Hydra, the water snake, goes on and on and on. It’s the longest of the 88 constellations. At midnight tonight, in fact, its head stands halfway up the southwestern sky, while its tail is just clearing the horizon in the southeast.
Yet the snake is hard to follow, because most of its stars are faint. The brightest one is only second magnitude. That puts it well down the list of the night sky’s leading lights.
Alphard looks so faint because it’s about 180 light-years away. In reality, it’s pretty impressive. The star is about four times the mass of the Sun, and about 50 times the Sun’s diameter. And it shines almost a thousand times brighter.
Alphard is more than 400 million years old — roughly one-tenth the age of the Sun. Because of its greater mass, though, it’s already nearing the end of its life. It’s “fused” the original hydrogen fuel in its core to make helium. Now, it’s fusing the helium to make oxygen and carbon.
Fairly soon, its core will shut down, and it will cast its outer layers into space. That will create a colorful bubble around the star’s core. The bubble will quickly dissipate, though, leaving only the now-dead core — and depriving the water snake of its brightest star.
Alphard is low in the east-southeast at nightfall, far to the upper right of the Moon. It’s a long way from the next bright star. That isolation earned the star its name: Alphard is an Arabic name that means “the solitary one.”
Script by Damond Benningfield