Why does Sirius B look brighter in x-ray?
Sirius B, the white dwarf companion of Sirius A, is a very hot, but very small, white dwarf. Because Sirius B is so small, its overall luminosity is small- it looks dim if you consider all its emitted wavelengths (including visible wavelengths). But because it so hot, it emits most of its radiation at the UV and x-ray wavelengths; on the other hand, Sirius A, a main sequence star, emits most of its radiation closer to the visible. So if you take an x-ray picture of this binary system (like Figure 18.1), the white dwarf will shine brightly and the main sequence star will look dim.
