Sirius double star. Chandra X-ray Telescope image of the double star Sirius (Alpha Canis Majoris). The bright star at centre is the white dwarf star Sirius B, with Sirius A above and just to the right of it. At optical wavelengths, Sirius A is the brightest star in the sky, and Sirius B is 10,000 times fainter. A white dwarf is a very hot, dense and compact star formed after a star has exhausted all its hydrogen fuel. Sirius B emits strongly in X-rays as it has a surface temperature of around 25,000 degrees Celsius. Sirius A has a surface temperature of around 10,000 degrees. The Sirius system lies only 8.6 light years from Earth.