Aussie astronomers find stellar heavyweights

  • Many huge stars in the Milky Way cannot be seen because of intervening dust
  • University of Sydney team develops method to spot them using X-ray data
  • These giant stars live fast and die young, often in violent explosions

AN ASTRONOMY TEAM at the University of Sydney has used the bright X-ray glow from our galaxy’s most massive stars to find where they are hiding.

There are over 400 million stars in the Milky Way but only a few are truly massive. These stars emits ‘winds’ that flash outwards at over 1,000 kilometres per second and at temperatures of up to 100 million degrees.

The search 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 they have very short life spans, and 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 … 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, easily shining through the galactic dust.

The ASCA satellite

The team used data from the ASCA satellite observatory.

Shocking discovery

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 [the object is] 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 1,000 kilometres per second and are often found in binary star 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,” added Anderson.

“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 previously 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,” Anderson says.

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 involved in the work include 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).

The results have been published in The Astrophysical Journal.

Adapted from information issued by the University of Sydney. Images courtesy NASA / ESA.

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