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Rigel is the brightest star in the constellation Orion, taking the position of the hunter's left foot. It outshines the other named star in the constellation, Betelgeuse, ranking as the 6th most luminous star in our night sky. The name Rigel was passed down to us from Arabic astronomers; a shortening of the phrase rijl al-jauza, meaning Left Foot of the Central One.
The most striking characteristic of Rigel is how bright it is. From the Earth it appears so bright because it's relatively close to us at somewhere between 700 and 900 light years. On top of that, Rigel is one of the most intrinsically briilliant stars in this part of the galaxy. The closest more powerful star is Naos, 1,100 light years away in the Puppis constellation. Direct measurement of Rigel's diameter tells us that it has a radius about 78 times bigger than our Sun and shines about 66,000 times more intensely. Our knowledge of stellar evolution puts Rigel as a blue supergiant.
In its 10 million year lifetime, Rigel has been fusing hydrogen into helium at its core. At some point this inert helium core became too large to continue fusing, and the star started to swell and cool. Eventually Rigel will start to contract, and the helium core will get hot enough to start fusing. This helium flash will make the star swell up even bigger, turning it into a red supergiant. Elements in Rigel's core like carbon and neon will form until it finally explodes as a supernova. While it was thought that Rigel had a mass of 17 solar masses, certain observational and theoretical uncertainties make it possible that Rigel actually has a lower mass of around 14 solar masses. This would mean that the star is already fusing helium after already having been a red giant, and has shrunken down and heated its surface to return to its blue supergiant phase.
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How Many Stars?
In 1831 the astronomer F.G.W. Struve observed that Rigel was actually a binary star system, designated as Rigel A and Rigel B. Rigel B is not a particularly faint star, having an apparent magnitude of 6.7, but Rigel A is 500 times brighter and only 2200 AU away and easily outshines its brother star. Using spectroscopy, modern astronomers were able to see that Rigel B itself was actually 2 main sequence stars that orbit each other every 9.8 days. There is also some speculation that Rigel might have a fourth star in its system. The 15th magnitude Rigel D would have an orbit of 11,500 AU and would take around 250,000 years to orbit the inner three stars.
In the past, Rigel has been classified in the Orion OB1 association of blue stars, which are stars that formed around the Orion nebula. In fact, Rigel appears to be too far from the Orion nebula to be an OB1. However, its age and path through space suggest that Rigel could have been formed in the Orion OB1 association, and ejected from the system.
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Witch Head http://antwrp.gsfc.nasa.gov/apod/fap/image/0102/witchhead_stevens_big.jpg
HR Diagram http://history.nasa.gov/EP-177/i4-3.jpg