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 |