Betelgeuse, a bright reddish star in the constellation Orion, some 600 light-years from the earth. 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.