampling arm extended, having previously excavated a small area on the surface. The circular dish on top is a combo high-gain antenna and camera mast, with stereo imaging cameras mounted on the back of the antenna. Three vertical shapes located around the top center of the lander are attachment points for cables that would lower the rover from a sky crane, the planned landing system for this mission concept. Credits: NASA/JPL-Caltech
The report lists three science goals for the mission. The primary goal is to search for evidence of life on Europa. The other goals are to determine the habitability of Europa by directly analyzing material from the surface, and to characterize the surface and subsurface to support future robotic exploration of Europa and its ocean.
This image from NASA’s Galileo spacecraft show the intricate detail of Europa’s icy surface. The red staining occurs in areas where briny waters from below — possibly mixed with sulfur — reach the surface. Radiation from Jupiter bombards the material, causing it to redden. Gravitational flexing of the moon as it orbits Jupiter fractures the icy crust into a chaotic landscape of snaking valleys and ice sheets. It also warms the ocean beneath the crust, potentially making it habitable. Credit: NASA/JPL-Caltech
The evidence is quite strong that Europa, with a diameter of 1,945 miles — slightly smaller than Earth’s moon — has a global saltwater ocean beneath its icy crust. This ocean has at least twice as much water as Earth’s oceans. Two things make Europa’s ocean unique and give the moon a greater chance of supporting microbial life compared to say, Ganymede and Enceladus, which also hold water reservoirs beneath their crusts.
Astronomers hypothesize that chloride salts bubble up from the icy moon’s global liquid ocean and reach the frozen surface where they are bombarded with sulfur from volcanoes on Jupiter’s innermost large moon Io. Molecular signs of life may be transported where they could be detected by a spacecraft. In this illustration, we see Europa (foreground), Jupiter (right) and Io (middle). Credit: NASA/JPL-Caltech
One: the ocean is relatively close to the surface, just 10-15 miles below the moon’s icy shell. Radiation from Jupiter (high-speed electrons and protons) bombards ice, sulfur and salts on the surface to create compounds that could trickle down into warmer regions and used by living things for growth and metabolism.
Broken plates and blocks of water ice now frozen in place in Europa’s crust suggest they floated freely for a time. Credit: NASA/JPL-Caltech
Two: While recent discoveries have shown that many bodies in the solar system either have subsurface oceans now, or may have in the past, Europa is one of only two places where the ocean appears to be in contact with a rocky seafloor (the other being Saturn’s moon Enceladus). This rare circumstance makes Europa one of the highest priority targets in the search for present-day life beyond Earth.
On Earth, chemical interactions between life and lifeless rock in deep oceans and within the outer crust provide the energy needed to power and sustain microbial life. For all we know, deep sea volcanoes belch essential elements into the salty waters spawned by the constant flexing and heating of the moon as it orbits Jupiter every 85 hours.
This mosaic of images includes the most detailed view of the surface of Jupiter’s moon Europa obtained by NASA’s Galileo mission. This observation was taken with the sun relatively high in the sky, so most of the brightness variations are due to color differences in the surface material rather than shadows. Ridge tops, brightened by frost, contrast with darker valleys, perhaps due to small temperature variations allow frost to accumulate in slightly colder, higher-elevation locations. Credit: NASA/JPL-Caltech
The SDT was tasked with developing a life-detection strategy, a first for a NASA mission since the Mars Viking mission era more than four decades ago. The report makes recommendations on the number and type of science instruments that would be required to confirm if signs of life are present in samples collected from the icy moon’s surface.
The team also worked closely with engineers to design a system capable of landing on a surface about which very little is known. Given that Europa has no atmosphere, the team developed a concept that could deliver its science payload to the icy surface without the benefit of technologies like a heat shield or parachutes.
This artist’s rendering shows NASA’s Europa mission spacecraft, which is being developed for a launch sometime in the 2020s. The spacecraft would orbit around Jupiter in order to perform a detailed investigation of Europa before a follow-up landing mission. The probe could look for “biosignatures” or molecular signs of life, such as the byproducts of metabolism, transported from the moon’s ocean to its surface. Credit: NASA/JPL-Caltech
The concept lander is separate from the solar-powered Europa multiple flyby mission, now in development for launch in the early 2020s. The spacecraft will arrive at Jupiter after a multi-year journey, orbiting the gas giant every two weeks for a series of 45 close flybys of Europa. The multiple flyby mission will investigate Europa’s habitability by mapping its composition, determining the characteristics of the ocean and ice shell, and increasing our understanding of its geology. The mission also will lay the foundation for a future landing by performing detailed reconnaissance using its powerful cameras.
We can’t help but be excited by the prospects of life-seeking missions to Europa. Sometimes wonderful things come in small packages.
The Moon occults Regulus of January 15th, 2017. Image credit and copyright: Lucca Ruggiero
The Moon occults Regulus of January 15th, 2017. Image credit and copyright: Lucca Ruggiero
In the southern hemisphere this weekend in the ‘Land of Oz?’ Are you missing out on the passage of Comet 45/P Honda-Mrkos-Pajdušáková, and the penumbral lunar eclipse? Fear not, there’s an astronomical event designed just for you, as the Moon occults (passes in front of) the bright star Regulus on the evening of Saturday, January 11th.
The entire event is custom made for the continent of Australia and New Zealand, occurring under dark skies. Now for the bad news: the waning gibbous Moon will be less than 14 hours past Full during the event, meaning that the ingress (disappearance) of Regulus will occur along its bright leading limb and egress (reappearance) will occur on the dark limb. We prefer occultations during waxing phase, as the star winks out on the dark limb and seems to slowly fade back in on the bright limb.
The footprint for the February 11th occultation of Regulus by the Moon. Image credit: Occult 4.2 software
The International Occultation Timing Association has a complete list of precise ingress/egress times for cities located across the continent. An especially interesting region to catch the event lies along the northern graze line across the sparsely populated Cape York peninsula, just north of Cairns.
The northern grazeline for the February 11th occultation of Regulus by the Moon. Graphic by author.
The Moon occults Aldebaran and then Regulus six days later during every lunation in 2017. This is occultation number three in a cycle of 19 running from December 18, 2016 to April 24, 2018. The Moon occults Regulus 214 times in the 21st century, and Regulus is currently the closest bright star to the ecliptic plane, just 27′ away.
We’ve also got a very special event just under 14 hours prior, as a penumbral lunar eclipse occurs, visible on all continents… except Australia. Mid-eclipse occurs at 00:45 Universal Time (UT, Saturday morning on February 11th), or 7:45 PM Eastern Standard Time (EST) on the evening of Friday, February 10th, when observers may note a dusky shading on the northern limb of the Moon as the Moon just misses passing through the dark edge of the Earth’s inner umbral shadow. Regulus will sit less than seven degrees off of the lunar limb at mid-eclipse Friday night.
How often does an eclipsed Moon occult a bright star? Well, stick around until over four centuries from now on February 22nd, 2445, and observers based around the Indian Ocean region can watch just such an event, as the eclipsed Moon also occults Regulus. Let’s see, I should have my consciousness downloaded into my second android body by then…
A graphic study of the simultaneous lunar eclipse and occultation of Regulus in 2445. Credit: NASA/GSFC/Fred Espenak/Occult 4.2/Stellarium.
We’ll be streaming the Friday night eclipse live from Astroguyz HQ here in Spring Hill, Florida starting at 7:30 PM EST/00:30 UT, wifi-willing. Astronomer Gianluca Masi of the Virtual Telescope Project will also carry the eclipse live starting at 22:15 UT on the night of Friday, February 10th.
This eclipse also marks the start of eclipse season one of two, which climaxes with an annular eclipse crossing southern Africa and South America on February 26th. The second and final eclipse season of 2017 starts with a partial lunar eclipse on August 7th, which sets us up for the Great American Eclipse crossing the United States from coast to coast on August 21st, 2017.
A lunar occultation of Regulus also provides a shot at a unique scientific opportunity. Spectroscopic measurements suggest that the primary main sequence star possesses a small white dwarf companion, a partner which has never been directly observed. This unseen white dwarf may – depending on the unknown orientation of its orbit – make a brief appearance during ingress or egress for a fleeting split second, when the dark limb of the Moon has covered dazzling Regulus. High speed video might just nab a double step occlusion, as the white dwarf companion is probably about 10,000 times fainter than Regulus at magnitude +11 at the very brightest. Regulus is located 79 light years distant.
Our best results for capturing an occultation of a star or planet by the Moon have always been with a video camera aimed straight through our 8” Schmidt-Cassegrain telescope. The trick is always to keep the star visible in the frame near the brilliant Full Moon. Cropping the Moon out of the field as much as possible can help somewhat. Set up early, to work the bugs out of focusing, alignment, etc. We also run WWV radio in the background for an audible time hack on the video.
The January 15th, 2017 occultation of Regulus by the Moon. Image credit and copyright: Lucca Ruggiero.
The best occultation of Regulus by the Moon for North America in 2017 occurs on October 15th, when the Moon is at waning crescent phase. Unfortunately, the occultation of Regulus by asteroid 163 Erigone back in 2014 was clouded out, though the planet Venus occults the star on October 1st, 2044. Let’s see, by then I’ll be…
Comets and eclipses and occultations, oh my. It’s a busy weekend for observational astronomy, for sure. Consider it an early Valentine’s Day weekend gift from the Universe.
Webcasting the eclipse or the occultation this weekend? Let us know, and send those images of either event to Universe Today’s Flickr forum.
Read about eclipses, occultations and more tales of astronomy in our yearly guide 101 Astronomical Events For 2017, free from Universe Today.
