Sirius B white dwarf star. Animation of the location in the night sky and the properties of the white dwarf star Sirius B (Alpha Canis Majoris B), 8.6 light years from Earth in the constellation of Canis Major. Sirius B is a white dwarf star, 0.0084 times the width of the Sun, 0.92 times the width of the Earth (11, 690 kilometres across) and 0.98 times the Sun's mass. Sirius B, the companion of Sirius A in the Alpha Canis Majoris binary system, became a white dwarf after passing through a red giant stage around 120 million years ago. It will steadily cool over the next 2 billion years. Sirius B is spectral type DA2 (degenerate class rich in hydrogen). It is some 0.056 times less luminous than the Sun and has an atmospheric temperature of 25, 200 Kelvin. Its spectrum has hydrogen lines only, with no helium or metal lines present. For Sirius A, see animation K005/3275.