Sirius binary star system, Hubble Space Telescope image. Sirius A (centre, Alpha Canis Majoris), in the constellation Canis Major, is the brightest star in the night sky and only 8.6 light years from Earth. It has a very small, faint companion, Sirius B (small dot at lower left). Sirius B, only 12,000 kilometres in diameter, is a white dwarf star, the collapsed core of a star. Normally, the companion star would be obscured by the brightness of its companion, but Hubble's Wide Field Planetary Camera 2 instrument was used to locate the star, and other instruments were then used to measure the gravitational redshift of its light, and hence its mass. Sirius B has a mass 98% of that of our Sun. The results were published in December 2005.