StarDate

The Sun and similar stars are losing weight – they blow some of their gas into space through strong “winds.” And at the end, they blow away all of their outer layers of gas. That leaves only their hot, dense cores, known as white dwarfs – tiny remnants of their once brilliant selves. An example is Sirius B, the faint companion of Sirius A, the brightest star in the night sky. Sirius climbs into view in the east-southeast by around 8:30 or 9, and arcs across the south during the night. Sirius B is too small and faint to see without a telescope. But long ago, that wouldn’t have been the case. The star probably was a few times as massive as the Sun, so it would have shined brighter than Sirius A is today. Such a hot, bright star produces a much thicker wind than the Sun does, so it loses mass at a higher rate. And because Sirius B was heavier than the Sun, it burned through the nuclear fuel in its core much faster – it fizzled out in a couple of hundred million years, while the Sun is still only half way through its 10-billion-year lifetime. As it neared the end of its life, Sirius B puffed up like a giant balloon, then ejected its outer layers. Some of that gas probably piled on the surface of Sirius A, increasing its mass. Today, Sirius B is as heavy as the Sun, but only as big as Earth. It still shines because it’s extremely hot. But it’s only a faint reminder of its former glory. Tomorrow: an early new year. Script by Damond Benningfield

Over the centuries, we’ve given all the visible stars many names – proper names, catalog designations, and others. But only one star is best known not by any of its formal names, but by its nickname: the Dog Star. Its proper name is Sirius, and it’s the leading light of the constellation Canis Major, the big dog – hence the nickname. Sirius is so well known because it’s the brightest star in the night sky – its closest competition is only about half as bright. Part of that is because Sirius itself is a couple of dozen times brighter than the Sun. But part of it is because Sirius is one of our closest neighbors – less than nine light-years away. And thanks to the relative motions of Sirius and the Sun, Sirius is moving closer, at about 12,000 miles per hour. It’ll continue to close in for tens of thousands of years. But the distances between stars are so vast that even at that speed, Sirius won’t grow much brighter in our sky. Astronomers discovered the star’s motion toward us by measuring its Doppler shift – a slight change in the wavelength of its light. The Doppler shift also allowed them to measure the orbit of a faint companion – a stellar corpse known as a white dwarf; we’ll have more about that tomorrow. In the meantime, look for Sirius climbing into good view in the east-southeast by around 8:30 or 9. It’s directly below the three stars of Orion’s Belt, so you can’t miss it. Script by Damond Benningfield

The most important thing to know about a star is its mass – how heavy it is. Among other things, the mass reveals how long the star will live and how it will die. Measuring the mass of a single star is tough. It’s a lot easier to get the masses of stars in binary systems – two stars that orbit each other. An example is Menkalinan, the second-brightest star of Auriga. It’s a third of the way up the northeastern sky at nightfall, below the charioteer’s brightest star, Capella. Menkalinan’s two stars are so close together that we can’t see them as individual points. But breaking the system’s light apart reveals the presence of both stars. The stars orbit each other every four days, at about one-tenth of the distance from Earth to the Sun. Combined, those numbers reveal the system’s total mass. A couple of other numbers complete the picture. One is the angle at which we’re seeing the system. In the case of Menkalinan, that’s easy – the stars pass in front of each other, so we see the system edge-on. The other is the orbital motions of the stars. Plugging those numbers into the formula provides a precise mass for the individual stars. The stars of Menkalinan are almost identical. Each is more than twice the mass of the Sun. Each is also bigger and brighter than the Sun. So even though Menkalinan is more than 80 light-years away, it’s easy to see – the combined glow of two big, well-understood stars. Script by Damond Benningfield

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 Francisco. 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