Orion is climbing into prominence in winter’s evening sky. The hunter clears the eastern horizon by about an hour and a half after sunset. He’s led by his shield. It’s not as easy to see as his belt or other features. But the shield’s brightest star does stand out. Pi-3 Orion is in the middle of the shield – where Orion’s hand is holding it. The star is a little bigger, heavier, and hotter than the Sun. That makes it about three times brighter than the Sun. There are a couple of ways to look at that brightness: apparent magnitude and absolute magnitude. Apparent magnitude is how bright a star looks. In that scale, Pi-3 shines at about magnitude 3.2 – not especially bright, but bright enough to see under even most light-polluted skies. But that number doesn’t tell you the star’s true brightness. It might be especially bright, but it might also be especially close. So that’s where absolute magnitude comes in. It’s how bright a star would look at a distance of 10 parsecs – 32.6 light-years. If you lined up every star at that distance, you could easily tell which ones are truly bright. Pi-3 is just 26 light-years away. If you moved it out to 10 parsecs, it would shine at magnitude 3.65 – half as bright as it looks now. In fact, if you moved all the stars in the shield to that distance, Pi-3 would be its faintest member – a middling middle for the shield. Script by Damond Benningfield

Not many planetary spacecraft get to shower off. But the Cassini spacecraft did – more than once. It flew through plumes of ice and water vapor from Enceladus, a moon of Saturn. The encounters helped scientists confirm that an ocean hides below the moon’s icy crust. Enceladus is a little more than 300 miles in diameter – roughly the distance from Los Angeles to San Diego. Its surface is completely coated with ice. That makes it the most reflective large body in the solar system, so it looks bright white. Much of that ice comes from more than a hundred geysers near the moon’s south pole. They erupt from deep cracks in the crust. They contain water vapor, water ice, hydrogen, grains of salt, and other compounds. Much of this material falls back on the surface. The rest of it escapes into space, where it forms a thin ring around Saturn. The geysers erupt from a global ocean. It’s buried about 20 to 25 miles below the surface, and it could be 10 miles deep or more. Hot, mineral-rich water could flow into the ocean through fissures on its floor. So the ocean appears to offer all the ingredients for life: liquid water, minerals, and a source of heat. That makes Enceladus a high-priority target in the hunt for life beyond Earth. Saturn is near our own moon this evening. It looks like a bright star, shining steadily through the lunar glare. But you need a good-sized telescope to pick out Enceladus. Script by Damond Benningfield

A look at the evening sky is a nice way to wrap up your Christmas. It features the Moon, two bright planets, and some of the brighter stars in all the night sky. As twilight drains from the sky, the Moon is well up in the southwest. The Sun lights up more than a quarter of the lunar hemisphere that faces our way, so it’s a fat crescent. It’s waxing toward first quarter, on Saturday. The planet Saturn is to the upper left of the Moon, and looks like a bright star. It shines so brightly for a couple of reasons: It’s the second-largest planet in the solar system – more than nine times the diameter of Earth – and it’s topped by clouds that reflect much of the sunlight that strikes them. The only planet that’s bigger than Saturn is Jupiter, and it climbs into good view, in the east-northeast, by 7 or 7:30. In all the night sky right now, only the Moon outshines it. The “twin” stars of Gemini – Pollux and Castor – stand to Jupiter’s left and upper left. At the same time, the brilliant constellation Orion is off to the upper right of Jupiter. Look for its three-star belt aiming straight up from the horizon, flanked by orange Betelgeuse and blue-white Rigel. Taurus perches well above Orion. It’s marked by its bright orange eye, Aldebaran. And the Dog Star, Sirius, climbs into good view by 8 or 8:30, below Orion’s Belt. It’s the brightest true star in the night sky – a beautiful decoration for Christmas night. Script by Damond Benningfield

For a star, showiness comes with a price. The most massive stars are far brighter than their punier cousins. But they live much shorter lives. An example is Alpha Camelopardalis. It’s the third-brightest star of Camelopardalis, the giraffe. It’s dimmed by its great distance – about 5500 light-years – so you need a dark sky to see it. Even so, it’s one of the most remote stars visible to the eye alone. The star is impressive. It’s more than 30 times the diameter of the Sun, and almost 40 times the Sun’s mass. Because of that great heft, Alpha Cam “burns” through the nuclear fuel in its core in a big hurry. That makes its surface tens of thousands of degrees hotter than the Sun’s, so the star shines blue-white. The combination of size and temperature makes Alpha Cam more than 600,000 times brighter than the Sun. The price for that showiness is a short lifespan. Stars like the Sun live for billions of years. But Alpha Cam will stick around for only a few million years. So even though it’s only about two million years old, its days are numbered. Before long – astronomically speaking – it will expire. Just how it will go out isn’t clear. Its core may collapse to form a black hole, with its outer layers exploding as a brilliant supernova. On the other hand, the entire star may collapse, forming a heavier black hole – a dark ending for a dazzling star. Script by Damond Benningfield

If you’d like to know how dark your night sky is, then look high in the northeast after the Moon sets this evening for the stars of Camelopardalis, the giraffe. If you can see any of them, then congratulations – your sky is pretty dark. Light pollution wipes out the view for most Americans. The glare of street lights, billboards, and other artificial sources overpowers the stars. None of the stars of Camelopardalis, for example, is brighter than fourth magnitude, which is pretty faint. So unless you’re under dark skies, there’s not much to see. That’s a little misleading, though. The giraffe’s brightest stars are all stunners. They look so faint only because they’re so far away. The giraffe’s brightest star is Beta Camelopardalis – Beta Cam for short. It’s a huge, massive star that shines roughly 1600 times brighter than the Sun. But it’s about 840 light-years away, so it’s a faint dot in the night sky. The next-brightest star is CS Cam. It is a supergiant star that’s perhaps 75,000 times the Sun’s brightness. But it’s 3400 light-years away. And the third-brightest, Alpha Cam, is the most impressive of all: more than 600,000 times the Sun’s brightness. At a distance of 5500 light-years, it’s one of the most remote stars visible to the unaided eye – but only under especially dark skies. More about Alpha Cam tomorrow. Script by Damond Benningfield

Space agencies are talking a lot these days about sending people to the Moon – and even setting up permanent bases there. But you might not want to be on the Moon seven years from today. A space rock that’s half the size of an NBA arena has a slight chance of slamming into the Moon. Asteroid 2024 YR4 was discovered last year, two days after Christmas, when the asteroid had flown just half a million miles from Earth. Early observations gave it more than a three percent chance of hitting Earth on December 22nd, 2032. As astronomers tracked it longer, they ruled out that chance. Instead, they calculated that it’ll pass about 7,000 miles from the Moon. But there’s a 45-thousand-mile margin of error. So there’s a better than four percent chance that it will hit the Moon. 2024 YR4 is so far away right now that we can’t see it. It won’t return to view until 2028. Once it reappears, astronomers will refine their calculations. That probably will rule out the chance of an impact. But for now, we can’t know for sure. The asteroid is about 200 feet in diameter. That’s about the size of the asteroid that gouged the famous meteor crater in Arizona. So an impact on the Moon probably would form a big crater. Debris from the impact could travel hundreds of miles – cosmic missiles crashing across much of the Moon. Script by Damond Benningfield

Today is the December solstice – the start of winter in the northern hemisphere. It’s the darkest time of the year – many hours of darkness for watching the stars. But it’s also a great time for space science in Antarctica, where it’s daylight around the clock. NASA launches high-altitude balloons from a base near McMurdo Station, the continent’s largest settlement. Their payloads can keep an eye on the heavens for weeks as they circle around the south pole. When their work is done, they parachute to the ice. Scientists from the United States, Japan, and other countries hunt for meteorites in Antarctica. There aren’t more meteorites there, but on the ice, there’s a good chance that almost any rock came from beyond Earth. Over the decades, tens of thousands of meteorites have been found there. Astronomers take advantage of the daylight to repair and upgrade telescopes at the south pole. The collection includes instruments that study the “afterglow” of the Big Bang. The instruments can operate even in daylight, but the southern summer is the only time to do most of the maintenance work. The south pole also is home to IceCube – a collection of thousands of light detectors frozen in the ice. They look for neutrinos – particles that tell us about some of the most energetic events in the universe. IceCube can also operate all year – even under the midnight sun at the south pole. Script by Damond Benningfield

If you don’t like winter but you live in the northern hemisphere, then give a little thanks to the laws of orbital mechanics. Because of Earth’s lopsided path around the Sun, winter is the shortest season north of the equator. Earth’s orbit around the Sun isn’t a perfect circle. Instead, it’s an ellipse. It looks like a flattened circle, with the Sun slightly away from the center. Because of that shape, our distance to the Sun changes. And that’s where the laws of orbital motion come into play. Johannes Kepler devised those laws more than four centuries ago. One of them says that if you draw a line from the center of the Sun to the center of a planet, as the planet orbits the Sun that line will sweep out equal areas over equal periods of time. To do that, a planet must move fastest when it’s closest to the Sun, and slowest when it’s farthest from the Sun. Earth is closest to the Sun in early January – the start of winter – and farthest at the start of summer, in July. So Earth moves around the Sun in a hurry during northern winter, making the season shorter. This winter, for example, starts at 9:03 a.m. Central Time tomorrow. That’s the moment of the December solstice, when the Sun stands farthest south for the year. The season ends 89 days later. By comparison, this past summer lasted almost 93 days – a longer season thanks to the science of orbits. More about the solstice tomorrow. Script by Damond Benningfield

The “she-goat” is a lot more than it seems. What looks like a single brilliant star is actually two sparklers. Both of them are much bigger, heavier, and brighter than the Sun. Capella is the brightest star of Auriga, the charioteer. Its name comes from a Latin phrase that means the she-goat. It’s 43 light-years away – just down the block by astronomical standards. Both stars of Capella are about two and a half times as massive as the Sun. And both are more than 70 times brighter than the Sun. But the stars are quite close together – less than the distance from Earth to the Sun. So we can’t see them as individual points of light, even with the biggest telescopes. Astronomers discovered Capella’s dual nature with a technique called spectroscopy. It breaks a star’s light into its individual wavelengths. The spectrum of Capella shows two patterns of light. The patterns move back and forth as the stars orbit each other. The patterns reveal details about both stars – their surface temperature, composition, and more. From that and other details, we know that both stars have moved beyond the prime phase of life. Now, they’re in the giant phase. Both stars have puffed up to about 10 times the diameter of the Sun – two big, brilliant stars for the she-goat. Capella is a third of the way up the northeastern sky at nightfall. It’s one of the brightest stars in the entire night sky, so you can’t miss it. Script by Damond Benningfield

The tales that describe many of the ancient constellations can be romantic, tragic, heroic, or majestic. Some, on the other hand, are just weird. An example is Auriga. The constellation is low in the eastern sky at nightfall, and climbs high across the sky later on. It’s marked by a pentagon of stars. It’s easy to pick out thanks to the brightest member of that figure, Capella – one of the brighter stars in the entire night sky. Although Auriga is described as a charioteer, the character usually isn’t depicted with a chariot. But he is shown with a goat and her two kids on his shoulder. There are several versions of his story. In one, he was an early king of Athens. He was raised by the goddess Athena. Among other things, she taught him how to tame horses. He was so good at it that he became the first person to harness four horses to a chariot, like the chariot that carried the Sun across the sky. Zeus, the king of the gods, was so impressed that he placed the charioteer in the stars. The goat is represented by Capella. It isn’t a part of any of the legends of Auriga from Greek or Roman mythology. It may represent the goat that suckled the infant Zeus, who placed her and her children in the sky in gratitude. The goat and kids may once have formed their own small constellation. Today, though, they ride on the shoulder of the charioteer – who rides on nothing at all. More about the charioteer tomorrow. Script by Damond Benningfield