Alpha Herculis

StarDate: April 19, 2009

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Hercules climbs the eastern sky this evening. Unfortunately, the constellation doesn't quite live up to its powerful name: It's faint and tough to find. To give it a try, though, look for a lopsided square of stars low in the northeast by mid evening.

Far to the lower right of this pattern is a star that's one of the biggest and heaviest in our region of the galaxy. Its proper name is Rasalgethi -- a name that refers to the hero's head. But it's also known as Alpha Herculis.

The star is a supergiant, which means that it's much larger and brighter than the Sun. In fact, if Alpha Herculis were at the center of the solar system, it would swallow Mercury, Venus, Earth, and Mars.

Alpha Herculis emits 500 times more visible light than the Sun does. As a result, you can see it with the unaided eye, even though it's about 360 light-years away.

Actually, Alpha Herculis is even more powerful than it looks. That's because the star is fairly cool, so it produces most of its radiation in the form of infrared energy, not visible light.

Alpha Herculis was once powered by the same process that powers the Sun. It "fused" together hydrogen atoms in its core to make heavier helium atoms. But it's used up the hydrogen, so today it's fusing together the helium to make even heavier elements. The change in the star's core set off a sequence of events that caused the star to expand and cool, making it the red supergiant we see today.

Script by Ken Croswell, Copyright 2009

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