Leonard E Parker

Center for Gravitation, Cosmology & Astrophysics

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The Leonard E Parker Center for Gravitation, Cosmology and Astrophysics is supported by NASA, the National Science Foundation, UW-Milwaukee College of Letters and Science, and UW-Milwaukee Graduate School. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of these organizations.

Hunting for the Milky Way's Heaviest Stars

Posted by Sara Gil-Casanova on April 28, 2011

A new study conducted by several astronomers, including UWM Assistant Professor David Kaplan, has found evidence for a plentiful population of X-ray emitting massive stars in the Milky Way. The study has been recently published in the Astrophysical Journal in a paper led by Gemma Anderson (University of Sydney).

This image shows infrared data from NASA's Spitzer Space Telescope near the plane of the Milky Way galaxy. Both outlined boxes contain an artificially darkened view of the Spitzer data, to highlight a bright X-ray source (blue) detected at the center of each square with NASA's Chandra X-ray Observatory. Each X-ray source coincides with a strong infrared signal Credit: X-ray: NASA/U. of Sydney/G.Anderson et al; IR: NASA/JPL-Caltech

The stars were identified using NASA's Chandra X-ray Observatory. A previous X-ray mission had already detected these X-ray sources, but the limited spatial resolution meant that astronomers did not know what the X-rays were coming from. But now, thanks to Chandra's incredible resolution (equivalent to reading a 1-inch newspaper headline at the distance of 2 miles), the scientists could identify the infrared, optical and radio counterparts of the X-ray sources.

The combined analysis of the data in different wavelengths revealed that these four bright sources are, in fact, extremely massive stars. All of these stars are thought to be at least 25 times more massive than the Sun and lie between 7,500 and 18,000 light years from Earth. They are rather rare, and are expected to last only a few million years before they end their lives with supernova explosions.

Finding these very massive stars is not easy. Dust and gas throughout the Milky Way obscures much of the view from optical telescopes. Infrared images suffer less obscuration but are extremely crowded with stars. However, these stars shine brightly in X-ray light and easily stand out from their neighbors.

Why are these massive stars so bright in X-rays? Some massive stars have winds that blow material away from their surface at over 2 million miles per hour. If this high-speed material collides with the wind from a companion star, it is decelerated so suddenly that acts like it has collided with a Solar System-sized brick wall. The shock waves resulting from this enormous collision generate temperatures up to 100 million degrees, and produce copious amounts of X-rays. There are many other unidentified Galactic X-ray sources with X-ray properties similar to these four sources, so a large population of massive stars may remain to be discovered with future Chandra observations.

More information:
- Astrophysical Journal
- Chandra's webpage

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