StarDate

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