"From an airplane you can see a diffuse glow from a city at night," said lead author Mikhail Revnivtsev of the Max Planck Institute for Astrophysics in Garching, Germany. "To simply say cities produce light is not enough. Only when you get closer do you see individual sources that make up that glow - the house lights, street lamps and automobile headlights. In this respect, we have identified the individual sources of local X-ray light. What we found will surprise many scientists."

The map is based on nearly 10 years of data collected by NASA's Rossi X-ray Timing Explorer, and it constitutes the most thorough map ever compiled of the galaxy in X-ray light. The team that created the map has concluded the Milky Way is teeming with X-ray sources, most of them not very bright, and scientists have vastly underestimated their number.

The origin of the X-ray component of the Milky Way, known as the galactic X-ray background, had been a long-standing mystery. Scientists eventually determined that the background is not diffuse, like a cloud, but rather emanates from untold hundreds of millions of individual sources, and dominated by a type of dead star called a white dwarf, along with stars with unusually strong coronas.

Surprising, but the regular suspects of X-ray emission, black holes and neutron stars are not implicated here. At higher X-ray energies, the glow arises nearly entirely from sources called cataclysmic variables - binary star systems containing a relatively normal star and a white dwarf, a stellar ember of a star like the Sun that has run out of fuel. On its own, a white dwarf is dim, but in a binary, it can pull away matter from its companion star to heat itself in a process called accretion. The accreted gas is very hot and emits a considerable amount of X-rays.

At lower X-ray energies, the glow is a mix of about one-third cataclysmic variables and two-thirds active stellar coronas. A corona is the outermost part of a star's atmosphere. Most of the stellar corona activity also takes place in binaries, where a nearby companion effectively stirs up the outer parts of the star. That energizes the stellar analog to solar flares, which emit X-rays. The research team said there are upwards of a million cataclysmic variables in our galaxy and close to a billion active stars. Both of these numbers reflect a major undercounting in previous estimates.

The scientists could not image individual objects. What they saw was a perfect match between X-rays and infrared light detected by NASA's Cosmic Background Explorer mission in the 1990's. This indicates that X-ray emissions match the galaxy's stellar mass distribution, and that the galactic X-ray background comprises a huge number of faint discrete sources.

X-rays are a high-energy light form, invisible but far more energetic than optical light. The X-ray background is more pervasive than the optical haze called the Milky Way, leading astronomers to think the X-ray haze is diffuse, not from point sources.

Previous observations have not revealed enough X-ray sources to account for the X-ray Milky Way, a finding that has led to theoretical problems. If the X-ray glow were from hot and diffuse gas, it ultimately would escape the confines of the galaxy. Also, all that hot gas would need to have originated from millions of past supernovae, which would imply estimates of star formation and star death were way off.

"X-ray telescopes that can resolve the emission into discrete sources looked but could not account for more than 30 percent of the emission," said Jean Swank, project scientist for the Rossi Explorer at NASA's Goddard Space Flight Center. "Many have thought that the lion's share was truly diffuse, for example, from hot gas between the stars."

Revnivtsey and colleagues at the Planck Institute, plus scientists at the Space Research Institute of the Russian Academy of Sciences in Moscow, will discuss their results in two papers to be published in future editions of the journal Astronomy & Astrophysics.

Related Links
NASA's Rossi X-ray Timing Explorer
Max Planck Institute for Astrophysics

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