Astronomy & Science

NexDome Observatory Dome

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Canadian Telescopes
3430 Brighton Ave.
Burnaby, BC Canada V5A 3H4

NexDome Observatory DomeCanadian Telescopes introduces the NexDome Observatory (starting at $1,795 for the dome only). This 2.2-meter (8-foot) dome is manufactured from multiple layers of impact-resistant ABS material that retains its shape under a wide range of temperatures. An additional, outer layer of Solarkote protects the structure from ultraviolet deterioration. The NexDome rides on two sets of wheels that prevent shifting and ensure smooth rotation while securing the dome during high winds. Its modular design allows a single person to assemble the dome in just a few hours with common household tools. See the manufacturer’s website for additional options.'s New Product Showcase is a reader service featuring innovative equipment and software of interest to amateur astronomers. The descriptions are based largely on information supplied by the manufacturers or distributors. Sky & Telescope assumes no responsibility for the accuracy of vendors statements. For further information contact the manufacturer or distributor. Announcements should be sent to Not all announcements will be listed.

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Sky & Telescope’s Pluto Globe is Here!

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This beautifully detailed 6-inch globe lets you explore the amazing geology revealed by New Horizons during its historic 2015 flyby.

Sky & Telescope's Pluto globe

Sky & Telescope's Pluto globe is 6 inches (15 cm) across and shows the informal names of 60 craters and other features. It comes with a clear plastic base and an information card with details about the globe and key facts about Pluto.
Sky & Telescope / Sean Walker

The editors of Sky & Telescope are proud to announce the arrival of the latest addition to our extensive line of planetary globes. After a year-long production effort, we received our first Pluto globes this week.

Now you can have one too!
Click here for the order page.

We first kicked around the idea of creating a Pluto globe within days of New Horizons' historic flyby of this frigid world in July 2015. Before the spacecraft got there, no one was sure how much detail might be seen on that icy surface. Of course, everyone was astounded by how interesting Pluto has turned out to be (some of the great images are here and here), and the planning for our globe began.

Unfortunately, the spacecraft couldn't capture all of Pluto with the same fidelity, and about a third of its surface (everything southward of latitude –30°) remained in shadow throughout the flyby. So we decided early on that our usual 12-inch globe format was a poor "fit" to the available data — it would just be too big. (Anyone who's ever seen an over-magnified telescopic view of Mars or Jupiter knows exactly what I mean!)

Also, we wanted as many people as possible to be able to explore Pluto's fascinating details firsthand, and switching to a 6-inch (15-cm) diameter made the final product much more affordable.

Details, Details

To make this globe a reality, about a year ago I started working with the New Horizons science team — notably principal investigator Alan Stern (Southwest Research Institute), Ross Beyer (NASA Ames Research Center), and Paul Schenk (Lunar & Planetary Institute). First we needed a detailed, full-color mosaic — and that took time because the flyby images were coming to Earth in a trickle that lasted for many months. Then S&T Illustration Director Gregg Dinderman worked with Kevin Dzurny and Dell Torgerson at Replogle Globes to make sure all those beautiful details would make it onto the finished product.

Pluto's northern half

Here's the raw material for S&T's Pluto globe: a mosaic of the dwarf planet's northern hemisphere, specially projected into polar coordinates.

Then there was the matter of feature names. All of them remain unofficial, because the New Horizons team and officials from the International Astronomical Union have only recently begun a series of meetings to hammer out what we'll ultimately call the peaks, valleys, and uniquely quirky features on Pluto's surface.

In the end we used a mosaic of more than 125 images acquired by the spacecraft during the 7 days prior to and during its close flyby on July 14, 2015. The resulting base map shows details as small as 1 mile (1½ km) across, and we included the informal names of 60 craters and other features.

We added a freestanding base, manufactured specifically for this project, so that you can pick up the globe and examine its details closely. We also include a 4-by-6-inch card that includes information about how the globe was made, key facts about Pluto, and a guide to the Latinized names used by the IAU to identify specific feature types.

We're very proud and happy with the result, and I hope you'll consider getting one of these for yourself or for some Pluto-lover in your life. Although Alan Stern is already lobbying NASA to build a Pluto orbiter, chances are this is the best view of Clyde Tombaugh's remarkable discovery we'll have for decades to come.

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This Week’s Sky at a Glance, December 23 – 31

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Venus and Mars, Dec. 23, 2016

The temporary "Southwest Triangle" of Venus, Mars and Fomalhaut continues to change, as Venus approaches Mars and the stars slide westward behind the planets.

Friday, December 23

Sirius and Procyon in the balance. Sirius, the Dog Star, sparkles low in the east-southeast after dinnertime. Procyon, the Little Dog Star, shines in the east about two fist-widths at arm's length to Sirius's left.

If you live around latitude 30° (Tijuana, New Orleans, Jacksonville), the two canine stars will be at the same height above your horizon soon after they rise. If you're north of that latitude, Procyon will be higher. If you're south of there, Sirius will be the higher one.

Saturday, December 24

• This is the time of year when M31, the Andromeda Galaxy, passes your zenith soon after dark (if you live in the mid-northern latitudes. It goes precisely across your zenith if you live at latitude 41° north.) Binoculars show M31 just off the upraised knee of the Andromeda constellation's stick figure; see the big evening constellation chart in the center of Sky & Telescope.

Sunday, December 25

• Right after dark you'll find the Pleiades high in the east, with Aldebaran and the Hyades below them. Far below these, Orion is beginning to clear the horizon. By about 9 p.m. Orion is much higher and Sirius is sparkling below it, completing this famous tall stack of December stars.

• Merry Sol Invictus! In late Roman times when the solstice fell on December 25th, the date was celebrated as Dies Natalis Solis Invicti, the Birthday of the Unconquered Sun — when the Sun began to reverse its long decline with the hopeful promise, in the cold and the dark, of a new spring and summer to come. When Christianity took over the Roman Empire, it took over the symbolism and the date.

Got a new scope for Christmas? Not exactly sure how to get going with it? Read What to See with Your New Telescope.

Monday, December 26

• Saturn already? Antares already? Before 2017 even begins, you can catch the very beginning of their apparition that will culminate in the warm evening sky next summer. In early dawn tomorrow the 27th, if air is very clear, go out and look very low in the southeast. Saturn is below the thin waning crescent Moon, and twinkly Antares is off to their right, as shown hereXX.

Binoculars will help. The best viewing will probably be about 45 to 35 minutes before your local sunrise time. (Though in a telescope, Saturn so low will look awfully blurred through the thick air.)

Tuesday, December 27

• Venus now stays up a full two hours after complete darkness (at least if you can see down to the west-southwest horizon). So you have plenty of time this evening to notice that just to Venus's left it little Delta Capricorni, magnitude 2.8. They're 1.1° apart (for North America). And below Venus by about 1.4° is fainter Gamma Cap, magnitude 3.6.

Wednesday, December 28

• After dinnertime at this time of year, the Great Square of Pegasus balances on one corner high in the west. The vast Andromeda-Pegasus constellation complex runs all the way from near the zenith (Andromeda's foot) down through the Great Square (Pegasus's body) to fairly low in the west (Pegasus's nose).

Thursday, December 29

• Orion is now up in the east-southeast soon after nightfall, with his three-star Belt nearly vertical. The Belt points up toward Aldebaran and, even higher, the Pleiades. In the other direction, it points down to where bright Sirius will rise around 7 or 8 p.m. (depending on your location) and twinkle furiously.

• New Moon (exact at 1:53 a.m. on the 29th Eastern Standard Time). A new lunar month begins. Unlike a calendar month, which averages 30.437 days long, a lunar month (from one new Moon to the next) averages 29.531 days. This means that, on average, you'll see the Moon in the same phase about one day earlier every calendar month.

Friday, December 30

• Can you spot the fingernail-thin crescent Moon in twilight? It's less than two days old as seen after sunset for North America. Look low in the southwest 30 to 50 minutes after the Sun goes down. The Moon is located about 30° (three fists at arm's length) to the lower right of shiny Venus.

A similar distance to the Moon's upper right sparkles Altair.

Saturday, December 31

• After the noise and whoopla at the turning of midnight tonight, step outside into the silent, cold dark. Shining in the south will be Sirius at its highest. The other bright stars of Canis Major will be to its right and below it. Sirius forms the bottom of the bright, equilateral Winter Triangle. The triangle's other stars are Betelgeuse in Orion's shoulder to Sirius's upper right, and Procyon the same distance to Sirius's upper left. The Winter Triangle now stands upright just about in balance.


Want to become a better astronomer? Learn your way around the constellations! They're the key to locating everything fainter and deeper to hunt with binoculars or a telescope.

This is an outdoor nature hobby. For an easy-to-use constellation guide covering the whole evening sky, use the big monthly map in the center of each issue of Sky & Telescope, the essential guide to astronomy.

Pocket Sky Atlas, jumbo edition

The Pocket Sky Atlas plots 30,796 stars to magnitude 7.6 — which may sound like a lot, but it's less than one per square degree on the sky. Also plotted are many hundreds of telescopic galaxies, star clusters, and nebulae. Shown above is the new Jumbo Edition for easier reading in the night. Click image for larger view.

Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of charts). The basic standard is the Pocket Sky Atlas (in either the original or new Jumbo Edition), which shows stars to magnitude 7.6.

Next up is the larger and deeper Sky Atlas 2000.0, plotting stars to magnitude 8.5, nearly three times as many. The next up, once you know your way around, is the even larger Uranometria 2000.0 (stars to magnitude 9.75). And read how to use sky charts with a telescope.

You'll also want a good deep-sky guidebook, such as Sue French's Deep-Sky Wonders collection (which includes its own charts), Sky Atlas 2000.0 Companion by Strong and Sinnott, or the bigger Night Sky Observer's Guide by Kepple and Sanner.

Can a computerized telescope replace charts? Not for beginners, I don't think, and not on mounts and tripods that are less than top-quality mechanically (meaning heavy and expensive). And as Terence Dickinson and Alan Dyer say in their Backyard Astronomer's Guide, "A full appreciation of the universe cannot come without developing the skills to find things in the sky and understanding how the sky works. This knowledge comes only by spending time under the stars with star maps in hand."

This Week's Planet Roundup Jupiter with Great Red Spot, Nov. 30, 2016

Jupiter's Great Red Spot side. . .

The non-Red-Spot side of Jupiter on Nov. 27, 2016

. . .and non-Red-Spot side, imaged by Christopher Go on November 30th and 27th, respectively. South is up. With Jupiter in fine telescopic view pre-dawn, it's showing a huge white zone south of the brownish South Equatorial Belt, but nothing like that in the northern hemisphere. A very small telescope should be able to show the difference.

Mercury has faded from view in the glow of sunset.

Venus (magnitude –4.4, in Capricornus) is the bright white "Evening Star" blazing in the southwest during and after twilight. In a telescope, it's a brilliant gibbous disc (about 60% sunlit) 20 arcseconds in diameter.

Mars (magnitude +0.8, in Aquarius) still glows in the south-southwest at dusk, about 15° upper left of Venus. In a telescope it's a tiny orange blob only 6 arcseconds in diameter.

Jupiter (magnitude –1.9, in Virgo) rises around 1 a.m. and shines brightly high the south-southeast by early dawn. Spot Spica 5° below it. In a telescope, Jupiter is 35 arcseconds in diameter: relatively small as Jupiter goes.

Saturn is deep in the glow of sunrise.

Uranus (magnitude 5.8, in Pisces) and Neptune (magnitude 7.9, in Aquarius) are in the southern sky right after dark. Info and finder charts.


All descriptions that relate to your horizon — including the words up, down, right, and left — are written for the world's mid-northern latitudes. Descriptions that also depend on longitude (mainly Moon positions) are for North America.

Eastern Standard Time (EST) is Universal Time (UT, UTC, or GMT) minus 5 hours.


"This adventure is made possible by generations of searchers strictly adhering to a simple set of rules. Test ideas by experiments and observations. Build on those ideas that pass the test. Reject the ones that fail. Follow the evidence wherever it leads, and question everything. Accept these terms, and the cosmos is yours."
— Neil deGrasse Tyson

The post This Week’s Sky at a Glance, December 23 – 31 appeared first on Sky & Telescope.

What to See with Your New Telescope

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Waxing moon

The waxing Moon just before first-quarter phase, as it appears in an amateur telescope magnified about 40 times. The Moon changes phase from night to night, revealing new features every step of the way. The Moon will next be at this particular phase, with the terminator running almost down the middle, on the evening of January 4, 2017.

Maybe this gift-giving season you got a shiny new telescope to call your own. Congratulations — you could be on your way to discovering many amazing far things in the night sky. Although most of them are so far and faint that just finding and detecting them is the challenge! Whether your new scope is a long, sleek tube or a compact marvel of computerized wizardry, surely you're itching to try it out.

"Here are three important tips for getting started," advises Alan MacRobert, a senior editor at Sky & Telescope magazine.

"First, get your scope all set up indoors, read the instructions, and get to know how it works — how it moves, how to change eyepieces, and so on — in warmth and comfort. So you don't have to figure out unfamiliar knobs, settings, and adjustments outside in the cold dark.

"Second, take it outside in the daytime and get familiar with how it works on distant scenes — treetops, buildings — to get a good sense of what it actually does. For instance, you'll find that a telescope's lowest magnification gives the brightest, sharpest, and widest views, with the least amount of the wiggles. The lowest power also makes it easiest to find what you're trying to aim at. So, you'll always want to start off with the lowest power. Switch to a higher power only after you've found your target, got it centered, and had a good first look.

"Also, if the telescope has a little finderscope on the side, daytime is the easiest time to 'align' the finderscope. Point the main telescope at a distant treetop or landmark, center it in the view, and then look through the finderscope. Use the finderscope's adjustment screws to get the crosshairs centered on the same treetop.

"Third," he adds, "be patient. Spend time with each sky object you're able to find, and really get to know it." Too many first-time telescope users expect Hubble-like brightness and color in the eyepiece — when in fact most astronomical objects are very dim to the human eye. And our night vision sees almost everything as shades of gray. Much of what the universe has to offer is subtle, and, once again, extremely far away. But the longer and more carefully you look, the more will come out.

On the other hand, the Moon and planets are bright and easy to find! They make excellent first targets for new telescopic observers. Sky & Telescope's This Week's Sky at a Glance has suggestions for both telescopic and naked-eye viewing of the brightest stars and planets.

Here are some suggestions for starting off:

New-Telescope Delight: The Moon

The Moon is one celestial object that never fails to impress in even the most humble scope. It’s our nearest neighbor in space — big, bright, starkly bleak, and just a quarter million miles away. An amateur telescope and a good Moon map can keep you busy forever.

Full Moon

See if you can identify these noteworthy features in your scope around the time of full Moon. Some of the most prominent craters display bright rays: splashes of impact debris.
Bob King

The crescent Moon will start returning to the evening sky around January 1st; look for it southwest in twilight. Each evening for nearly week after that the Moon will thicken and shine higher. It waxes all the way to full on the night of January 11th. But in some respects, the full Moon is actually the worst time for telescopic Moon viewing, because its full sunlit face lacks shadows to cast mountains and craters into sharp relief. The waxing and waning phases are better, especially for features along the terminator — the lunar sunrise or sunset line — when hills, mountains and craters cast long shadows and stand out in stark relief. The terminator moves quite a bit from night to night, revealing new lunar landscapes.

Evening Planets

You can't miss the brilliant "Evening Star" Venus in the southwest these evenings during and after dusk. Venus is by far the brightest planet. Its permanent white clouds forever hide its surface, but watch Venus go through Moon-like phases as it swings around the Sun from our viewpoint. Right now in a telescope, Venus is a smallish (but dazzling) gibbous ball. Venus will enlarge in the coming months as it swings toward us, while waning in phase to become half-lit and finally, in March, a larger, ever thinner eerie crescent.

Mars glows orange to Venus's upper left all winter, but don't expect much. Mars is a small planet to begin with, and this season it's on the far side of its orbit from us, making it just a very tiny blob in any telescope.

Dawn Adventure: Jupiter

The solar system's largest planet awaits night owls and early risers. The best time for Jupiter now is before the first light of dawn, which means about 1½ to 2 hours before your local sunrise time. Make an adventure of it!

Around that time, Jupiter is by far the brightest point on the southern side of the sky. "Jupiter is the king of the planets and the most interesting one for a small telescope," says MacRobert. "It's big, it's bright, it has cloud belts, and it has four moons that do interesting things."

Even at 50× or 100×, you should be able to make out two dusky-tan bands girding Jupiter's midsection: the North and South Equatorial Belts. These, and the bright Equatorial Zone between them and Jupiter's lesser belts and zones, are cloud features akin to jet streams high in the Jovian atmosphere. (Jupiter is a gas giant with no solid surface.)

Larger telescopes — those whose main mirror or lens is at least 6 inches across — might bring a few more belts and zones into view, along with an assortment of spots and streaks. The famous Great Red Spot, a huge cyclonic storm larger than Earth, is more strongly colored now than it has been in recent decades, so you may detect it even in a small scope if you're looking at a time when it's facing Earth. Jupiter completes a rotation in just under 10 hours — causing its globe to bulge out visibly at the equator — and the Red Spot is easiest to see around when it crosses the midline of Jupiter's Earthward face. You can find the times when this happens using our online app.

With your first look at Jupiter, you'll immediately notice the array of bright moons on either side of it, roughly aligned with the belts. These are the four "Galilean satellites," named for Galileo, who discovered them from Italy in 1610. "From night to night you'll see their movement as they shuttle around Jupiter," notes MacRobert. "Sometimes not all four are visible: occasionally one of them ducks behind Jupiter or is hidden in its shadow." Their own tiny black shadows sometimes cross Jupiter's face. How can you tell which moon is which? We've got an app for that too.

For more about what to look for on and around Jupiter, check out our Jupiter observing guide.

Other New-Telescope Sights

There's more to the night sky than the nearby Moon and planets, of course. Winter evenings often bring crisp, transparent skies with a grand canopy of stars. But with so many inviting targets overhead, where should you point first?

How to find the Orion Nebula

This chart shows where to find the Orion Nebula, in Orion's Sword below the trio of stars forming Orion's Belt. Only the brightest stars (the largest dots) on this chart are readily visible to the unaided eye.

The familiar constellation Orion climbs in the southeast after dark. In its middle, look for the three-star line of Orion's Belt. It's currently nearly vertical in early evening, and it's diagonal (like on the chart at right) late at night.

Just a few degrees south of the Belt runs a smaller, dimmer line: Orion's Sword. Within it lies the Orion Nebula, a luminous cloud of gas and dust where stars are forming by the hundreds. It shows pink in photographs, but dim gray with a hint of green to the human eye. The nebula is plain in any telescope once you get pointed at it, and so is the tight quartet of stars near its center, called the Trapezium. Astronomers sometimes refer to this nebula as Messier 42 (M42), and you might see it labeled that way on star charts. Located about 1,400 light-years away, it's the closest massive star-forming nebula to Earth.

You can use Orion's Belt as a pointer to other things. Extend the line far upward, past the relatively bright star Aldebaran (the orange-red eye of Taurus, the Bull) and you'll reach a little cluster of stars called the Pleiades. It's about the size of your fingertip held at arm's length.

Through binoculars or a telescope at its lowest magnification, the Pleiades cluster shows dozens of stars. Astronomers have found that the cluster has about 500 in all. Like other star clusters, the Pleiades are bound together by their mutual gravity. Collectively called an open cluster for their relatively uncrowded arrangement, the Pleiades move together through space as a swarm. They're about 440 light-years away.

Researchers have determined that the Pleiades began to shine roughly 70 to 100 million years ago. This makes the stars mere toddlers compared to our Sun and solar system, age 4.6 billion years. M45’s youthful suns are astonishingly energetic. Alcyone (al-SIGH-oh-nee), the brightest, is at least 350 times as luminous as our Sun. Like the other Pleiads it gleams with an intense blue-white light — a sign that it’s unusually hot and massive.

Next Steps in Astronomy

To find much else in the night sky, you'll need to start learning the bright naked-eye constellations. They're the key to locating everything fainter and deeper to hunt with binoculars or a telescope — the same way you'd need to know the continents and countries on a globe of Earth before you could pinpoint Thimphu, Bhutan, in south Asia. For an easy-to-use constellation guide covering the whole evening sky, use the big monthly map in the center of each issue of Sky & Telescope magazine, the essential guide to astronomy (ahem).

You'll also want a good, detailed star atlas (set of maps), such as the widely used Pocket Sky Atlas; a good deep-sky guidebook; and some practice in how to use the maps to pinpoint the aim of your telescope on something. (There are a few key tricks to this — see Using a Map at the Telescope.)

For more tips on skywatching and how to get the most out of your telescope, see our Observing section and Getting Started section.

Whatever else, stick with it! Nobody is born knowing this stuff. Everyone has to work their way into the hobby at their own comfortable pace, finding things to know and do and understand and not worrying about everything they don't yet. Living in the universe is like that.

The post What to See with Your New Telescope appeared first on Sky & Telescope.

The Year 2016 Will Be One Second Longer

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Do you think 2016 has seemed unusually long? An international agency has decided to make it even longer. A leap second will be added to December 31st.

On July 6, 2016, the International Earth Rotation and Reference Systems Service (IERS) issued an inobtrusive bulletin addressed to the “authorities responsible for the measurement and distribution of time.” It states that “a positive leap second will be introduced at the end of December 2016.” In other words, the last minute of the last day of 2016 will have 61 seconds rather than the usual 60 seconds.

Leap second June 30, 2015

This is what the U.S. time website looked like when the last leap second was added.
NIST / USNO / Wikimedia Commons

This happens just before midnight on December 31st Universal Time, which is 7 p.m. Eastern Standard Time, 6 p.m. Central Standard Time, 5 p.m. Mountain Standard Time, and 4 p.m. Pacific Standard Time. Assuming that the Official U.S. Time website doesn't crash due to overuse, you can actually watch the time progress from 6:59:59 to 6:59:60 and then to 7:00:00 EST (or the equivalent for your time zone).

What Is a Leap Second?

Let's take a deeper look at what's going on. The first thing to realize is that the real intent is to alter the length of the day; changing the length of the year is an unintended side-effect. Leap seconds are needed because the average length of a day is a little longer than 24 hours. To put that another way, Earth rotates a little more slowly than it would need to for days to average out to 24 hours apiece. If there were no leap seconds, the times of sunrise, noon, and sunset would gradually drift later and later due to the fact that clocks run faster than Earth does.

When the IERS was established, its name was simply the International Earth Rotation Service, which sounds as though they're the people who actually make the world go around. If that were true, the IERS could eliminate the mismatch between Earth's rotation and clock time just by pushing a little harder! Presumably, they added that bit about “Reference Systems” to their name to make it crystal clear that their job is actually to measure Earth's rotation, not to make it happen.

Among its other duties, the IERS is charged with keeping clock time in sync with Earth's rotation. Since they can't speed Earth up, their only other option is to slow clocks down, which they do by adding leap seconds.

Length of Day, 1972 to 2015

The actual length of the day, as determined by Earth's rotation, has fluctuated from about four milliseconds more than 24 hours to one millisecond less than 24 hours in the past few decades.

Unfortunately, Earth's rotation isn't predictable, which is why leap seconds have to be added based on actual observations rather than using a formula similar to the one used for leap years. Over the very long run, the average length of the day is increasing one or two milliseconds per century due to tidal interactions between Earth and the Moon. For historical reasons, the Standard International second was defined as 1/86400th of the presumed average day length in 1900, and it's unlikely that the average day length will ever again be that short for any length of time. That's why leap seconds are always added, never subtracted.

But as the graph above shows, the actual day length fluctuates quite a lot on shorter time scales. Contrary to the long-term trend, it has actually decreased since the leap-second system was instituted in 1972, from about 24 hours plus 3 milliseconds in the 1970s to 24 hours plus 1 millisecond right now. That's why the IERS added one leap second every year during the 1970s, but has added only four during the last decade. That means that the average clock day during the 3,652-day period between January 1, 2006 and January 1, 2016 will be 4/3652 ~= 0.0011 seconds longer than 24 hours, matching the actual day length shown in the graph.

The short-term fluctuations come about because Earth isn't actually solid; the relatively rigid crust that we live on is the exception rather than the rule. The quickest fluctuations are probably due to changing wind patterns transferring angular momentum between the crust and the atmosphere. Others may be due to ocean currents. The longer-period oscillations are presumably due to currents deep inside our planet — either the convection currents in the 1800-mile-thick mantle that drive continental drift, the currents in the iron-nickel core that generate Earth's magnetic field, or some interaction between then.

For more information about how time is defined and measured, see our article Time in the Sky and the Amateur Astronomer.

The post The Year 2016 Will Be One Second Longer appeared first on Sky & Telescope.

Did Betelgeuse Swallow Its Companion?

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The red supergiant marking Orion's shoulder seems to be spinning too fast. Did it get a boost when merged with a smaller companion star 100,000 years ago?

Betelgeuse and Rigel in Orion

Orion, the Hunter, is a hallmark of northern winter skies. The red supergiant star Betelgeuse marks one of his shoulders.
Akira Fujii

This much is clear: Someday Betelgeuse will explode as a supernova. It's roughly 650 light-years away — and when it goes, it'll be spectacular.

But astronomers can't estimate when that might happen, because virtually everything known about this star is uncertain. Its surface temperature, mass, luminosity, and even its distance aren't pinned down very well.

Those loose ends have nagged at J. Craig Wheeler, a colorful theorist and supernova specialist at the University of Texas, who has long been "obsessed with the uncertainty in the evolutionary state of Betelgeuse." But to narrow the date range for the star's eventual demise, he needs tighter constraints on its basic characteristics. Wheeler and an international team of undergraduate students have mounted an effort called the "Betelgeuse Project" to try to do that, and their results appear in an article published December 19th in the Monthly Notices of the Royal Astronomical Society.

Best guesses, as put forward earlier this year by Michelle Dolan (University of Notre Dame) and others, are that Betelgeuse has 19 times the Sun's mass (somewhat larger than previously assumed), 126,000 times its energy output, a surface temperature of 3500 kelvin, and a diameter of at least 1.2 billion kilometers — big enough to gobble up everything to the outer fringe of our asteroid belt and maybe even Jupiter too.

Size of Betelgeuse

Astronomers first determined the diameter of Betelgeuse in 1920, but it wasn't until 1995 that the Hubble Space Telescope recorded the supergiant star's swollen disk.
A. Dupree / R. Gilliland / NASA / ESA

These guesstimates are all tied to how far away the star is, and for now we don't know that more accurately than to about ±25%. Another complication is that astronomers can resolve the disk of Betelgeuse (it's about 0.05 arcsecond across) — but, again, the true value is uncertain.

And don't expect the European Space Agency's Gaia astrometry mission to pinpoint this star's distance. Betelgeuse is too bright for the spacecraft's sensitive sensors. Gaia can use a small filter to dim the light of stars brighter than magnitude 3, explains project scientist Timo Prusti, but the stars' centers still saturate. The "extremely challenging" analysis and reduction of these small images will take time, Prusti says, with "quite some patience needed."

Another key characteristic is how fast Betelgeuse's bloated outer envelope spins. The blue- and redshifting of its spectrum (one limb is moving toward us, while the one on the opposite side is moving away), combined with a tilt to our line of sight of about 20°, yields a rotation speed along the equator of about 15 km per second. In its youth Betelgeuse must have spun much faster, perhaps as fast as 250 km/s. But like a spinning figure skater who flings her arms outward to slow down, the swelling of Betelgeuse as it became a red supergiant and the gradual loss of angular momentum via outflowing stellar winds have slowed the rotation rate dramatically.

Betelgeuse is Spinning (Too) Fast

Now, 15 km/s might not seem all that fast, but for comparison the Sun's equator crawls along at just 2 km/s. Having modeled the star's evolution with various masses and spin rates, Wheeler and his student team find the 15 km/s spin to be a particularly challenging constraint that only fits "at a very special, short-lived point in the evolution" lasting a mere 1,000 years. Given that this soon-to-die star is perhaps 8 or 8½ million years old, it's highly unlikely we're seeing it at such a short blip in its lifetime.

According to Wheeler's team, one "out" could be that Betelgeuse formed as part of a binary system and that it gobbled up a 1-solar-mass companion while ballooning to its present size.

“Suppose Betelgeuse had a companion when it was first born," he muses in a University of Texas press release. "And let’s just suppose it is orbiting around Betelgeuse at an orbit about the size that Betelgeuse is now. And then Betelgeuse turns into a red supergiant and absorbs it — swallows it."

Actually, this hypothesis isn't far-fetched. Most beefy stars, those in spectral classes O or B, do form as binaries, and there's a 1-in-5 chance that the massive, solitary stars seen today are actually mergers of two paired suns into one.

Bubble around Betelgeuse

This 2012 infrared image of Betelgeuse by the orbiting Herschel telescope shows shells of matter on one side of the star.
Leen Decin / ESA

There might even be circumstantial evidence for Betelgeuse's gluttonous act. We've known for two decades that this star is surrounded by a double-rimmed shell of matter that lies about 7 arcminutes away. Some astronomers think it's a shock front created as the star plows through the interstellar medium. But Wheeler calculates that the shell is just about where it should be if Betelgeuse had "burped" while devouring its companion roughly 100,000 years ago.

The next step for the Betelgeuse Project will be to be to probe the star's interior using a technique called asteroseismology. This might reveal, for instance, whether a relatively dense, undissolved remnant of the sacrificial companion lies within the star's enormous volume. Stay tuned!

The post Did Betelgeuse Swallow Its Companion? appeared first on Sky & Telescope.


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