StarDate

The Moon and Mercury line up low in tomorrow’s dawn twilight. Mercury looks like a moderately bright star. It’s to the left of the Moon by more than the width of your fist held at arm’s length. The little planet will climb slightly higher into the sky through the end of the month. Even so, it’ll remain low and tough to find.

That highlights a problem astronomers have had from the beginning. Mercury is the closest planet to the Sun, so it never stands far from the Sun in our sky. This month’s appearance is one of the best of the year, yet Mercury is in view only during twilight, which veils its surface. Astronomers also are seeing it through a thick layer of air, so the view is blurry. The combination makes it almost impossible to see any surface features.

As a result, scientists didn’t even know how fast Mercury turns on its axis until the 1960s. They saw the same features every time Mercury was closest to Earth, so they thought the same hemisphere always faced the Sun. That would make the day and the year the same length — about 88 Earth days. In the ’60s, though, they bounced radar beams off Mercury’s surface. That revealed that a day lasts about 59 Earth days.

We learned most of what we know about Mercury from the only two spacecraft to study it. But we should learn much more from a European mission that will orbit Mercury later in this decade — teaching us about a hard-to-see little planet.

Script by Damond Benningfield

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Antares is one of the more impressive stars in the galaxy. It’s about a dozen times more massive than the Sun, 700 times wider, and almost a hundred thousand times brighter. Yet it has some brothers and sisters that are even heavier. They don’t look as impressive right now, but they will before long.

Antares is the orange heart of Scorpius. It stands just a whisker from the crescent Moon at dawn tomorrow. It’s flanked by two of those impressive siblings. Tau Scorpii is close below Antares, with Sigma Scorpii above it.

All three systems were born in a region known as Upper Scorpius. Over the past 12 million years or so, it’s given birth to thousands of stars. And quite a few of them are big and heavy.

Tau Scorpii, for example, is more massive than Antares. It’s not nearly as bright, though, because it’s millions of years younger. Antares has gone through a series of changes that have caused it to puff up to become a supergiant. Tau hasn’t reached that stage of life. But it will in the next couple of million years or so. When it does, it’ll shine even brighter than Antares does today.

Sigma Scorpii is a system of at least four stars — one of which is about 18 times the Sun’s mass. So when it puffs up, it’ll be even brighter than Tau.

Antares, Tau, and Sigma all face the same fate. Each star will explode as a supernova — blasting itself to bits and briefly shining as brightly as billions of Suns.

Script by Damond Benningfield

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From here on Earth, the big dog among the stars is Sirius, the brightest star in the night sky. It’s the leading light of Canis Major, the big dog, so it’s also known as the Dog Star.

And it really is a bright star — a couple of dozen times brighter than the Sun. Yet it looks so bright mainly because it’s close — less than nine light-years away.

In fact, if you lined up all the stars in the outline of Canis Major at the same distance, Sirius would look like a candle in a field of searchlights. The constellation is home to some truly impressive stars. And perhaps the most impressive of all is Aludra, at the tip of the dog’s tail.

The details are a bit blurry, because there’s a wide range in measurements of the star’s distance. Even so, we know that Aludra is a blue supergiant. It’s more than a hundred thousand times brighter than the Sun. It’s also more than 50 times wider than the Sun, and 15 to 20 times more massive.

We also know that Aludra is quite young, but it’s already nearing the end of its life. Because of its great mass, it “burns” through its nuclear fuel in a hurry. Within a few million years, it’ll no longer be able to sustain the reactions in its core. The core will collapse to form a neutron star or a black hole, while the outer layers explode as a supernova. For a brief moment of cosmic time, that’ll leave no doubt about the big dog among the stars of the big dog — Aludra will outshine them all.
 

Script by Damond Benningfield

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The hindquarters of the big dog face a painful future. The stars that outline his tail and back legs are destined to explode within the next few million years.

Canis Major, the big dog, clears the southeastern horizon by about 9 o’clock. The brightest star in all the night sky, Sirius, is at the front of the dog. The hindquarters are outlined by a triangle of three moderately bright stars well below Sirius: Wezen at the top of the triangle, Adhara to its right or lower right, and Aludra, the faintest of the three, at the bottom.

The stars are quite similar. They’re all much bigger, brighter, and hotter than the Sun, and more than 10 times heavier. And they’re all in about the same stage of life. They’ve used up the hydrogen fuel in their cores to make helium, so the fusion reactions in their cores have stopped.

Now, the cores are shrinking and getting hotter, so they’re about to start fusing the helium to make oxygen and carbon. As that happens, the stars will get even bigger and brighter.

And all three stars likely face the same fate. Within a few million years, they’ll all reach a point where they can no longer sustain any reactions. When that happens, each star’s core will collapse, and its outer layers probably will explode as a supernova.

Although it looks fainter than the other two, the most impressive of the three stars appears to be Aludra, at the tip of the dog’s tail. More about that tomorrow.
 

Script by Damond Benningfield

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Caroline Herschel spent thousands of nights helping her brother William study the stars. She polished the mirrors for his telescopes, helped operate them, and recorded his observations. And when William was away, she often made her own observations of the sky. Over the years, she discovered eight comets — the big prizes for astronomers of the day. She also discovered 13 objects that are far beyond our own solar system.

The first of those discoveries, made in 1783, is in view on winter nights, in Canis Major, the big dog. You need help to see it, though.

Caroline’s Cluster spans about 15 light-years, and it’s home to several dozen stars. That’s a bit surprising, because the cluster is more than two billion years old. Over that span, most clusters like this one are pulled apart by the gravity of the rest of the galaxy’s stars and gas clouds. The individual stars survive, but they go their own ways. So it’s rare to find clusters of this type that are so old.

To find the cluster — also known as NGC 2360 — first find Sirius, the brightest star in the night sky. It’s in good view in the southeast by 8 o’clock. Caroline’s Cluster is to the lower left of Sirius by less than the width of your fist held at arm’s length. Under dark skies, binoculars reveal a smudge of light with a few individual stars — discovered by one of history’s first female astronomers.

More about the big dog tomorrow.
 

Script by Damond Benningfield

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A faint constellation with an ironic name is climbing into view in the evening sky. Lynx is in view in the northeast by the time the sky gets fully dark. But “in view” is relative. The constellation is above the horizon, but most of its stars are so faint that you need especially clear, dark skies to see them. Skywatchers in light-polluted cities have no chance.

And that’s where the irony about its name comes in. Although it’s formally named for a small wildcat, the name was picked because you need the eyes of a lynx to see it.

Lynx was created in 1687 by Johannes Hevelius. He drafted several constellations by picking regions of the sky with few bright stars. Although most astronomers were using telescopes by then, Hevelius did most of his observing with his eyes alone. They were sharp eyes, and Hevelius was proud of his vision. So he named this constellation in part as a tribute to his own eyesight.

The brightest star is Alpha Lyncis. It’s about 200 light-years away. It’s more than 50 times the diameter of the Sun, and close to 700 times the Sun’s brightness. It’s that big and bright because it’s past the prime of life. It’s puffed up to giant proportions, which makes it especially bright.

Lynx is a zig-zagging trail of stars between the Big Dipper and the twins of Gemini. Alpha Lyncis is at the bottom of that trail, so it clears the horizon last — one of the few stars that you don’t need the eyes of a lynx to see.
 

Script by Damond Benningfield

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The planet Mars is ready to shift gears. It’s been driving in reverse since the end of October — moving backward compared to its usual motion across the sky. Today, it’ll put on the brakes and stand still. Over the coming days, it’ll shift back into drive and slowly resume its normal eastward path against the background of stars.

In astrology, the time a planet is in reverse is usually considered a bad sign — a time of disruption. And in astrology’s early days, that was understandable. It was hard to explain a celestial body’s reverse gear as anything but a disruption in the order of the heavens.

Today, though, astronomy tells us there’s nothing to it — Mars moving eastward or westward or standing still is simply a result of our changing viewing angle.

Earth moved past Mars early last month. As it did so, our viewing angle switched. It looked like Mars had shifted into that reverse gear. But the planet didn’t change direction at all — it kept up its usual pace and direction around the Sun. Today, we reach the distance from Mars where the planet appears to stop in its tracks. And after this, the angle will be such that Mars will resume its normal easterly course. It won’t have any impact on human behavior — unless you want it to.

Mars is high in the east at nightfall, and looks like a bright orange star. The bright orange eye of the bull — the star Aldebaran — stands to the lower right of Mars.
 

Script by Damond Benningfield

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The rumors of the death of Betelgeuse are greatly exaggerated. The star did stage a big eruption. But it’s still alive and well — although it won’t be for long: It’ll explode as a supernova sometime in the next hundred thousand years.

On the human timescale, that’s pretty much forever. On the timescale of the stars, though, it’s the blink of an eye. The Sun, for example, is four and a half billion years old.

Betelgeuse is only about 10 million to 12 million years old, but the clock is ticking rapidly toward its demise. That’s because Betelgeuse is 15 to 20 times the mass of the Sun. Such heavy stars “burn” through their nuclear fuel in a hurry. They shine brightly but die young.

Betelgeuse has consumed its original hydrogen fuel to make helium. Now, it’s probably burning the helium to make carbon. Over time, it’ll burn through the carbon, then a series of even heavier elements. That will leave it with an onion-like structure, with shells of the different elements around its core.

The core will be converted to iron, but not for long. It won’t be able to “fuse” the iron, so the nuclear reactions will stop. The core will collapse, probably forming a neutron star. But the bulk of the star will be blasted into space — a brilliant ending for a big, beautiful star.

Betelgeuse is the shoulder of Orion. The bright orange star is in the east at nightfall, with the rest of the hunter spreading to its right and lower right.
 

Script by Damond Benningfield

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Betelgeuse is a long way from home. The supergiant star probably was born in Orion OB1 — a region that’s given birth to many supergiants. Today, though, it’s a long way from there, and it’s moving farther by the second. And astronomers are still trying to explain how it was kicked out of its birthplace.

Betelgeuse is one of the biggest and most massive stars in our region of the galaxy. It’s moving through space at a speedy clip. It’s leaving a wake behind it like a ship racing across the sea. That makes it a runaway — a star that’s moving much faster than the other stars around it.

The leading idea says that Betelgeuse was born with an even more massive companion. It aged out quickly, then exploded as a supernova. Without the companion’s gravity to keep it in place, Betelgeuse went speeding away.

Another idea says Betelgeuse and a less-massive companion were kicked out by the gravity of other stars in their birthplace.

But Betelgeuse doesn’t have a companion today, so something must have happened to it. A team led by University of Texas astronomer Craig Wheeler suggested that Betelgeuse ate it. That would explain the star’s rapid rotation — it spins at about a quarter of the speed needed to rip it apart. It would also explain a set of rings of gas and dust that encircle the star.

The scenario is gaining support from other astronomers — suggesting a violent history for a supergiant.

More about Betelgeuse tomorrow.
 

Script by Damond Benningfield

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Betelgeuse has lived a short but spectacular life — and it’s almost over. Within the next hundred thousand years or so, it should blast itself to bits as a supernova. Even then, it’ll leave an amazing corpse: a neutron star.

Betelgeuse marks the shoulder of Orion the hunter. The constellation is in the east and southeast at nightfall. It forms a rectangle of bright stars with a short line of three bright stars in its middle — Orion’s Belt. The belt aims straight up from the horizon in early evening. Betelgeuse is to the left of the belt.

Betelgeuse shines bright orange. That color indicates that its surface is thousands of degrees cooler than the Sun’s.

That “coolness” is only skin deep, though. Deep in its core, Betelgeuse is tens of millions of degrees hotter than the Sun. The core probably is “fusing” helium atoms to make carbon. That process is much hotter than the reactions taking place in the Sun. It produces tremendous amounts of radiation, which inflate the star’s outer layers like a giant balloon. Betelgeuse is so puffy that, if it took the Sun’s place, it would engulf the four inner planets — including Earth — and push toward the next planet, Jupiter.

As the star’s outer layers expanded, they got thinner and cooler, making them look orange. But because Betelgeuse is so big, it’s quite bright, even though it’s probably 550 to 600 light-years away — the bright shoulder of the hunter.

More tomorrow.
 

Script by Damond Benningfield

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