X-ray image from the Chandra Observatory showing Sirius B, a small companion star to Sirius A (Alpha Canis Majoris) the brightest star in the sky. Sirius B, although hotter, is much smaller and dimmer, and was not sighted until 1862. Sirius A and B are part of a a binary star system eight light-years away from Earth. Sirius B is the closest known white dwarf star; its surface temperature is 25,000 kelvins, its mass is the same as that of the Sun, and its radius is slightly less than taht that of the Earth's.
© Science Museum / Science & Society Picture Library