Astronomy Archives

April 3, 2017April 28, 2017 by Matt Williams

Mars’ Trojans Show Remains Of Ancient Planetoid

A new study led by researchers from OU indicates that the outer planets could be why Mars is significantly smaller than Earth. Credit: NASA

Trojan asteroids are a fascinating thing. Whereas the most widely known are those that orbit Jupiter (around its L4 and L5 Lagrange Points), Venus, Earth, Mars, Uranus and Neptune have populations of these asteroids as well. Naturally, these rocky objects are a focal point for a lot of scientific research, since they can tell us much about the formation and early history of the Solar System.

And now, thanks to an international team of astronomers, it has been determined that the Trojan asteroids that orbit Mars are likely the remains of a mini-planet that was destroyed by a collision billions of years ago. Their findings are detailed in a paper that will be published in The Monthly Notices of the Royal Astronomical Society later this month.

For the sake of their study, the team – which was led by Galin Borisov and Apostolos Christou of the Armagh Observatory and Planetarium in Northern Ireland, examined the composition of Marian Trojans. This consisted of using spectral data obtained by the XSHOOTER spectrograph on the Very Large Telescope (VLT) and photometric data from the National Astronomical Observatory‘s two-meter telescope, and the William Herschel Telescope.

*Diagram of Jupiter and the inner Solar System, showing the Jupiter and Martian Trojans (light green) and the Main Belt (teal). Credit: Wikipedia Commons/AndrewBuck*

Specifically, they examined two members of the Eureka family – a group of Martian Trojans located at the planet’s L5 point. It is here that eight of Mars’ nine known Trojans exist in stable orbits (the other being at L4), and which are named after the first Martian Trojan ever discovered – 5261 Eureka. Like all Trojans, the Eurekas are thought to have orbited Mars ever since the formation of the Solar System.

In fact, astronomers have suspected for some time that the Martian Trojans could be the survivors of an early generation of planetesimals from which the inner Solar System formed. As Dr. Christou told Universe Today via email:

“[The Trojan family] is unique in the Solar System, in more ways than one. Unlike every other family that exists in the Main Asteroid Belt between Mars and Jupiter, it is made up of olivine-rich asteroids. Also, the asteroids are < 2km across, much smaller than we can see at other families, basically because they are much closer to the Earth than other asteroids. Finally, it is the closest family we know to the Sun, and this has implications on how it formed in that the tiny but continuous action of sunlight may have played a role.”

After combining spectrographic and photometric data on these asteroids, the team found that they were rich in the mineral olivine – a magnesium iron silicate that is a primary component of the Earth’s mantle and (it is believed) other terrestrial planets. This was unusual find as far as asteroids go, but it was even more interesting when compared to 5261 Eureka itself – which also has an olivine-rich composition.

*The first X-ray view of Martian soil by Curiosity rover at the “Rocknest” (October 17, 2012), showing traces of feldspar, pyroxenes, and olivine. Credit: NASA/JPL-Caltech/Ames*

Given that the Eureka asteroids also have similar orbits, the team concluded that every member of this family is likely to have a common composition – and hence, a common origin. These findings could have drastic implications for both the origin of Martian Trojans, and the origin of the inner Solar System. As Dr. Christou explained:

“The presence of asteroids with exposed olivine on their surfaces constrains the sequence of events that led to Mars’ formation. Olivine forms within objects that grew large enough to differentiate into a crust, mantle and core. Therefore, these objects must have formed before Mars did and were available to participate in Mars’ formation. To expose the olivine, it is necessary to break these objects up through collisions. Our ongoing work indicates that this is unlikely to have happened after the Solar System settled down in its current configuration, therefore there must have been period of intense collisional evolution during the planet formation process.”

In other words, if Mars formed from several types of material that was mixed together, these asteroids would be samples of the original source – i.e. planetesimals. By examining these asteroids further, scientists will be able to learn more about the process through which Mars came to be and (as Christou says) help us “unscramble the Martian omelette.”

This research is also likely to reveal much about the formation of Earth and the other terrestrial planets of the Solar System. Similar efforts will be made with NASA’s upcoming Lucy mission, which is scheduled to launch in October of 2021. Between 2027 and 2033, this probe will study Jupiter’s Trojan population, obtaining information on six of the asteroid’s geology, surface features, compositions, masses and densities to learn more about their origins.

Further Reading: MNRAS, Armagh Observatory

April 3, 2017April 4, 2017 by David Dickinson

Surprise: Comet E4 Lovejoy Brightens

Credit and copyright: The Virtual Telescope Project

Had your fill of binocular comets yet? Thus far this year, we’ve had periodic comets 2P/Encke, 45P/Honda-Mrkos-Pajdušáková and 41P/Tuttle-Giacobini-Kresák all reach binocular visibility above +10^th magnitude as forecasted. Now, we’d like to point out a surprise interloper in the dawn sky that you’re perhaps not watching, but should be: Comet C/2017 E4 Lovejoy.

If that name sounds familiar, that’s because E4 Lovejoy is the sixth discovery by prolific comet hunter Terry Lovejoy. Comets that have shared the Lovejoy moniker include the brilliant sungrazer C/2011 W3 Lovejoy, which amazed everyone by surviving its 140,000 kilometer (that’s about 1/3 the Earth-Moon distance!) pass near the blazing surface of the Sun on December 16^th, 2011 and went on to be a great comet for southern hemisphere skies.

The path of Comet E4 Lovejoy through the end of April. Credit: Starry Night.

Unfortunately, E4 Lovejoy won’t get quite that bright, but it’s definitely an over achiever. Shining at a faint +15^th magnitude when it was first discovered last month on March 9^th, 2017, it has since jumped up to +7^th magnitude (almost 160 times in brightness) in just a few short weeks. We easily picked it out near the +2.4 magnitude star Enif (Epsilon Pegasi) on Saturday morning April 1^st in the pre-dawn sky. E4 Lovejoy was an easy catch with our Canon 15×45 image-stabilized binocs, and looked like a tiny +7 magnitude globular (similar to nearby Messier 15) that stubbornly refused to snap into focus. In fact, I’d say that E4 Lovejoy was a much easier comet to observe than faint Comet 41P/Tuttle-Giacobini-Kresák, which made its closest pass 0.142 Astronomical Units (21.2 million kilometers) from the Earth on the same day.

Comet E4 Lovejoy from the morning of April 4th. Image credit and copyright: Gerald Rhemann/Sky Vistas.

Prospects and Prognostications

E4 Lovejoy will remain an early pre-dawn object through April for northern hemisphere observers as it glides through the constellations Pegasus, Andromeda and Triangulum. If current predictions hold true, the comet should reach a maximum brightness of magnitude +6 around April 15^th. On an estimated ~ 600,000 year orbit, Comet E4 Lovejoy may be a first time visitor to the inner solar system, and its current outburst may also be short-lived. In fact, there’s lots of speculation that Comet E4 Lovejoy may disintegrate altogether, very soon. Plus, the Moon is headed towards Full next week on April 11^th, making this week the best time to catch a glimpse of this fleeting comet.

The projected light curve for Comet E4 Lovejoy. Credit: Seiichi Yoshida’s *Weekly Information About Bright Comets*.

And to think: we just missed having a bright naked eye comet! That’s because Comet E4 Lovejoy very nearly passed through the space that the Earth will occupy just next month. In fact, the comet passed just 0.11 AU (17 million kilometers) interior to the Earth’s orbit on March 22^nd, 2017. Had it done the same on May 4^th, it would have been 5 times closer and 25 (about 3 to 4 magnitudes) times brighter!

The orbit of Comet E4 Lovejoy through the inner solar system. NASA/JPL

A tantalizing miss, for sure. Comet C/2017 E4 Lovejoy reaches perihelion at 0.5 AU (77.5 million kilometers) from the Sun on April 23^rd, and passed 0.6 AU (93 million kilometers) from the Earth on March 31^st. This week, it will be moving through Pegasus at a rate of about four degrees (8 Full Moon diameters) a day. With an orbital inclination of 88 degrees, Comet E4 Lovejoy’s path is very nearly perpendicular to the ecliptic path traced out by the Earth. The comet swung up from the south during discovery, and is now headed northward towards perihelion.

Here are some key dates for Comet C/2017 E4 Lovejoy to watch out for in April:

April 7^th: Passes less than one degree from the +3.5 magnitude star Sadal Bari (Lambda Pegasi).

April 9^th: Passes less than 10′ from the +2.4 magnitude star Scheat (Beta Pegasi).

April 13^th: Crosses into the constellation Andromeda.

April 19^th: Photo-op, as the comet passes 4 degrees from the Andromeda Galaxy M31.

April 22^nd: Passes between the +2^nd magnitude star Mirach and the +4^th magnitude star Mu Andromedae.

April 27^th: Passes five degrees from the Pinwheel Galaxy M33.

April 28^th: Crosses into the constellation Triangulum.

Looking to the northeast at 6 pm local on the morning of April 19th from latitude 30 degrees north. Credit: Stellarium.

Teaser for 2017 Comets

We’re barely a quarter of the way through 2017, with more cometary action to come. We’re expecting 2015 ER51 PanSTARRS (May), and 2015 V2 Johnson (June) to reach binocular visibility. You can read about comets, occultations, and more in our guide to 101 Astronomical Events for 2017, a free e-book from Universe Today.

We’re due for the next big one, for sure. It always seems like there’s a “Great Comet” per every generation or so, and its been 20 years now since comets Hale-Bopp and Hyakutake graced northern skies.

Binoculars are the best tool for observing comets like E4 Lovejoy, as they offer a generous true (i.e. not inverted) field of view. A good finder chart and dark skies also help. We like to find a good nearby ‘anchor’ object such as a bright star, then hop into the suspected comet area and start sweeping.

One thing’s for sure: we need more comets with names like Lovejoy… if nothing else, it’s much easier to pronounce, and us science writers don’t have to keep hunting through the ‘insert’ menu for those strange letter symbols that grace many of these icy denizens of the Oort Cloud as they pay a visit to the inner solar system.

March 31, 2017April 4, 2017 by Matt Williams

Deepest X-ray Image Ever Made Contains Mysterious Explosion

A mysterious flash of X-rays has been discovered by NASA’s Chandra X-ray Observatory in the deepest X-ray image ever obtained. Credit: NASA/Chandra/Harvard

For over sixty years, astronomers have been exploring the Universe for x-ray sources. Known to be associated with stars, clouds of super heated gas, interstellar mediums, and destructive events, the detection of cosmic x-rays is challenging work. In recent decades, astronomers have been benefited immensely from by the deployment of orbital telescopes like the Chandra X-ray Observatory.

Since it was launched on July 23rd, 1999, Chandra has been NASA’s flagship mission for X-ray astronomy. And this past week (on Thurs. March 30th, 2017), the Observatory accomplished something very impressive. Using its suite of advanced instruments, the observatory captured a mysterious flash coming from deep space. Not only was this the deepest X-ray source ever observed, it also revealed what could be an entirely new phenomenon.

Located in the region of the sky known as the Chandra Deep Field-South (CDF-S), this X-ray emission source appeared to have come from a small galaxy located approximately 10.7 billion light-years from Earth. It also had some remarkable properties, producing more energy in the space of a few minutes that all the stars in the galaxy combined.

*Artist illustration of the Chandra X-ray Observatory, the most sensitive X-ray telescope ever built. Credit: NASA/CXC/NGST*

Originally detected in 2014 by a team of researchers from Penn State University and the Pontifical Catholic University of Chile in Santiago, Chile, this source was not even detected in the X-ray band at first. However, it quickly caught the team’s attention as it erupted and became 1000 brighter in the space of a few hours. At this point, the researchers began gathering data using Chandra’s Advanced CCD Imaging Spectronomer.

A day after the flare-up, the X-ray source had faded to the point that Chandra was no longer able to detect it. As Niel Brandt – the Verne M. Willaman Professor of Astronomy and Astrophysics at Penn State and part of the team that first observed it – described the discovery in a Penn State press release:

“This flaring source was a wonderful surprise bonus that we accidentally discovered in our efforts to explore the poorly understood realm of the ultra-faint X-ray universe. We definitely ‘lucked out’ with this find and now have an exciting new transient phenomenon to explore in future years.”

Thousands of hours of legacy data from the Hubble and Spitzer Space Telescopes was then consulted in order to determine the location of the CDF-S X-ray source. And though scientists were able to determine that the image of the X-ray source placed it beyond any that had been observed before, they are not entirely clear as to what could have caused it.

*X-ray (left) and optical (right) images of the space around the X-ray source, made with Chandra and the Hubble Space Telescope, respectively. Credit: NASA/CXC/F. Bauer et al.*

On the one hand, it could be the result of some sort of destructive event, or something scientists have never before seen. The reason for this has to do with the fact that X-ray bursts also come with a gamma-ray burst (GRB), which appears to be missing here. Essentially, GRBs are jetted explosions that are triggered by the collapse of a massive star or by the merger of two neutron stars (or a neutron star with a black hole).

Because of this, three possible explanations have been suggested. In the first, the CDF-S X-ray source is indeed the result of a collapsing star or merger, but the resulting jets are not pointed towards Earth. In the second, the same scenario is responsible for the x-ray source, but the GRB lies beyond the small galaxy. The third possible explanation is that the event was caused by a medium-sized black hole shredding a white dwarf star.

Unfortunately, none of these explanations seem to fit the data. However, these research team also noted that these possibilities are not that well understood, since none have been witnessed in the Universe. As Franz Bauer – an astronomer from the Pontifical Catholic University of Chile – said: “Ever since discovering this source, we’ve been struggling to understand its origin. It’s like we have a jigsaw puzzle but we don’t have all of the pieces.”

Not only has Chandra not observed any other X-ray sources like this one during the 17 years it has surveyed the CDF-S region, but no similar events have been observed by the space telescope anywhere in the Universe during its nearly two decades of operation. On top of that, this event was brighter, more short-lived, and occurred in a smaller, younger host galaxy than other unexplained X-ray sources.

*Still image of the X-ray source observed by Chandra, showing the captured flare up at bottom Credit: NASA/CXC/Pontifical Catholic Univ./F.Bauer et al.*

From all of this, the only takeaway appears to be that the event was likely the result of a cataclysmic event, like a neutron star or a white dwarf being torn apart. But the fact that none of the more plausible explanations seem to account for it’s peculiar characteristics would seem to suggest that astronomers may have witnessed an entirely new kind of cataclysmic event.

The team’s study – “A New, Faint Population of X-ray Transient“- is available online and will be published in the June 2017 issue of the Monthly Notices of the Royal Astronomical Society. In the meantime, astronomers will be sifting through the data acquired by Chandra and other X-ray observatories – like the ESA’s XMM-Newton and NASA’s Swift Gamma-Ray Burst Mission – to see if they can find any other instances of this kind of event.

And of course, future surveys conducted using Chandra and next-generation X-ray telescopes will also be on the lookout for these kind of short-lived, high-energy X-ray bursts. It’s always good when the Universe throws us a curve ball. Not only does it show us that we have more to learn, but it also teaches us that we must never grow complacent in our theories.

Be sure to check out this animation of the CDF-S X-ray source too, courtesy of the Chandra X-ray Observatory:

Further Reading: Chandra, PennState

March 30, 2017April 16, 2017 by Matt Williams

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

Uranus as seen by NASA's Voyager 2. Credit: NASA/JPL

Uranus is a most unusual planet. Aside from being the seventh planet of our Solar System and the third gas giant, it is also classified sometimes as an “ice giant” (along with Neptune). This is because of its peculiar chemical composition, where water and other volatiles (i.e. ammonia, methane, and other hydrocarbons) in its atmosphere are compressed to the point where they become solid.

In addition to that, it also has a very long orbital period. Basically, it takes Uranus a little over 84 Earth years to complete a single orbit of the Sun. What this means is that a single year on Uranus lasts almost as long as a century here on Earth. On top of that, because of it axial tilt, the planet also experiences extremes of night and day during the course of a year, and some pretty interesting seasonal changes.

Orbital Period:

Uranus orbits the Sun at an average distance (semi-major axis) of 2.875 billion km (1.786 billion mi), ranging from 2.742 billion km (1.7 mi) at perihelion to 3 billion km (1.86 billion mi) at aphelion. Another way to look at it would be to say that it orbits the Sun at an average distance of 19.2184 AU (over 19 times the distance between the Earth and the Sun), and ranges from 18.33 AU to 20.11 AU.

*Images of Uranus taken over a four year period using the Hubble Space Telescope. Credit: NASA/ESA/HST*

The difference between its minimum and maximum distance from the Sun is 269.3 million km (167.335 mi) or 1.8 AU, which is the most pronounced of any of the Solar Planets (with the possible exception of Pluto). And with an average orbital speed of 6.8 km/s (4.225 mi/s), Uranus has an orbital period equivalent to 84.0205 Earth years. This means that a single year on Uranus lasts as long as 30,688.5 Earth days.

However, since it takes 17 hours 14 minutes 24 seconds for Uranus to rotate once on its axis (a sidereal day). And because of its immense distance from the Sun, a single solar day on Uranus is about the same. This means that a single year on Uranus lasts 42,718 Uranian solar days. And like Venus, Uranus’ rotates in the direction opposite of its orbit around the Sun (a phenomena known as retrograde rotation).

Axial Tilt:

Another interesting thing about Uranus is the extreme inclination of its axis (97.7°). Whereas all of the Solar Planets are tilted on their axes to some degree, Uranus’s extreme tilt means that the planet’s axis of rotation is approximately parallel with the plane of the Solar System. The reason for this is unknown, but it has been theorized that during the formation of the Solar System, an Earth-sized protoplanet collided with Uranus and tilted it onto its side.

A consequence of this is that when Uranus is nearing its solstice, one pole faces the Sun continuously while the other faces away – leading to a very unusual day-night cycle across the planet. At the poles, one will experience 42 Earth years of day followed by 42 years of night.

This is similar to what is experienced in the Arctic Circle and Antarctica. During the winter season near the poles, a single night will last for more than 24 hours (aka. a “Polar Night”) while during the summer, a single day will last longer than 24 hours (a “Polar Day”, or “Midnight Sun”).

Meanwhile, near the time of the equinoxes, the Sun faces Uranus’ equator and gives it a period of day-night cycles that are similar to those seen on most of the other planets. Uranus reached its most recent equinox on December 7th, 2007. During the Voyager 2 probe’s historic flyby in 1986, Uranus’s south pole was pointed almost directly at the Sun.

Seasonal Change:

Uranus’ long orbital period and extreme axial tilt also lead to some extreme seasonal variations in terms of its weather. Determining the full extent of these changes is difficult because astronomers have yet to observe Uranus for a full Uranian year. However, data obtained from the mid-20th century onward has showed regular changes in terms of brightness, temperature and microwave radiation between the solstices and equinoxes.

These changes are believed to be related to visibility in the atmosphere, where the sunlit hemisphere is thought to experience a local thickening of methane clouds which produce strong hazes. Increases in cloud formation have also been observed, with very bright cloud features being spotted in 1999, 2004, and 2005. Changes in wind speed have also been noted that appeared to be related to seasonal increases in temperature.

*Close up of Uranus Dark Spot, taken by the Hubble Telescope. Credit: NASA/ESA/HST*

Uranus’ “Great Dark Spot” and its smaller dark spot are also thought to be related to seasonal changes. Much like Jupiter’s Great Red Spot, this feature is a giant cloud vortex that is created by winds – which in this case are estimated to reach speeds of up to 900 km/h (560 mph). In 2006, researchers at the Space Science Institute and the University of Wisconsin observed a storm that measured 1,700 by 3,000 kilometers (1,100 miles by 1,900 miles).

Interestingly enough, while Uranus’ polar regions receive more energy on average over the course of a year than the equatorial regions, the equatorial regions have been found to be hotter than the poles. The exact cause of this remains unknown, but is certainly believed to be due to something endogenic.

Yep, Uranus is a pretty weird place! On this planet, a single year lasts almost a century, and the seasons are characterized by extreme versions of Polar Nights and Midnight Suns. And of course, an average year brings all kinds of seasonal changes, complete with extreme winds, massive storms, and thickening methane clouds.

We have written many articles about the length of a year on other planets here at Universe Today. Here’s How Long is a Year on the Other Planets?, How Long is a Year on Mercury?, How Long is a Year on Venus?, How Long is a Year on Earth?, How Long is a Year on Mars?, How Long is a Year on Jupiter?, How Long is a Year on Saturn?, How Long is a Year on Neptune? and How Long is a Year on Pluto?

If you’d like more info on Uranus, check out Hubblesite’s News Releases about Uranus. And here’s a link to the NASA’s Solar System Exploration Guide to Uranus.

We have recorded an episode of Astronomy Cast just about Uranus. You can access it here: Episode 62: Uranus.

Sources:

March 29, 2017 by Bob King

Finite Light — Why We Always Look Back In Time

Beads of rainwater on a poplar leaf act like lenses, focusing light and enlarging the leaf’s network of veins. Moving at 186,000 miles per second, light from the leaf arrives at your eye 0.5 nanosecond later. A blink of an eye takes 600,000 times as much time! Credit: Bob King

My attention was focused on beaded water on a poplar leaf. How gemmy and bursting with the morning’s sunlight. I moved closer, removed my glasses and noticed that each drop magnified a little patch of veins that thread and support the leaf.

Focusing the camera lens, I wondered how long it took the drops’ light to reach my eye. Since I was only about six inches away and light travels at 186,000 miles per second or 11.8 inches every billionth of a second (one nanosecond), the travel time amounted to 0.5 nanoseconds. Darn close to simultaneous by human standards but practically forever for positronium hydride, an exotic molecule made of a positron, electron and hydrogen atom. The average lifetime of a PsH molecule is just 0.5 nanoseconds.

Light takes about 35 microseconds to arrive from a transcontinental jet and its contrail. Credit: Bob King

In our everyday life, the light from familiar faces, roadside signs and the waiter whose attention you’re trying to get reaches our eyes in nanoseconds. But if you happen to look up to see the tiny dark shape of a high-flying airplane trailed by the plume of its contrail, the light takes about 35,000 nanoseconds or 35 microseconds to travel the distance. Still not much to piddle about.

The space station orbits the Earth in outer space some 250 miles overhead. During an overhead pass, light from the orbiting science lab fires up your retinas 1.3 milliseconds later. In comparison, a blink of the eye lasts about 300 milliseconds (1/3 of a second) or 230 times longer!

The Lunar Laser Ranging Experiment placed on the Moon by the Apollo 14 astronauts. Observatories beam a laser to the small array, which reflects a bit of the light back. Measuring the time delay yields the Moon’s distance to within about a millimeter. At the Moon’s surface the laser beam spreads out to 4 miles wide and only one photon is reflected back to the telescope every few seconds. Credit: NASA

Light time finally becomes more tangible when we look at the Moon, a wistful 1.3 light seconds away at its average distance of 240,000 miles. To feel how long this is, stare at the Moon at the next opportunity and count out loud: one one thousand one. Retroreflecting devices placed on the lunar surface by the Apollo astronauts are still used by astronomers to determine the moon’s precise distance. They beam a laser at the mirrors and time the round trip.

Venus as a super-thin crescent only 10 hours before conjunction on March 25. The planet was just 2.3 light minutes from the Earth at the time. Credit: Shahrin Ahmad

Of the eight planets, Venus comes closest to Earth, and it does so during inferior conjunction, which coincidentally occurred on March 25. On that date only 26.1 million miles separated the two planets, a distance amounting to 140 seconds or 2.3 minutes — about the time it takes to boil water for tea. Mars, another close-approaching planet, currently stands on nearly the opposite side of the Sun from Earth.

With a current distance of 205 million miles, a radio or TV signal, which are both forms of light, broadcast to the Red Planet would take 18.4 minutes to arrive. Now we can see why engineers pre-program a landing sequence into a Mars’ probe’s computer to safely land it on the planet’s surface. Any command – or change in commands – we might send from Earth would arrive too late. Once a lander settles on the planet and sends back telemetry to communicate its condition, mission control personnel must bite their fingernails for many minutes waiting for light to limp back and bring word.

Before we speed off to more distant planets, let’s consider what would happen if the Sun had a catastrophic malfunction and suddenly ceased to shine. No worries. At least not for 8.3 minutes, the time it takes for light, or the lack of it, to bring the bad news.

Pluto and Charon lie 3.1 billion miles from Earth, a long way for light to travel. We see them as they were more than 4 hours ago. NASA/JHUAPL/SwRI

Light from Jupiter takes 37 minutes to reach Earth; Pluto and Charon are so remote that a signal from the “double planet” requires 4.6 hours to get here. That’s more than a half-day of work on the job, and we’ve only made it to the Kuiper Belt.

Let’s press on to the nearest star(s), the Alpha Centauri system. If 4.6 hours of light time seemed a long time to wait, how about 4.3 years? If you think hard, you might remember what you were up to just before New Year’s Eve in 2012. About that time, the light arriving tonight from Alpha Centauri left that star and began its earthward journey. To look at the star then is to peer back in time to late 2012.

The Summer Triangle rises fully in the eastern sky around 3 o’clock in the morning in late March. Created with Stellarium

But we barely scrape the surface. Let’s take the Summer Triangle, a figure that will soon come to dominate the eastern sky along with the beautiful summer Milky Way that appears to flow through it. Altair, the southernmost apex of the triangle is nearby, just 16.7 light years from Earth; Vega, the brightest a bit further at 25 and Deneb an incredible 3,200 light years away.

We can relate to the first two stars because the light we see on a given evening isn’t that “old.” Most of us can conjure up an image of our lives and the state of world affairs 16 and 25 years ago. But Deneb is exceptional. Photons departed this distant supergiant (3,200 light years) around the year 1200 B.C. during the Trojan War at the dawn of the Iron Age. That’s some look-back time!

Rho Cassiopeia, currently at magnitude +4.5, is one of the most distant stars visible with the naked eye. Its light requires about 8,200 years to reach our eyes. This star, a variable, is enormous with a radius about 450 times that of the Sun. Credit: IAU/Sky and Telescope (left); Anynobody, CC BY-SA 3.0 / Wikipedia

One of the most distant naked eye stars is Rho Cassiopeiae, yellow variable some 450 times the size of the Sun located 8,200 light years away in the constellation Cassiopeia. Right now, the star is near maximum and easy to see at nightfall in the northwestern sky. Its light whisks us back to the end of the last great ice age at a time and the first cave drawings, more than 4,000 years before the first Egyptian pyramid would be built.

This is the digital message (annotated here) sent by Frank Drake to M13 in 1974 using the Arecibo radio telescope.

On and on it goes: the nearest large galaxy, Andromeda, lies 2.5 million light years from us and for many is the faintest, most distant object visible with the naked eye. To think that looking at the galaxy takes us back to the time our distant ancestors first used simple tools. Light may be the fastest thing in the universe, but these travel times hint at the true enormity of space.

Let’s go a little further. On November 16, 1974 a digital message was beamed from the Arecibo radio telescope in Puerto Rico to the rich star cluster M13 in Hercules 25,000 light years away. The message was created by Dr. Frank Drake, then professor of astronomy at Cornell, and contained basic information about humanity, including our numbering system, our location in the solar system and the composition of DNA, the molecule of life. It consisted of 1,679 binary bits representing ones and zeroes and was our first deliberate communication sent to extraterrestrials. Today the missive is 42 light years away, just barely out the door.

Galaxy GN-z11, shown in the inset, is seen as it was 13.4 billion years in the past, just 400 million years after the big bang, when the universe was only three percent of its current age. The galaxy is ablaze with bright, young, blue stars, but looks red in this image because its light has been stretched to longer spectral wavelengths by the expansion of the universe. Credit: NASA, ESA, P. Oesch, G. Brammer, P. van Dokkum, and G. Illingworth

Let’s end our time machine travels with the most distant object we’ve seen in the universe, a galaxy named GN-z11 in Ursa Major. We see it as it was just 400 million years after the Big Bang (13.4 billion years ago) which translates to a proper distance from Earth of 32 billion light years. The light astronomers captured on their digital sensors left the object before there was an Earth, a Solar System or even a Milky Way galaxy!

Thanks to light’s finite speed we can’t help but always see things as they were. You might wonder if there’s any way to see something right now without waiting for the light to get here? There’s just one way, and that’s to be light itself.

From the perspective of a photon or light particle, which travels at the speed of light, distance and time completely fall away. Everything happens instantaneously and travel time to anywhere, everywhere is zero seconds. In essence, the whole universe becomes a point. Crazy and paradoxical as this sounds, the theory of relativity allows it because an object traveling at the speed of light experiences infinite time dilation and infinite space contraction.

Just something to think about the next time you meet another’s eyes in conversation. Or look up at the stars.

March 29, 2017March 29, 2017 by David Dickinson

Watch Rotating Horns of Venus at Dawn

Venus inferior conjunction — Venus, just 10.5 hours before inferior conjunction on March 25th. Image credit and copyright: Shahrin Ahmad (@Shahgazer)

Have you seen it yet? An old friend greeted us on an early morning run yesterday as we could easily spy brilliant Venus in the dawn, just three days after inferior conjunction this past Saturday on March 25^th.

This was an especially wide pass, as the planet crossed just over eight degrees (that’s 16 Full Moon diameters!) north of the Sun. We once managed to see Venus with the unaided eye on the very day of inferior conjunction back in 1998 from the high northern latitudes of the Chena Flood Channel just outside of Fairbanks, Alaska.

The planet was a slender 59.4” wide, 1% illuminated crescent during this past weekend’s passage, and the wide pass spurred many advanced imagers to hunt for the slim crescent in the daytime sky. Of course, such a feat is challenging near the dazzling daytime Sun. Safely blocking the Sun out of view and being able to precisely point your equipment is key in this endeavor. A deep blue, high contrast sky helps, as well. Still, many Universe Today readers rose to the challenge of chronicling the horns of the slender crescent Venus as they rotated ’round the limb and the nearby world moved once again from being a dusk to dawn object.

Venus rotating horns — A daily sequence showing the ‘Horns of Venus’ rotate as it approaches inferior conjunction. Image credit and copyright: Shahrin Ahmad (@ShahGazer)

The orbit of Venus is tilted 3.4 degrees with respect to the Earth, otherwise, we’d get a transit of the planet like we did on June 5-6^th, 2012 once about every 584 days, instead of having to wait again until next century on December 10^th, 2117.

The joint NASA/European Space Agency’s SOlar Heliospheric Observatory (SOHO) mission also spied the planet this past weekend as it just grazed the 15 degree wide field of view of its Sun-observing LASCO C3 camera:

Venus SOHO — The glow of Venus (arrowed) just barely bleeding over into the field of view of SOHO’s LASCO C3 camera. Credit: SOHO/NASA/LASCO

Venus kicks off April as a 58” wide, 3% illuminated crescent and ends the month at 37” wide, fattening up to 28% illumination. On closest approach, the planet presents the largest apparent planetary disk possible as seen from the Earth. Can you see the horns? They’re readily readily apparent even in a low power pair of hunting binoculars. The coming week is a great time to try and see a crescent Venus… with the naked eye. Such an observation is notoriously difficult, and right on the edge of possibility for those with keen eyesight.

One problem for seasoned observers is that we know beforehand that (spoiler alert) that the Horns of Venus, like the Moon, always point away from the direction of the Sun.

True Story: a five year old girl at a public star party once asked me “why does that ‘star’ look like a tiny Moon” (!) This was prior to looking at the planet through a telescope. Children generally have sharper eyes than adults, as the lenses of our corneas wear down and yellow from ultraviolet light exposure over the years.

Still, there are tantalizing historical records that suggest that ancient cultures such as the Babylonians knew something of the true crescent nature of Venus in pre-telescopic times as well.

The Babylonian frieze of Kudurru Melishipak on display at the Louvre, depicting the Sun Moon and Venus. According to some interpretations, the goddess Ishtar (Venus) is also associated with a crescent symbol… possibly lending credence to the assertion that ancient Babylonian astronomers knew something of the phases of the planet from direct observation. Credit: Wikimedia Commons/Image in the Public Domain.

Another fun challenge in the coming months is attempting to see Venus in the daytime. This is surprisingly easy, once you know exactly where to look for it. A nearby crescent Moon is handy, as occurs on April 23^rd, May 22^nd, and June 20^th.

Daytime Venus — Venus (arrowed) near the daytime Moon. Photo by author.

Strangely enough, the Moon is actually darker than dazzling Venus in terms of surface albedo. The ghostly daytime Moon is just larger and easier to spot. Many historical ‘UFO’ sightings such as a ‘dazzling light seen near the daytime Moon’ by the startled residents of Saint-Denis, France on the morning on January 13^th, 1589 were, in fact, said brilliant planet.

The Moon near Venus on May 22nd. Credit: Stellarium.

Venus can appear startlingly bright to even a seasoned observer. We’ve seen the planet rise as a shimmering ember against a deep dark twilight sky from high northern latitudes. Air traffic controllers have tried in vain to ‘hail’ Venus on more than one occasion, and India once nearly traded shots with China along its northern border in 2012, mistaking a bright conjunction of Jupiter and Venus for spy drones.

The third brightest object in the sky behind the Sun and the Moon, Venus is even bright enough to cast a shadow as seen from a dark sky site, something that can be more readily recorded photographically.

Watch our nearest planetary neighbor long enough, and it will nearly repeat the same pattern for a given apparition. This is known as the eight year cycle of Venus, and stems from the fact that 13 Venusian orbits (8x 224.8 days) very nearly equals eight Earth years.

Follow Venus through the dawn in 2017, and it will eventually form a right triangle with the Earth and the Sun on June 3^rd, reaching what is known as greatest elongation. This can vary from 47.2 to 45.4 degrees from the Sun, and this year reaches 45.9 degrees elongation in June. The planet then reaches half phase known as dichotomy around this date, though observed versus theoretical dichotomy can vary by three days. The cause of this phenomenon is thought to be the refraction of light in Venus’ dense atmosphere, coupled with observer bias due to the brilliance of Venus itself. When do you see it?

Also, keep an eye out for the ghostly glow on the night-side of Venus, known as Ashen Light. Long thought to be another trick of the eye, there’s good evidence to suggest that this long reported effect actually has a physical basis, though Venus has no large reflecting moon nearby… how could this be? The leading candidate is now thought to be air-glow radiating from the cooling nighttime side of the planet.

Cloud enshrouded Venus held on to its secrets, right up until the Space Age less than a century ago… some observers theorized that the nighttime glow on Venus was due to aurorae, volcanoes or even light pollution from Venusian cities (!). This also fueled spurious sightings of the alleged Venusian moon Neith right up through the 19^th century.

Venus should also put in a showing 34 degrees west of the Sun shining at magnitude -4 during the August 21^st, 2017 total solar eclipse. Follow that planet, as it makes a complex meet up with Mars, Mercury, and the Moon in late September of this year.

More to come!

-Read about planets, occultations, comets and more for the year in our 101 Astronomical Events for 2017, out as a free e-book from Universe Today.

March 29, 2017September 12, 2017 by Matt Williams

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

New Horizon's July 2015 flyby of Pluto captured this iconic image of the heart-shaped region called Tombaugh Regio. Credit: NASA/JHUAPL/SwRI.

Discovered in 1930 by Clyde Tombaugh, Pluto was once thought to be the ninth and outermost planet of the Solar System. However, due to the formal definition adopted in 2006 at the 26th General Assembly of the International Astronomical Union (IAU), Pluto ceased being the ninth planet of the Solar System and has become alternately known as a “Dwarf Planet”, “Plutiod”, Trans-Neptunian Object (TNO) and Kuiper Belt Object (KBO).

Despite this change of designation, Pluto remains one of the most fascinating celestial bodies known to astronomers. In addition to having a very distant orbit around the Sun (and hence a very long orbital period) it also has the most eccentric orbit of any planet or minor planet in the Solar System. This makes for a rather long year on Pluto, which lasts the equivalent of 248 Earth years!

Orbital Period:

With an extreme eccentricity of 0.2488, Pluto’s distance from the Sun ranges from 4,436,820,000 km (2,756,912,133 mi) at perihelion to 7,375,930,000 km (4,583,190,418 mi) at aphelion. Meanwhile, it’s average distance (semi-major axis) from the Sun is 5,906,380,000 km (3,670,054,382 mi). Another way to look at it would be to say that it orbits the Sun at an average distance of 39.48 AU, ranging from 29.658 to 49.305 AU.

New Horizons trajectory and the orbits of Pluto and 2014 MU69.

At its closest, Pluto actually crosses Neptune’s orbit and gets closer to the Sun. This orbital pattern takes place once every 500 years, after which the two objects then return to their initial positions and the cycle repeats. Their orbits also place them in a 2:3 mean-motion resonance, which means that for every two orbits Pluto makes around the Sun, Neptune makes three.

The 2:3 resonance between the two bodies is highly stable, and is preserved over millions of years. The last time this cycle took place was between 1979 to 1999, when Neptune was farther from the Sun than Pluto. Pluto reached perihelion in this cycle – i.e. its closest point to the Sun – on September 5th, 1989. Since 1999, Pluto returned to a position beyond that of Neptune, where it will remain for the following 228 years – i.e. until the year 2227.

Sidereal and Solar Day:

Much like the other bodies in our Solar System, Pluto also rotates on its axis. The time it takes for it to complete a single rotation on its axis is known as a “Sidereal Day”, while the amount of time it takes for the Sun to reach the same point in the sky is known as a “Solar Day”. But due to Pluto’s very long orbital period, a sidereal day and a solar day on Pluto are about the same – 6.4 Earth days (or 6 days, 9 hours, and 36 minutes).

*View from the surface of Pluto, showing its large moon Charon in the distance. Credit: New York Time*

It is also worth noting that Pluto and Charon (its largest moon) are actually more akin to a binary system rather than a planet-moon system. This means that the two worlds orbit each other, and that Charon is tidally locked around Pluto. In other words, Charon takes 6 days and 9 hours to orbit around Pluto – the same amount of time it takes for a day on Pluto. This also means that Charon is always in the same place in the sky when seen from Pluto.

In short, a single day on Pluto lasts the equivalent of about six and a half Earth days. A year on Pluto, meanwhile, lasts the equivalent of 248 Earth years, or 90,560 Earth days! And for the entire year, the moon is hanging overhead and looming large in the sky. But factor in Pluto’s axial tilt, and you will come to see just how odd an average year on Pluto is.

Seasonal Change:

It has been estimated that for someone standing on the surface of Pluto, the Sun would appear about 1,000 times dimmer than it appears from Earth. So while the Sun would still be the brightest object in the sky, it would look more like a very bright star that a big yellow disk. But despite being very far from the Sun at any given time, Pluto’s eccentric orbit still results in some considerable seasonal variations.

On the whole, the surface temperature of Pluto does not change much. It’s surface temperatures are estimated to range from a low of 33 K (-240 °C; -400 °F ) to a high of 55 K (-218 °C; -360°F) – averaging at around 44 K (-229 °C; -380 °F). However, the amount of sunlight each side receives during the course of a year is vastly different.

Compared to most of the planets and their moons, the Pluto-Charon system is oriented perpendicular to its orbit. Much like Uranus, Pluto’s very high axial tilt (122 degrees) essentially means that it is orbiting on its side relative to its orbital plane. This means that at a solstice, one-quarter of Pluto’s surface experiences continuous daylight while the other experiences continuous darkness.

This is similar to what happens in the Arctic Circle, where the summer solstice is characterized by perpetual sunlight (i.e. the “Midnight Sun”) and the winter solstice by perpetual night (“Arctic Darkness”). But on Pluto, these phenomena affect nearly the entire planet, and the seasons last for close to a century.

Even if it is no longer considered a planet (though this could still change) Pluto still has some very fascinating quarks, all of which are just as worthy of study as those of the other eight planets. And the time it takes to complete a full year on Pluto, and all the seasonal changes it goes through, certainly rank among the top ten!

We have written many interesting articles about a year on other planets here at Universe Today. Here’s How Long is a Year on the Other Planets?, Which Planet has the Longest Day?, How Long is a Year on Mercury?, How Long is a Year on Venus?, How Long is a Year on Earth?, How Long is a Year on Mars?, How Long is a Year on Jupiter?, How Long is a Year on Saturn?, How Long is a Year on Uranus?, and How Long is a Year on Neptune?.

For more information, be sure to check out NASA’s Solar System Exploration page on Pluto, and the New Horizon’s mission page for information on Pluto’s seasons.

Astronomy Cast also has some great episodes on the subject. Here’s Episode 1: Pluto’s Planetary Identity Crisis and Episode 64: Pluto and the Icy Outer Solar System.

Sources:

March 28, 2017March 28, 2017 by Matt Williams

What is the Color of Pluto?

Pluto was re-classified as a dwarf planet based on our growing understanding of its nature. Will Schlaufman's new study help us more accurately classify gas giants and brown dwarfs? NASA's New Horizons spacecraft captured this high-resolution enhanced color view of Pluto on July 14, 2015. Credit: NASA/JHUAPL/SwRI

When Pluto was first discovered by Clybe Tombaugh in 1930, astronomers believed that they had found the ninth and outermost planet of the Solar System. In the decades that followed, what little we were able to learn about this distant world was the product of surveys conducted using Earth-based telescopes. Throughout this period, astronomers believed that Pluto was a dirty brown color.

In recent years, thanks to improved observations and the New Horizons mission, we have finally managed to obtain a clear picture of what Pluto looks like. In addition to information about its surface features, composition and tenuous atmosphere, much has been learned about Pluto’s appearance. Because of this, we now know that the one-time “ninth planet” of the Solar System is rich and varied in color.

Composition:

With a mean density of 1.87 g/cm³, Pluto’s composition is differentiated between an icy mantle and a rocky core. The surface is composed of more than 98% nitrogen ice, with traces of methane and carbon monoxide. Scientists also suspect that Pluto’s internal structure is differentiated, with the rocky material having settled into a dense core surrounded by a mantle of water ice.

*The Theoretical structure of Pluto, consisting of 1. Frozen nitrogen 2. Water ice 3. Rock. Credit: NASA/Pat Rawlings*

The diameter of the core is believed to be approximately 1700 km, which accounts for 70% of Pluto’s total diameter. Thanks to the decay of radioactive elements, it is possible that Pluto contains a subsurface ocean layer that is 100 to 180 km thick at the core–mantle boundary.

Pluto has a thin atmosphere consisting of nitrogen (N₂), methane (CH₄), and carbon monoxide (CO), which are in equilibrium with their ices on Pluto’s surface. However, the planet is so cold that during part of its orbit, the atmosphere congeals and falls to the surface. The average surface temperature is 44 K (-229 °C), ranging from 33 K (-240 °C) at aphelion to 55 K (-218 °C) at perihelion.

Appearance:

Pluto’s surface is very varied, with large differences in both brightness and color. Pluto’s surface also shows signs of heavy cratering, with ones on the dayside measuring 260 km (162 mi) in diameter. Tectonic features including scarps and troughs has also been seen in some areas, some as long as 600 km (370 miles).

Mountains have also been seen that are between 2 to 3 kilometers (6500 – 9800 ft) in elevation above their surroundings. Like much of the surface, these features are believed to be composed primarily of frozen nitrogen, carbon monoxide, and methane, which are believed to sit atop a “bedrock” of frozen water ice.

*Color mosaic map of Pluto’s surface, created from the New Horizons many photographs. Credit: NASA/JHUAPL/SwRI*

The surface also has many dark, reddish patches due to the presence of tholins, which are created by charged particles from the Sun interacting with mixtures of methane and nitrogen. Pluto’s visual apparent magnitude averages 15.1, brightening to 13.65 at perihelion. In other words, the planet has a range of colors, including pale sections of off-white and light blue, to streaks of yellow and subtle orange, to large patches of deep red.

Overall, its appearance could be described as “ruddy”, given that the combination can lend it a somewhat brown and earthy appearance from a distance. In fact, prior to the New Horizon‘s mission, which provided the first high-resolution, close-up images of the planet, this is precisely what astronomers believed Pluto looked like.

Major Surface Features:

Several different regions (“regio”) have been characterized based on the notable features they possess. Perhaps the best known is the large, pale area nicknamed the “Heart” – aka. Tombaugh Regio (named after Pluto’s founder). This large bright area is located on the side of Pluto that lies opposite the side that faces Charon, and is named because of its distinctive shape.

Tombaugh Regio is about 1,590 km (990 mi) across and contains 3,400 m (11,000 ft) mountains made of water ice along its southwestern edge. The lack of craters suggests that its surface is relatively young (about 100 million years old) and hints at Pluto being geologically active. The Heart can be subdivided into two lobes, which are distinct geological features that are both bright in appearance.

This new global mosaic view of Pluto was created from the latest high-resolution images to be downlinked from NASA’s New Horizons spacecraft and released on Sept. 11, 2015. Credits: NASA/Johns Hopkins APL/SwRI/Marco Di Lorenzo/Ken Kremer

The western lobe, Sputnik Planitia, is vast plain of nitrogen and carbon monoxide ices measuring 1000 km in width. It is divided into polygonal sections that are believed to be convection cells, which carry blocks of water ice and sublimation pits along towards the edge of the plain. This region is especially young (less than 10 million years old), which is indicated by its lack of cratering.

Then there is the large, dark area on the trailing hemisphere known as Cthulhu Regio (aka.the “Whale”). Named for its distinctive shape, this elongated, dark region along the equator is the largest dark feature on Pluto – measuring 2,990 km (1,860 mi) in length. The dark color is believed to be the result methane and nitrogen in the atmosphere interacting with ultraviolet light and cosmic rays, creating the dark particles (“tholins”) common to Pluto.

And then there are the “Brass Knuckles”, a series of equatorial dark areas on the leading hemisphere. These features average around 480 km (300 mi) in diameter, and are located along the equator between the Heart and the tail of the Whale.

New Horizons Mission:

The NH mission launched from Cape Canaveral Air Force Station in Florida on January 19th, 2006. After swinging by Jupiter for a gravity boost and to conduct some scientific studies in February of 2007, it reached Pluto in the summer of 2015. Once there, it conducted a six month-long reconnaissance flyby of Pluto and its system of moons, culminating with a closest approach that occurred on July 14th, 2015.

A portrait from the final approach of the New Horizons spacecraft to the Pluto system on July 11th, 2015. Pluto and Charon display striking color and brightness contrast in this composite image. Credit: NASA-JHUAPL-SWRI.

The first images of Pluto acquired by NH were taken on September 21st to 24th, 2006, during a test of the Long Range Reconnaissance Imager (LORRI). At the time, the probe was still at a distance of approximately 4.2 billion km (2.6 billion mi) or 28 AU, and the photos were released on November 28th, 2006. Between July 1st and 3rd, the first images were taken that were able to resolved Pluto and its largest moon, Charon, as separate objects.

Between July 19th–24th, 2014, the probe snapped 12 images of Charon revolving around Pluto, covering almost one full rotation at distances ranging from 429 to 422 million kilometers (267,000,000 to 262,000,000 mi). After a brief hibernation during its final approach, New Horizons “woke up” on Dec. 7th, 2014. Distant-encounter operations began on January 4th, 2015, and NH began taking images of Pluto as it grew closer.

During its closest approach (July 14th, 2015, at at 11:50 UTC), the NH probe passed within 12,500 km (7,800 mi) of Pluto. About 3 days before making its closest approach, long-range imaging of Pluto and Charon took place that were 40 km (25 mi) in resolution, which allowed for all sides of both bodies to be mapped out.

Close-range imaging also took place twice a day during this time to search for any indication of surface changes. The NH probe also analyzed Pluto’s atmosphere using its suite of scientific instruments. This included it’s ultraviolet imaging spectrometer (aka. Alice) and the Radio Science EXperiment (REX), which analyzed the composition and structure of Pluto’s atmosphere.

Haze with multiple layers in the atmosphere of Pluto. Part of the plain Sputnik Planitia with nearby mountains is seen below. Photo by New Horizons, taken 15 min after the closest approach to Pluto. Credit: NASA/JHUAPL/SwRI

It’s Solar Wind Around Pluto (SWAP) and Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) examined the interaction of Pluto’s high atmosphere with solar wind. Pluto’s diameter was also resolved by measuring the disappearance and reappearance of the radio occultation signal as the probe flew by behind Pluto. And the gravitational tug on the probe were used to determine Pluto’s mass and mass distribution.

All of this information has helped astronomers to make the first detailed maps of Pluto, and led to numerous discoveries about Pluto’s structure, composition, and the kinds of forces that actively shape its surface. The mission also led to the first true images of what Pluto looks like up close, revealing its true colors, it’s famous “Heart” region, and the many other now-famous features.

We have written many interesting articles about the colors of astronomical bodies here at Universe Today. Here’s What Color is the Sun?, What are the Colors of the Planets?, What Color is Mercury?, What Color is Venus?, What Color is the Moon?, Why is Mars Red?, What Color is Jupiter?, What Color is Saturn?, What Color is Uranus?, and What Color is Neptune?

Sources:

March 28, 2017March 28, 2017 by Matt Williams

What is an Astronomical Unit?

Apsis — The Earth revolves around the Sun like this.

When it comes to dealing with the cosmos, we humans like to couch things in familiar terms. When examining exoplanets, we classify them based on their similarities to the planets in our own Solar System – i.e. terrestrial, gas giant, Earth-size, Jupiter-sized, Neptune-sized, etc. And when measuring astronomical distances, we do much the same.

For instance, one of the most commonly used means of measuring distances across space is known as an Astronomical Unit (AU). Based on the distance between the Earth and the Sun, this unit allows astronomers to characterize the vast distances between the Solar planets and the Sun, and between extra-solar planets and their stars.

Definition:

According to the current astronomical convention, a single Astronomical Unit is equivalent to 149,597,870.7 kilometers (or 92,955,807 miles). However, this is the average distance between the Earth and the Sun, as that distance is subject to variation during Earth’s orbital period. In other words, the distance between the Earth and the Sun varies in the course of a single year.

Earth’s orbit around the Sun, showing its average distance (or 1 AU). Credit: Huritisho/Wikipedia Commons

During the course of a year, the Earth goes from distance of 147,095,000 km (91,401,000 mi) from the Sun at perihelion (its closest point) to 152,100,000 km (94,500,000 mi) at aphelion (its farthest point) – or from a distance of 0.983 AUs to 1.016 AUs.

History of Development:

The earliest recorded example of astronomers estimating the distance between the Earth and the Sun dates back to Classical Antiquity. In the 3rd century BCE work, On the Sizes and Distances of the Sun and Moon – which is attributed to Greek mathematician Aristarchus of Samos – the distance was estimated to be between 18 and 20 times the distance between the Earth and the Moon.

However, his contemporary Archimedes, in his 3rd century BCE work Sandreckoner, also claimed that Aristarchus of Samos placed the distance of 10,000 times the Earth’s radius. Depending on the values for either set of estimates, Aristarchus was off by a factor of about 2 (in the case of Earth’s radius) to 20 (the distance between the Earth and the Moon).

The oldest Chinese mathematical text – the 1st century BCE treatise known as Zhoubi Suanjing – also contains an estimate of the distance between the Earth and Sun. According to the anonymous treatise, the distance could be calculated by conducting geometric measurements of the length of noontime shadows created by objects spaced at specific distances. However, the calculations were based on the idea that the Earth was flat.

*Illustration of the Ptolemaic geocentric conception of the Universe, by Bartolomeu Velho (?-1568), from his work Cosmographia, made in France, 1568. Credit: Bibilotèque nationale de France, Paris*

Famed 2nd century CE mathematician and astronomer Ptolemy relied on trigonometric calculations to come up with a distance estimate that was equivalent to 1210 times the radius of the Earth. Using records of lunar eclipses, he estimated the Moon’s apparent diameter, as well as the apparent diameter of the shadow cone of Earth traversed by the Moon during a lunar eclipse.

Using the Moon’s parallax, he also calculated the apparent sizes of the Sun and the Moon and concluded that the diameter of the Sun was equal to the diameter of the Moon when the latter was at it’s greatest distance from Earth. From this, Ptolemy arrived at a ratio of solar to lunar distance of approximately 19 to 1, the same figure derived by Aristarchus.

For the next thousand years, Ptolemy’s estimates of the Earth-Sun distance (much like most of his astronomical teachings) would remain canon among Medieval European and Islamic astronomers. It was not until the 17th century that astronomers began to reconsider and revise his calculations.

This was made possible thanks to the invention of the telescope, as well as Kepler’s Three Laws of Planetary Motion, which helped astronomers calculate the relative distances between the planets and the Sun with greater accuracy. By measuring the distance between Earth and the other Solar planets, astronomers were able to conduct parallax measurements to obtain more accurate values.

*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.*

By the 19th century, determinations of about the speed of light and the constant of the aberration of light resulted in the first direct measurement of the Earth-Sun distance in kilometers. By 1903, the term “astronomical unit” came to be used for the first time. And throughout the 20th century, measurements became increasingly precise and sophisticated, thanks in part to accurate observations of the effects of Einstein’s Theory of Relativity.

Modern Usage:

By the 1960s, the development of direct radar measurements, telemetry, and the exploration of the Solar System with space probes led to precise measurements of the positions of the inner planets and other objects. In 1976, the International Astronomical Union (IAU) adopted a new definition during their 16th General Assembly. As part of their System of Astronomical Constants, the new definition stated:

“The astronomical unit of length is that length (A) for which the Gaussian gravitational constant (k) takes the value 0.01720209895 when the units of measurement are the astronomical units of length, mass and time. The dimensions of k² are those of the constant of gravitation (G), i.e., L³M^-1T^–². The term “unit distance” is also used for the length A.”

In response to the development of hyper-precise measurements, the International Committee for Weights and Measures (CIPM) decided to modify the the International System of Units (SI) in 1983. Consistent with this, they redefined the meter to be measured in terms of the speed of light in vacuum.

*Infographic comparing the orbit of the planet around Proxima Centauri (Proxima b) with the same region of the Solar System. Credit: ESO*

However, by 2012, the IAU determined that the equalization of relativity made the measurement of AUs too complex, and redefined the astronomical unit in terms of meters. In accordance with this, a single AU is equal to 149597870.7 km exactly (92.955807 million miles), 499 light-seconds, 4.8481368×10^-6 of a parsec, or 15.812507×10^-6 of a light-year.

Today, the AU is used commonly to measure distances and create numerical models for the Solar System. It is also used when measuring extra-solar systems, calculating the extent of protoplanetary clouds or the distance between extra-solar planets and their parent star. When measuring interstellar distances, AUs are too small to offer convenient measurements. As such, other units – such as the parsec and the light year – are relied upon.

The Universe is a huge place, and measuring even our small corner of it producing some staggering results. But as always, we prefer to express them in ways that are as relatable and familiar.

We’ve written many interesting articles about distances in the Solar System here at Universe Today. Here’s How Far are the Planets from the Sun?, How Far is Mercury from the Sun?, How Far is Venus from the Sun?, How Far is Earth from the Sun?, How Far is Mars from the Sun?, How Far is Jupiter from the Sun?, How Far is Saturn from the Sun?, How Far is Uranus from the Sun?, How Far is Neptune from the Sun?, How Far is Pluto from the Sun?

If you’d like more information about the Earth’s orbit, check out NASA’s Solar System Exploration page.

We’ve also recorded an episode of Astronomy Cast dedicated to the measurement of distances in astronomy. Listen here, Episode 10: Measuring Distance in the Universe.

Sources:

March 27, 2017March 27, 2017 by Tammy Plotner

Messier 38 – The Starfish Cluster

The open star cluster Messier 38, in proximity to Messier 36 and Messier 37. Credit: Wikisky

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the Starfish Cluster, otherwise known as Messier 38. 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 it the Starfish Cluster, also known as Messier 38 (or M38). This open star cluster is located in the direction of the northern Auriga constellation, along with the open star clusters M36 and M37. While not the brightest of the three, the location of the Starfish within the polygon formed by the brightest stars of Auriga makes it very easy to find.

Description:

Cruising around our Milky Way some 4200 light years from our solar system, this 220 million year old group of stars spreads itself across about 25 light years of space. If you’re using a telescope, you may have noticed its not alone… Messier 38 might very well be a binary star cluster! As Anil K. Pandey (et al) explained in a 2006 study:

“We present CCD photometry in a wide field around two open clusters, NGC 1912 and NGC 1907. The stellar surface density profiles indicate that the radii of the clusters NGC 1912 and NGC 1907 are 14′ and 6′ respectively. The core of the cluster NGC 1907 is found to be 1′.6±0′.3, whereas the core of the cluster NGC 1912 could not be defined due to its significant variation with the limiting magnitude. The clusters are situated at distances of 1400±100 pc (NGC 1912) and 1760±100 pc (NGC 1907), indicating that in spite of their close locations on the sky they may be formed in different parts of the Galaxy.”

*The Starfish Cluster also known as Messier 38. Credit: Wikisky*

So what’s happening here? Chances are, when you’re looking at M38, you’re looking at a star cluster that’s currently undergoing a real close encounter! Said M.R. de Oliveira (et al) said in their 2002 study:

“The possible physical relation between the closely projected open clusters NGC 1912 (M 38) and NGC 1907 is investigated. Previous studies suggested a physical pair based on similar distances, and the present study explores in more detail the possible interaction. Spatial velocities are derived from available radial velocities and proper motions, and the past orbital motions of the clusters are retrieved in a Galactic potential model. Detailed N-body simulations of their approach suggest that the clusters were born in different regions of the Galaxy and presently experience a fly-by.”

However, it was Sang Hyun Lee and See-Woo Lee who gave us the estimates of M38’s distance and age. As they wrote in their 1996 study, “UBV CCD Photometry of Open Cluster NGC 1907 and NGC 1912“: “The distance difference of the two clusters is 300pc and the age difference is 150 Myr. These results imply that the two clusters are not physically connected.”

So, how do we know they are two clusters passing in the night? The credit for that goes to de Oliveira and colleagues, who also asserted in their 2002 study:

“These simulations also shows that the faster the clusters approach the weaker the tidal debris in the bridge region, which explain why there is, apparently, no evidence of a material link between the clusters and why it should not be expected. It would be necessary to analyse deep wide field CCD photometry for a more conclusive result about the apparent absence of tidal link between the clusters.”

*Atlas image mosaic of the Starfish Cluster (Messier 38), obtained as part of the Two Micron All Sky Survey (2MASS). Credit: NASA/NSF/Caltech/UofMass/IPAC*

History of Observation:

This wonderful star cluster was originally discovered by Giovanni Batista Hodierna before 1654 and independently rediscovered by Le Gentil in 1749. However, it was Charles Messier’s catalog which brought it to attention:

“In the night of September 25 to 26, 1764, I have discovered a cluster of small stars in Auriga, near the star Sigma of that constellation, little distant from the two preceding clusters: this one is of square shape, and doesn’t contain any nebulosity, if one examines it with a good instrument: its extension may be 15 minutes of arc. I have determined its position: its right ascension was 78d 10′ 12″, and its declination 36d 11′ 51″ north.”

By correcting cataloging its position, M38 could later be studied by other astronomers who would also add their own notes. Caroline, then William Herschel would observe it, where the good Sir William would add to his private notes: “A cluster of scattered, pretty large [bright] stars of various magnitudes, of an irregular figure. It is in the Milky Way.”

Messier Object 38 would then later be added to the New General Catalog by John Herschel, who wasn’t particularly descriptive, either. However, there was an historic astronomer who was determined to examine this star cluster and it was Admiral Symth:

“A rich cluster of minute stars, on the Waggoner’s left thigh, of which a remarkable pair in the following are here estimated. A [mag] 7, yellow; and B 9, pale yellow; having a little companion about 25″ off in the sf [south following, SE] quarter. Messier discovered this in 1764, and described it as ‘a mass of stars of a square form without any nebulosity, extending to about 15′ of a degree;’ but it is singular that the palpable cruciform shape of the most clustering part did not attract his notice. It is an oblique cross, with a pair of large [bright] stars in each arm, and a conspicuous single one at the centre; the whole followed by a bright individual of the 7th magnitude. The very unusual shape of this cluster, recalls the sagacity of Sir William Herschel’s speculations upon the subject, and very much favours the idea of an attractive power lodged in the brightest part. For although the form be not globular, it is plainly to be seen that there is a tendency toward sphericity, by the swell of the dimensions as they draw near the most luminous place, denoting, as it were, a stream, or tide, of stars, setting toward the centre. As the stars in the same nebula must be very merely all at the same relative distance from us, and they appear to be about the same size [brightness], Sir William infers that their real magnitudes must be nearly equal. Granting, therefore, that these nebulae and clusters of stars are formed by their mutual attraction, he concludes that we may judge of their relative age, by the disposition of their component parts, those being the oldest which are the most compressed.”

*Open Cluster M38, photographed on Feb 19, 2015. Credit: Wikipedia Commons/Miguel Garcia*

Perhaps by taking his time and really observing, Smyth gained some insight into the true nature of M38! Observe it yourself, and see if you can also locate NGC 1907. It’s quite a pair!

Locating Messier 38:

Locating Messier 38 is relatively easy once you understand the constellation of Auriga. Looking roughly like a pentagon in shape, start by identifying the brightest of these stars – Capella. Due south of it is the second brightest star which shares its border with Beta Tauri, El Nath. By aiming binoculars at El Nath, go north about 1/3 the distance between the two and enjoy all the stars!

You will note two very conspicuous clusters of stars in this area, and so did Le Gentil in 1749. Binoculars will reveal the pair in the same field, as will telescopes using lowest power. The dimmest of these is the M38, and will appear vaguely cruciform in shape. At roughly 4200 light years away, larger aperture will be needed to resolve the 100 or so fainter members. About 2 1/2 degrees to the southeast (about a finger width) you will see the much brighter M36.

More easily resolved in binoculars and small scopes, this “jewel box” galactic cluster is quite young and about 100 light years closer. If you continue roughly on the same trajectory about another 4 degrees southeast you will find open cluster M37. This galactic cluster will appear almost nebula-like to binoculars and very small telescopes – but comes to perfect resolution with larger instruments.

*The location of Messier 38 open star cluster in the Auriga constellation. Credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)*

While all three open star clusters make fine choices for moonlit or light polluted skies, remember that high sky light means less faint stars which can be resolved – robbing each cluster of some of its beauty. Messier 38 is faintest and northernmost of the trio and located almost in the center of the Auriga pentagon. Binoculars and small telescopes will easily spot its cross-shaped pattern.

And here are the quick facts on the Starfish Nebula to help you get started:

Object Name: Messier 38
Alternative Designations: M38, NGC 1912
Object Type: Galactic Open Star Cluster
Constellation: Auriga
Right Ascension: 05 : 28.4 (h:m)
Declination: +35 : 50 (deg:m)
Distance: 4.2 (kly)
Visual Brightness: 7.4 (mag)
Apparent Dimension: 21.0 (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: