The conventional view of the galactic X-ray background is that it is produced by clouds of gas that have been heated to extremely high temperatures by supernova explosions. But there are problems with this picture: calculations suggest that there are not enough supernovae in our galaxy to replace the gas as it speeds out of the Milky Way.

Now, however, Revnivstev and colleagues think that the X-ray glow is produced by individual stars that are not very bright and so were missed in previous searches, like those made by the Chandra X-ray Observatory. The team came to their conclusion by analyzing a decade's worth of data produced by NASA's Rossi X-Ray Timing Explorer between 1995 and 2005. In doing so, they have also produced the most detailed ever map of the Milky Way at X-ray wavelengths.

According to the astronomers, the glow comes from two main sources: "cataclysmic variables" and "active stellar coronas". Cataclysmic variables consist of a normal star and a white dwarf -- an Earth-sized remnant of a star like our Sun that has run out if its nuclear fuel. Normally, a white dwarf is very dim and so cannot be detected, but in a binary it can pull matter from its companion and heat this gas up, which releases X-rays. Active stellar coronas are binary systems in which a normal star "stirs up" another star's outer atmosphere (or corona) causing flares like those produced by our Sun. These also release X-rays.

The study suggests that there are about one million cataclysmic variables and around one billion active coronas, which suggests the number of stars in our galaxy has been greatly underestimated until now. The team now plans to confirm its results by making more sensitive measurements of the central part of the galaxy with Chandra.

According to the researchers, the X-ray background is like seeing the glow of a city by night when looked at from an aeroplane. "Only when you get closer do you see individual sources that make up the glow," explains Revnivtsev.