Heavy, fast and furious: stars of the Milky Way

18 April 2011

The astronomy team developed a method of finding the truly massive stars in the Milky Way.
The astronomy team developed a method of finding the truly massive stars in the Milky Way.

There are over 400 million stars in the Milky Way but only a few are truly massive. An astronomy team at the University of Sydney has used the bright X-ray glow from these stars to find where they are hiding in our galaxy.

These massive stars have winds that reach over 1000km per second and burn up to 100 million degrees.

Results of a search for massive stars in the Milky Way produced never before seen X-ray images published recently in The Astrophysical Journal, an international publication. The project known as "ChIcAGO" (Chasing the Identification of ASCA Galactic Objects) is lead by Gemma Anderson from the School of Physics.

"ChIcAGO was designed to explore the unidentified X-ray sources detected with the Advanced Satellite for Cosmology and Astrophysics (ASCA), an older generation orbital X-ray telescope," says Anderson.

Ms Anderson says the massive stars they found can be 50 times heavier than our sun but have a very short life span, which may end in a supernova explosion that produces enough light to outshine the entire galaxy.

"We asked how do we find these rare and distant supernova progenitors, hidden deep in the Milky Way?

"These stars are nearly invisible to traditional optical telescopes, because the dust in the plane of our galaxy absorbs their light," Anderson explains.

Recent observations with NASA's Chandra X-ray Observatory have discovered that these massive stars can be some of the brightest sources of X-ray radiation in our galaxy easily shining through the galactic dust.

Taking it a step further, the ChIcAGO project asked what possible process could cause a star to produce such high-energy radiation?

Anderson explains: "When X-ray radiation is detected from an astronomical object it means that they are extremely hot and that particles are being accelerated to speeds near the speed of light.

"In this case the massive stars have winds that blow over 1000km per second and are often found in binary pairs.

"Such systems are known as colliding-wind binaries as the strong winds from these stars collide, creating extremely strong shocks that heat the stellar material to temperatures up to 100 million degrees, resulting it the production of bright and powerful X-rays.

"The collisions in colliding-wind binaries are some of the most violent in our universe, only surpassed by extreme events like the death of one of these massive stars in a supernova.

"The detection of their X-ray emission is a new way of discovering massive stars that eluded discovery in extensive infrared and optical surveys of our galaxy.

"By searching for such high energy X-rays with Chandra we have devised an efficient way of finding the most massive stars in our galaxy."

In the future, the ChIcAGO project is aiming to discover the identity of other massive stars in colliding-wind binaries, as well as their supernova remnants, allowing us the explore the life, death and evolution of these stellar giants in the Milky Way.

Other authors included Bryan Gaensler (University of Sydney), David Kaplan (University of Wisconsin, Milwaukee), Bettina Posselt, Patrick Slane and Stephen Murray (Harvard-Smithsonian Center for Astrophysics, or CfA), Jon Mauerhan (California Institute of Technology), Robert Benjamin (University of Wisconsin, Whitewater), Crystal Brogan (National Radio Astronomy Observatory), Deepto Chakrabarty (Massachusetts Institute of Technology), Jeremy Drake (CfA), Janet Drew (University of Hertfordshire), Jonathan Grindlay and Jaesub Hong (CfA), Joseph Lazio (Naval Research Laboratory), Julia Lee (CfA), Danny Steeghs (University of Warwick), and Marten van Kerkwijk (University of Toronto).

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