New observations using NASA’s Far Ultraviolet Spectroscopic Explorer (FUSE) satellite have given astronomers evidence that their assumptions and calculations about white dwarf stars are correct. FUSE made detailed observations of Sirius B, which is 10,000 times dimmer than its companion Sirius A (the brightest star in the sky). You can only measure the mass of a star in a binary system like this; you can observe the two stars’ orbit, get the period, and then find the sum of the two star masses. These new observations helped astronomers determine Sirius B’s size and mass within 1%.