An “X-ray star,” as its name implies, is a star that emits a great deal of X-ray radiation. Our Sun, as with most typical stars, emits lots of X-rays compared to terrestrial sources. As a percentage of the total radiation emitted by the Sun, however, its an X-ray star’s emission is very small. X-ray stars may emit thousands of times more X-rays than visible light radiation.

X-ray stars are almost always binary star systems or multiple star systems. The interaction between the two or more stars in the systems—one of which is usually a compact object like a white dwarf, neutron star, or black hole—is what causes the strong X-ray emission. Astronomers usually use the terms “low-mass X-ray binary” (LMXRB) or “high-mass X-ray binary” (HMXRB) to describe the two main classes of X-ray star systems.