As night falls this evening, the southern sky is a big blob of darkness. The brilliant planet Jupiter is off in the southeast, with the star Spica close by. And Sirius, the brightest star in the night sky, is in the southwest, with some of the other leading lights of winter around it. But the space between them looks pretty empty. In fact, there’s only one modestly bright star in the whole region: Alphard, a name that means “the solitary one.”
Alphard is the brightest star of Hydra, the water snake, the longest of the 88 constellations. At nightfall, it stretches halfway across the southern sky.
Alphard’s appearance isn’t misleading — it really is bright. In fact, when you add up all wavelengths of light, it’s almost a thousand times brighter than the Sun.
That’s because the star is nearing the end of its life. It’s converted the original hydrogen fuel in its core to helium, the next-heavier element. Now, it’s converting the helium to carbon and oxygen. That’s caused the core to get much hotter. Its radiation pushes outward on the star’s outer layers, causing Alphard to puff up to giant proportions — more than 50 times the Sun’s diameter.
In the not-too-distant future, those outer layers will puff away into space. For perhaps 50,000 years or so, that’ll create a colorful bubble known as a planetary nebula. After that, though, only the star’s dead core will remain — and Hydra’s solitary bright light will disappear.
Script by Damond Benningfield