Betelgeuse, a bright reddish star in the constellation Orion. It is about 495 light-years from the earth, though some measurements put it at about 640 light-years away. Its color indicates a relatively low surface temperature of about 5,000° F. (2,800° C.), about half that of the sun. Betelgeuse is a variable star that fluctuates irregularly in size and brightness. At its smallest, Betelgeuse has a diameter some 300 times that of the sun; its greatest diameter is about one-third larger than this. Betelgeuse is classified in the group of largest known stars, the supergiants.