The bright star in the southwest corner of the constellation Orion (apparent magnitude +0.12), also referred to as β Orionis. It is a blue-white supergiant of spectral type B8, one of the most luminous stars known. Its intrinsic brightness is estimated to be more than 50,000 times that of the Sun. Rigel is a very young star by stellar standards, with such an enormous rate of energy output that its life span is only a few million years. By comparison, the Sun is approximately 5 × 10⁹ years old, and is still only half way through its main-sequence evolution. Rigel is the brightest member of the Orion OB1 Association, a large molecular cloud complex in which active star formation is currently taking place. In addition to its high luminosity, Rigel has a photosphere which is so large that if the star were placed where the Sun is, it would almost fill the orbit of Mercury.