Figure 11-27 RIVUXG Sirius A and Its White-Dwarf Companion Sirius, the brightest-appearing star in the sky, is actually a binary star. The secondary star, called Sirius B, is a white dwarf. In this Hubble Space Telescope image, Sirius B is almost obscured by the glare of the overexposed primary star, Sirius A, which is about 104 times more luminous than Sirius B. The halo and rays around Sirius A are the result of optical effects within the telescope.