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Alpha Herculis

Alpha Herculis 1

Alpha Herculis


Distance to Earth: 360 light years
Surface temperature: 2,800 K
Mass: 2.15E22 kg
Radius: 387,000,000 km
Right ascention: 17h 14m 38.8s
Constellation: Hercules


Alpha Herculis  is a multiple star system in the constellation Hercules.  The traditional name “Head” comes from the fact that in antiquity Hercules was depicted upside down on the constellation maps.
The angular diameter of the red giant, α1 Her, has been measured with an interferometer as 34 ± 0.8 mass, or 0.034 arcseconds. At an estimated distance of 110 parsecs from Hipparcos,  this corresponds to a radius of about 280 million kilometers (or 170 million miles). If Rasalgheti were at the center of our Solar System its radius would extend past the orbit of Mars, but not quite as far as the asteroid belt. The red giant is estimated to have started its life with about 7-8 solar masses. Like most type M stars near the end of their lives, Rasalgethi is experiencing a high degree of stellar mass loss creating a sparse, gaseous envelope that extends at least 90 astronomical units.

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

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