Nearly 400 years after Galileo determined the wispy Milky Way actually
comprises myriad individual stars, scientists using NASA's Rossi
X-ray Timing Explorer have done the same for the "X-ray Milky Way."

The origin of this X-ray counterpart to the Milky Way, known to
scientists as the galactic X-ray background, has been a long-standing
mystery. Scientists have determined the background is not diffuse, as
many have thought. Rather, it emanates from untold hundreds of
millions of individual sources dominated by a type of dead star
called a white dwarf, along with stars with unusually strong coronas.


If confirmed, this new finding would have a profound impact on our
understanding of the history of our galaxy, from star-formation and
supernova rates to stellar evolution. The result solves major
theoretical problems yet points to a surprising undercounting of
stellar objects, perhaps by a hundredfold.

Scientists from the Max Planck Institute for Astrophysics in Garching,
Germany and the Space Research Institute of the Russian Academy of
Sciences in Moscow discuss these results in two papers that will be
published in future editions of Astronomy & Astrophysics.

"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. "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."

X-rays are a high-energy light form, invisible to our eyes and 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." This has led to theoretical
problems. If the X-ray glow were from hot and diffuse gas, it would
ultimately rise and escape the confines of the galaxy. Also, all that
hot gas would need to have come from millions of past star explosions
called 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, Greenbelt, Md. "Many have
thought that the lion's share was truly diffuse, for example, from
hot gas between the stars."

The new study is based on nearly 10 years of data collected by the
Rossi Explorer and constitutes the most thorough map of the galaxy in
X-rays. The science team concluded the Milky Way galaxy is teeming
with X-ray stars, most of them not very bright, and that scientists
over the years had underestimated their numbers.

Surprisingly, 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. A cataclysmic variable is a binary star system containing
a relatively normal star and a white dwarf, which is a stellar ember
of a star like the Earth's sun that has run out of fuel. On its own,
a white dwarf is dim. 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, a source of considerable 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
science 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
X-ray emission traces the stellar mass distribution and implies that
the galactic X-ray background comprises a huge number of faint
discrete sources.

Goddard manages the Rossi Explorer, which was launched in December
1995. For more information about the research on the Web, visit: