StarDate

A big visitor from the fringes of the solar system is headed for its closest-ever encounter with the Sun. It won’t reach that point for another nine years. And even then, it’ll be a long way out. But it should be bright enough to allow astronomers to learn a lot about it.

Comet Bernardinelli-Bernstein was discovered in 2014.

When astronomers first saw it, the comet was as far from the Sun as Neptune, the most-remote major planet. For it to be visible from that range, it must be unusually bright — which means unusually big. Astronomers aren’t quite sure how big, but it’s at least several dozen miles wide — compared to only a few miles for most comets.

The comet came from the Oort Cloud — a shell of icy bodies far from the Sun. In fact, it may be the biggest Oort Cloud comet yet seen. It probably formed closer to the Sun, then was hurled far away by the gravity of one of the giant planets.

The comet approaches the Sun every few million years. It appears to get a little closer with each visit. This time, it’ll pass a billion miles from the Sun — more than 10 times Earth’s distance.

Because it’s stayed so far away, Bernardinelli-Bernstein probably hasn’t changed much since it formed, billions of years ago. So it may reveal details about conditions when Earth and the other planets were born — details from a visitor that won’t be seen again for millions of years.
 

Script by Damond Benningfield

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Pollux and Castor, the twins of Gemini, stay close to the gibbous Moon tonight. They line up to the upper left of the Moon as night falls, with Pollux closer to the Moon. The twins are to the right of the Moon as they set, in the wee hours of the morning.

Although they’re known as twins, Pollux and Castor aren’t related at all. Pollux is about 34 light-years from Earth, while Castor is half-again that far — 51 light-years. That means they’re more than 17 light-years apart.

There’s no indication that the stars are related by birth, either. Pollux is a good bit older than Castor. That means they probably didn’t come from the same original star cluster. So their only real connection is their chance alignment in our sky.

The stars also have different characters.

Pollux is a single star — a bloated giant near the end of its life. Castor, on the other hand, is a system of six stars — one of only a dozen or so known sextuple stars. Two of its stars are bigger, brighter, and heavier than the Sun. The other four are almost identical to each other. They’re red dwarfs — the most-common variety of star in the galaxy. They’re smaller, fainter, and less massive than the Sun.

Seen from Earth, Pollux is about half-again as bright as Castor. If you lined them up at the same distance, though, Castor would be the brighter star, and by almost exactly the same amount — making it the outstanding “twin” of Gemini.
 

Script by Damond Benningfield

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A cosmic missile blazed above the Sikhote-Alin Mountains of Siberia 75 years ago today. It exploded three and a half miles above the ground, raining tons of iron shrapnel across a forest. The event was seen and heard up to 200 miles away.

The “missile” was a small asteroid — probably a chip off a larger object in the asteroid belt. It weighed a hundred tons or more when it hit the atmosphere. Much of its outer layers burned away as it streaked through the sky at tens of thousands of miles per hour. What was left exploded, showering debris across half a square mile.

It took months for scientists to reach the remote impact site. They recorded more than a hundred craters. The largest was 85 feet across and 20 feet deep. And they gathered about 25 tons of meteorites — more debris than from any other single meteor in history. The meteorites are made almost entirely of iron, with a little bit of nickel and a tiny smattering of other elements.

Today, some of the Sikhote-Alin meteorites are in museums in Russia, the U.K., and the United States. Small pieces are in private hands, where they’re sometimes sold. Dealers offer samples that weigh a few grams for a hundred dollars or so. Larger bits fetch larger prices. In 2016, for example, Christie’s auction house sold a half-pound sample for about $7,000. And Sotheby’s sold a five pounder for 10 thousand — valuable pieces of a cosmic missile.
 

Script by Damond Benningfield

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The Sun spins on its axis at a leisurely rate — it takes about four weeks to complete a single turn. Some other stars, though, are like electric blenders on high. They can make a full turn in days or even hours — even though they might be a lot bigger than the Sun.

An example is Beta Cassiopeia. It’s about half way up the northwestern sky at nightfall. It forms the lower left point of the prominent letter “M” outlined by five bright stars of the constellation Cassiopeia.

Beta Cass is about twice the Sun’s mass, and about 30 times its brightness. And a spot at its equator spins dozens of times faster than the Sun — a full turn in just 1.1 days. In fact, Beta Cass is rotating just about as fast as it possibly could — just a few percent faster and it would fly apart.

That high-speed rotation has some interesting effects. For one thing, it pushes gas at the star’s equator outward. As a result, Beta Cass is about 25 percent fatter through the equator than through the poles. And since the poles are closer to the star’s core, where it produces energy, they’re a couple of thousand degrees hotter than the equator. The higher temperature makes the poles brighter than the equator as well.

Beta Cass is less than a quarter of the age of the Sun, but it’s already at the end of its “normal” lifetime. It’s getting bigger and cooler. That process may slow the star’s rotation a little — turning down the speed on a stellar blender.
 

Script by Damond Benningfield

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When you have one of those days that seems to last forever, just be grateful you’re not on the planet Mercury. There, a single day lasts for two years.

Mercury is the closest planet to the Sun. The Sun’s gravitational pull has locked the planet in such a way that it spins on its axis three times for every two orbits it makes around the Sun. Because of the Sun’s changing position in the Mercury sky, that means that a “day” on Mercury — the time from one noon to the next — lasts for two full Mercury years — about 176 Earth days.

That doesn’t necessarily mean that sunsets or sunrises are 176 days apart, though. Mercury’s orbit is lopsided, so the planet’s orbital speed varies by quite a bit. Combined with the speed of its rotation, that creates some odd effects. From parts of the planet, during parts of its orbit, the Sun can rise, then reverse direction and set, then rise again. And there are double sunsets as well.

Mercury is putting in a decent showing as the Sun rises in our own sky. Next week, in fact, it’ll stand farthest from the Sun for its current morning appearance. It’s in the southeast as twilight paints the dawn, and looks like a fairly bright star. It’s so low that you need a clear horizon to spot it. Don’t confuse it with brilliant Venus, the “morning star,” well to the upper right, or with fainter Mars, which is closer to Venus. Mercury will slip from view in the dawn’s early light by late February.
 

Script by Damond Benningfield

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The Moon looks down on the eye of the bull tonight. Aldebaran, the brightest star of Taurus, is below the Moon as night falls, and to the left or lower left of the Moon as they set, in the wee hours of the morning.

It might be hard to notice through the glare of the Moon, but Aldebaran shines bright orange. The color is an indication of its surface temperature, which is much lower than the surface of the Sun. Because of that, Aldebaran is designated as a class “K” star.

Astronomers use seven letters to classify stars — O, B, A, F, G, K, and M.

O stars are the hottest. They blaze tens of thousands of degrees hotter than the Sun, so they shine blue-white. They’re also among the biggest, heaviest, and brightest of all stars.

At the other end are the M stars, which are the coolest. They look orange or red. Almost all M stars are so small and faint that they’re invisible to the unaided eye. The only ones we can see are bloated and bright — stars that have swelled to giant proportions at the ends of their lives.

The Sun is class G. It’s bigger and heavier than most stars, but nowhere near the monsters at the top of the scale.

As it nears the end of its life, though, the Sun will look much like Aldebaran does today — a bloated orange giant, designated as class K or M — at the red end of the stellar classification system.

Tomorrow: A planet with double sunrises and sunsets.
 

Script by Damond Benningfield

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If you’re cruising through space, stay away from pulsars — they’re nasty. They produce deadly radiation, and their gravity rips apart anything that gets close. From many light-years away, though, pulsars could be handy.

A pulsar is the crushed core of a once-mighty star. It’s heavier than the Sun, but only as big as a city. It spins rapidly, beaming energy into space like a cosmic lighthouse. We see these beams as pulses of radio waves or other forms of energy.

Young pulsars rotate dozens to hundreds of times per second. Their pulses are as regular as clockwork. And astronomers are using that precise timing to look for gravitational waves — “jiggles” in space-time caused by the motions of heavy objects. In particular, they’re looking for waves produced by the mergers of supermassive black holes.

As the waves move through space, they cause space itself to shrink and expand by tiny amounts. That changes the distances between objects by a tiny amount.

And that’s what astronomers are looking for. They’re using arrays of radio telescopes to watch many pulsars. By changing the distance between Earth and the pulsars, gravitational waves would change the timing of their “pulses” of energy.

The change is tiny. And so far, astronomers haven’t detected any gravitational waves this way. But they’re adding more telescopes and better detectors to the effort — using the “pulses” of dead stars to hunt for giant black holes.
 

Script by Damond Benningfield

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“Megamaser” sounds like something an alien warship might use to destroy planets. The reality is a little less dramatic, but it’s no less interesting. A megamaser is a beam of energy from another galaxy. And astronomers use these beams to study galaxies — especially the supermassive black holes at their hearts.

A megamaser fires up when a bright energy source lights up a cloud of gas and dust. The energy “pumps up” certain molecules in the cloud — water is one of the most common — amplifying the amount of energy they produce. The result is a tightly focused beam of microwaves. The beam can be anywhere from 10 to a million times as bright as the Sun.

Some megamasers are produced by galaxies that are merging with other galaxies. Such galaxies are giving birth to many new stars, which provide some of the energy to power the masers.

Many water masers are found in galaxies with active black holes in their hearts. Those masers are powered by giant disks of super-heated gas and dust around the black holes. Astronomers use the masers to measure the mass of the black hole, study the disk around the black hole, and more.

And in one recent study, they probed a galaxy that’s almost 13 billion light-years away. That means we see the galaxy as it looked when the universe was less than a billion years old. The study provided the most detailed look at the conditions in gas clouds in the early universe to date — thanks to the power of a megamaser.
 

Script by Damond Benningfield

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Spiral galaxies are a bit like snowflakes — they’re all beautiful, but no two look just alike.

Consider NGC 2217. It’s tens of millions of light-years away, in Canis Major, the big dog. It’s about the same size as our home galaxy, the Milky Way. But it looks much different — like sunlight reflecting off a silver hubcap. It consists of a big oval “bulge” of stars in the middle, with bars of stars extending off its ends. They connect with tightly wound spiral arms that enfold the center of the galaxy. More spiral arms form an almost perfect circle around those.

That layout may suggest that the galaxy was hit by a smaller galaxy. As they collided, NGC 2217 was stirred up. That might have transformed its layout from something similar to the Milky Way to what we see now.

The bars are feeding gas and dust into the center of the galaxy. Along the way, some of that material may give birth to new stars. And the stuff that doesn’t make new stars may be funneled into the supermassive black hole at the galaxy’s heart.

Whatever is happening, it’s molded NGC 2217 into its current form — a unique spiral galaxy — just like all the rest.

Canis Major is in the southeast at nightfall. Look for its brightest star, Sirius — the brightest star in the night sky. The body of the dog extends below and to the lower right of Sirius. NGC 2217 is near one of the dog’s back feet. But you need a telescope to pick it out.
 

Script by Damond Benningfield

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A couple of “nu” stars trot along with the Dog Star, Sirius, in Canis Major, the big dog. Both stars are actually old — they’ve entered one of the final stages of life. The “nu” comes from their names — Nu-2 and Nu-3 Canis Majoris. The word is n-u — the 13th letter of the Greek alphabet.

Nu-2 is a single star that’s the same age as the Sun — 4.6 billion years. But it’s about 30 percent heavier than the Sun. The extra mass revved up the nuclear reactions in its core, so the star has puffed up to become a giant.

The star has two known planets. Both are big and heavy, and not likely to harbor life. But if they have moons, it’s possible they could sustain life. As Nu-2 puffed up, though, it got a lot brighter. Since both of the planets are close to the star, the extra energy could be bad news for any life there.

Nu-3 consists of two stars. The main star is about three-and-a-half times the mass of the Sun. So even though it’s less than 400 million years old, it’s already moved into the giant phase of life. It’s so big that it’s 400 times brighter than the Sun, so it’s visible even though it’s more than 400 light-years away.

Sirius is in the southeast in early evening. It’s the brightest star in the night sky. Nu-2 and -3 are off to its right. They’re much fainter than Sirius, so you need fairly dark skies to see them. And in case you’re wondering, there’s also a Nu-1, but it’s so faint that you need really dark skies to see it.
 

Script by Damond Benningfield

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