White Dwarf star Sirius B, c 2000.
© National Aeronautics & Space Administration / Science & Society
This burned-out stellar remnant is a faint companion to the brilliant Dog Star, Sirius. Astronomers, using NASA's Hubble Space Telescope to isolate the light from the white dwarf, called Sirius B, measured precisely the white dwarf's mass based on how its intense gravitational field alters the wavelengths of light emitted by the star. Sirius B is smaller than Earth, but much denser with a gravitational field 350,000 times greater than Earth's, meaning that a 150-pound person would weigh 50 million pounds standing on its surface. Light from the surface has to climb out of this gravitational field and is stretched to longer, redder wavelengths of light in the process. This effect, predicted by Einstein's theory of General Relativity in 1916, is called gravitational redshift. Based on the Hubble measurements of the redshift, Sirius B has a mass that is 98 percent that of our own Sun.