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For astronomers, it's always been a source of frustration that the nearest white-dwarf star is buried in the glow of the brightest star in the nighttime sky. This burned-out stellar remnant is a faint companion of the brilliant blue-white Dog Star, Sirius, located in the winter constellation Canis Major. Now, an international team of astronomers has used the keen eye of NASA's Hubble Space Telescope to isolate the light from the white dwarf, called Sirius B. The new results allow them to measure precisely the white dwarf's mass based on how its intense gravitational field alters the wavelengths of light emitted by the star.

This Hubble Space Telescope image shows Sirius, the brightest star in our nighttime sky, along with its faint, tiny stellar companion, Sirius B. Astronomers overexposed the image of Sirius [at center] so that the dim Sirius B [tiny dot at lower left] could be seen. The cross-shaped diffraction spikes and concentric rings around Sirius, and the small ring around Sirius B, are artifacts produced within the telescope's imaging system. Sirius B is a white dwarf that orbits around Sirius every 50 years. Sirius, only 8.6 light-years from Earth, is the fifth closest star system known.