REVELATIONS in the Milky Way
Being able to to see through the whole galaxy and count
X-ray sources,
scientists
have discovered something very important for astronomers who calculate the lives of stars. SK Bhasin reports
CLOSE on 400 years since Galileo determined that the wispy Milky Way actually comprised a myriad individual stars,
scientists using the National Aeronautics and Space Administration’s Rossi X-ray Timing Explorer have done much the same for the “X-ray Milky Way”. The origin of this counterpart to the Milky Way, known to scientists as the galactic X-ray
background, has been a long-standing mystery. They have now determined that the background is not diffuse, as many have thought, but that it emanates from 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 is bound to 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 and 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 are scheduled to discuss these results in two papers that will be published in Astronomy & Astrophysics.
Says lead author Mikhail Revnivtsev of the Max Planck Institute for Astrophysics, “From an airplane you can see a diffuse glow from a city at night. To simply say cities produce light is not enough. Only when you get closer do you see individual sources that make up that glow — 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.
“Like a medical X-ray, the chart of the galactic X-ray background reveals details of the Milky Way’s structure,” he says. “We can see through the whole galaxy and count X-ray sources. This is very important to astronomers who calculate the lives of stars.”
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”, and 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 that 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 per cent of the emission,” says Jean Swank, project scientist for the Rossi Explorer at Nasa’s Goddard Space Flight Centre. “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 team of scientists concluded that the Milky Way galaxy was teeming with X-ray stars, most of them not very bright, and that scientists over the years had underestimated their numbers.
In what may come as a surprise, 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 whitedwarf 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 of cataclysmic variables and two-thirds of active stellar coronas. A corona is the outermost part of a star’s atmosphere, and most of the stellar corona activity also takes place in binaries, where a nearby companion effectively stirs up the outer parts of the star. This energises the stellar analog to solar flares, which emit X-rays. The team of scientists says there are upwards of a million cataclysmic variables in our galaxy and close to a billion active stars. These numbers reflect a major undercounting in previous estimates.
The scientists could not image individual objects but what they saw was a perfect match between X-rays and infrared light detected by Nasa’s Cosmic Background Explorer mission in the 1990s. This indicates that X-ray emission traces the stellar mass distribution and implies that the galactic X-ray background comprises a huge number of faint discrete sources.