StarDate

The weather can be a bit blustery at this time of year, with strong winds lofting dust into the sky. The dust can make the setting Sun look especially orange or red.

Astronomers see that same “redness” when they look at the stars beyond the Sun, and for the same reason: clouds of dust. In fact, the space between the stars is far from empty. It’s filled with gas and dust that form the “interstellar medium” – the ISM. This material accounts for about 15 percent of all the “normal” matter in our home galaxy, the Milky Way.

The interstellar medium is an extreme vacuum by any standard – it’s far emptier than any vacuum created in the laboratory. But there’s a lot of space between the stars, so the gas and dust add up.

Most of the ISM consists of hydrogen – atoms that were created in the Big Bang. But there’s also a smattering of other atoms and molecules, plus the tiny solid particles known as dust.

Many of the dust grains are about the size of the particles in cigarette smoke. That’s the right size to scatter blue wavelengths of light. The effect is similar to what makes the Sun look redder when it’s viewed through a thicker layer of air.

Interstellar dust makes far-away stars look redder than they really are, too. So to fully understand the stars, astronomers need to know how much dust they’re looking through – dust that reddens the view of the distant universe.

Script by Damond Benningfield

Outer space is one of those things you know when you see it. The sky is dark and quiet, with not enough air to sustain life. But just where space begins is hard to say – there’s no single definition that’s accepted by one and all.

In the last century, the National Advisory Committee for Aeronautics – the forerunner of NASA – said that space begins 50 miles up. At that altitude, the air pressure is only about a hundred-thousandth of the pressure at sea level. NASA and the Air Force have used that definition to award astronaut wings, beginning with the pilots of the X-15 rocketplane in the 1960s.

Later, engineer Theodore von K‡rm‡n devised a slightly higher boundary: 100 kilometers – about 62 miles. At that altitude, the air pressure is just one-millionth of the pressure at sea level. In such thin air, a craft must move at orbital velocity to stay aloft.

That altitude is known as the K‡rm‡n Line. Today, it’s the most commonly used definition for the edge of space. Anything that stays below that line is an aircraft, while anything that goes above it is a spacecraft. Hundreds of people have flown above that line – qualifying them as space travelers.

A smidgen of atmosphere extends above even the Karman Line – up to several thousand miles. It’s so thin, though, that it’s basically a vacuum – a region that certainly qualifies as “outer space.”

We’ll go deep into outer space tomorrow.

Script by Damond Benningfield

Things are changing on Mars. The Red Planet is about half way through spring in the northern hemisphere and autumn in the southern hemisphere. The switch triggers a kind of “see-saw” effect. As the north gets warmer, its polar ice cap gets smaller. And as the south gets colder, its ice cap gets bigger. That transition can cause the air pressure to vary by as much as a third across the entire planet.

The ice caps are made mainly of frozen water. They’re up to a couple of miles thick. They’re marked by spiral patterns of deep valleys – perhaps carved by strong winds.

A layer of dry ice forms atop each cap every autumn, and vanishes every spring. The carbon dioxide is pulled from the atmosphere, which is much thinner than Earth’s. So as the dry-ice cap moves from pole to pole, the atmospheric pressure changes by quite a bit.

There’s disagreement about what lies below the southern ice cap. A few studies have suggested there could be liquid water – perhaps in a thin layer, or mixed with the dust and rock in the ground. But other studies say that liquid water couldn’t survive in the conditions below the ice at the south pole of Mars.

Mars is in a crowded region of the sky tonight. It’s close to the upper right of the Moon at nightfall, and looks like a bright orange star. The true star Pollux is closer to the lower left of the Moon, with Castor, the other twin of Gemini, to their upper left.

Script by Damond Benningfield

The Big Bear is more than just the Big Dipper. The dipper outlines the body and tail of Ursa Major, the big bear. But the constellation covers much more territory. Some fainter stars outline the bear’s legs and head. But it’s the stars of the dipper that we most notice – not only because they’re fairly bright, but because they form an easy-to-see pattern.

The dipper is an “asterism” – a group of stars that forms a discernible pattern, but that doesn’t form a whole constellation. An asterism can be part of a constellation, as the dipper is, or it can incorporate stars from two or more constellations.

A couple of examples are in good view this evening. A small one is the Circlet of Pisces – five stars that outline the head of one of the fish of Pisces. The stars aren’t that bright, but they form a compact pattern, making the circlet easier to pick out. And it has a bright pointer: It’s close to the lower right of Venus, the “evening star.”

On the other end of the spectrum is the Winter Hexagon or Winter Circle. It is outlined by seven stars – most of them among the brightest in the night sky. The pattern spans more than six times the width of your fist held at arm’s length. It ranges from Sirius, the brightest star in the night sky, which is low in the southeast at nightfall, to yellow-orange Capella high in the east. It incorporates stars from six constellations – the largest of all the well-known asterisms.

Script by Damond Benningfield

The gibbous Moon passes across a special spot tonight. It lines up opposite the center of the Milky Way Galaxy – the galactic anti-center. In that direction, we’re looking toward the rim of the galaxy’s disk. The rim is about 25,000 light-years away, with intergalactic space beyond.

The center of the galaxy is in Sagittarius. When we look in that direction we stare into thick clouds of stars, along with dark clouds of dust – places where more stars are being born.

But when we look in the opposite direction the view is much less impressive. There aren’t as many stars or dust clouds. And the stars thin out as you get closer to the rim.

Beyond that, we’re looking into the galaxy’s “halo” – a region that extends hundreds of thousands of light-years into space. It contains a few giant star clusters, and a smattering of individual stars. Almost all of them are ancient – dating to the earliest days of the Milky Way itself.

And beyond the halo, there’s not much at all – some wisps of gas, and an occasional star or free-ranging planet. It’s millions of light-years to the next galaxy – through the vastness of intergalactic space.

The closest star to the anticenter is Elnath, the second-brightest star of Taurus. It marks the tip of one of the bull’s horns. It’s just above the Moon at nightfall. The brilliant planet Jupiter and the star Aldebaran – the bull’s eye – stand farther to the right or upper right of the Moon.

Script by Damond Benningfield

The Moon probably was born when the young Earth was hit by another planet. That blasted a lot of debris into space. Much of it came together to form the Moon.

But Earth wasn’t the only world to get smacked around in the early solar system. One of the others might have been Ganymede, the largest moon of Jupiter. A recent study suggested that a giant asteroid slammed into it, making it wobble for a thousand years.

Ganymede is the largest moon in the solar system – bigger than the planet Mercury. It probably has an ocean of liquid water below its icy crust. The ocean might be 60 miles deep, and hold more water than all of Earth’s oceans combined.

The new study looked at some long ripples on the surface, the way Ganymede spins, and other evidence. Researchers then used computer models to simulate Ganymede’s history. Their work suggested that Ganymede was hit by an asteroid about four billion years ago. The asteroid was almost 200 miles in diameter, and hit near the north pole. That caused Ganymede to wobble. Eventually, it flipped over on its side. What had been the equator became the poles as Ganymede settled down after a massive impact.

Jupiter is close to our moon at nightfall. It looks like a brilliant star. Aldebaran, the brightest star of Taurus, is to the lower right of Jupiter. Binoculars reveal Jupiter’s four big moons, including Ganymede – a world that might have suffered a “big whack” billions of years ago.

Script by Damond Benningfield

An odd “zombie” star has grabbed its companion in a magnetic embrace. It keeps the two stars synchronized, and it pulls gas from the companion.

The system is A-M Herculis. It’s almost 300 light-years away, in the constellation Hercules.

The main star in the system is a white dwarf — the corpse of a once-normal star. It’s about two-thirds as massive as the Sun, but only about as big as Earth. That means it’s extremely dense, so its gravity is strong.

The star’s magnetic field is millions of times stronger than the Sun’s. And that’s bad for the companion – a cool, faint ember known as a red dwarf. The stars are so close that they orbit each other every three hours. At that range, the magnetic field of the white dwarf exerts a powerful pull. It’s made the two stars rotate in such a way that the same hemisphere of each star always faces the other – just as the same hemisphere of the Moon always faces Earth.

The field also pulls gas from the companion. In most systems like this, the infalling gas forms a wide, spinning disk. But in the case of A-M Herculis, it plunges directly onto the white dwarf, guided by the magnetic field. The streamer varies – thicker at some times, thinner at others. But it piles onto the poles of the white dwarf, making the star hotter and brighter – renewed vigor for a stellar zombie.

Hercules is high in the sky at dawn. But A-M Herc is much too faint to see without a telescope.

Script by Damond Benningfield

One of the most amazing star systems in the galaxy is only 7500 light-years away. In the 1800s, it shined brighter than only one other star in the entire night sky. And combined, its two stars are at least five million times brighter than the Sun. Yet no one has ever seen either star directly – they’re encased in a cocoon of their own making.

Eta Carinae appears to consist of two supergiants. One of them may be a hundred times the mass of the Sun – one of the heaviest stars yet discovered. The other is perhaps 30 times the Sun’s mass or more.

Almost two centuries ago, the system began to flare up. Astronomers still aren’t quite sure why. Perhaps one of the stars engulfed a third member of the system. Or perhaps the two stars passed especially close together, and their gravity ripped out big torrents of hot gas.

Whatever the reason, over a couple of decades Eta Carinae grew especially bright. Then it faded from view. That’s because the two stars had expelled a huge amount of gas – enough to make 20 stars like the Sun. The material cooled, wrapping the stars in a dense nebula. It’s shaped like an hourglass, with a sheet extending from its waist like a ballerina’s tutu

The stars are only about three million years old. But the heavier one could expire at any time. It could explode as an especially bright supernova. Or it might collapse to form a black hole – a dark fate for a brilliant star.

Script by Damond Benningfield

Clouds made of sand may scud across the skies of a pair of brown dwarfs – “failed stars” that are among the Sun’s closest neighbors. The clouds move in a hurry – they can change the appearance of the brown dwarfs over just a few hours.

The system is known as Luhman 16. It’s just six and a half light-years away – about two light-years farther than the closest star system, Alpha Centauri. But it wasn’t discovered until about a decade ago because it’s extremely faint. It’s in the constellation Vela, which just peeks into view late tonight for skywatchers in the far-southern United States.

Luhman 16 is a binary, so the two brown dwarfs are locked in orbit around each other. Both of them are about 30 to 35 times the mass of Jupiter, the largest planet in the solar system. That makes them too massive to be planets, but not massive enough to shine as true stars.

The surfaces of both objects are hot, but not nearly as hot as the Sun. That allows molecules to condense and form clouds.

Researchers examined the system with instruments aboard Webb Space Telescope. The instruments are sensitive to different wavelengths of light, so they reveal different details. They found clouds made of silicate – grains of sand. They also found water, carbon monoxide, and methane. The compounds were found at different depths in the atmosphere. So the observations provided a 3-D look at the busy skies of these faint neighbors.

Script by Damond Benningfield

If you just can’t wait for spring, you might want to use one of the calendar markers of long-ago Europe and the British Isles. In their traditions, early February marked the end of winter and the start of spring. The days were getting longer and warmer, with the promise of even more sunlight to come.

Early February falls roughly half way between the winter solstice and the spring equinox. Such midpoints are known as cross-quarter days. That means they cross between the quarter days – the solstices and equinoxes.

Early Christians celebrated February 2nd as Candlemas. It was the 40th day of Christmas. People brought their candles to church to have them blessed for the whole year.

In Ireland and Scotland, the February celebration came on the 1st. It was known as Imbolc. The name may indicate the start of lambing season, although it could have other meanings as well.

One of the traditions of Imbolc concerned the weather. If the day was sunny and cold, it meant winter would maintain its icy grip until the equinox, about six weeks away. But if the day was warm and cloudy, then winter was just about over. Some people relied on badgers or other animals to come out of their dens to check out the conditions.

That tradition was carried to the United States by immigrants. And it’s still with us today as Groundhog Day – a modern celebration of a cross-quarter day – one that marked the beginning of spring.

Script by Damond Benningfield