Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer. Part of a binary star system only 8 light-years away, it was known in modern times to have a small companion star, Sirius B. Sirius B is much dimmer and appears so close to the brilliant Sirius A that it was not actually sighted until 1862, during Alvan Clark's testing of a large, well made optical refracting telescope . For orbiting x-ray telescopes , the Sirius situation is exactly reversed, though. The fainter source seen at the position of Sirius A may be largely due to ultraviolet light from the star leaking into the x-ray detector. With a surface temperature of 25,000 kelvins , the mass of the Sun, and a radius just less than Earth's, Sirius B is the closest known white dwarf star .
Specific rights apply .