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AAO Press Releases

Thursday, 7 January 1993

Hot massive young stars

Astronomers have discovered a cluster of hot, massive young stars at the very centre of our Galaxy, the Milky Way. The discovery of so many massive stars so close together is unprecedented, and challenges current under-standing of the evolution of our Galaxy &emdash; it leads some astronomers to cast doubt on whether there really is a massive black hole in the centre of the Galaxy, as many scientists have believed.

Using the Anglo-Australian Telescope (AAT), located in northwest NSW, and and its new state-of-the-art infrared camera, IRIS (the Infrared Imaging Spectrometer designed and built at the Observatory), astronomers Michael Burton and David Allen of the Anglo-Australian Observatory have completed the latest chapter in a detective story involving collaborative groups from all over the world.

For this project, observations at infrared wavelengths are essential as there is so much obscuring dust between the Earth and the centre of the Galaxy that astronomers are unable to see the visible light from the stars and gas there. The glow of the infrared emissions, however, is able to penetrate this veil. The new technologies of infrared astronomy enable astronomers to learn what exists in the very heart of our Galaxy.

There have been several phases to this project. In 1986 Allen, with Harry Hyland (now of the University of New South Wales) and John Hillier (now of Munich University), discovered a hot massive star near the Galactic centre using the AAT. An intriguing discovery on its own.

Four years later a group of German researchers from the Max Planck Institute in Munich, led by Dr A Krabbe, brought an infrared camera to the AAT and learnt that this star was the brightest member of a cluster of some dozen massive stars in the Galactic centre.

Finally, using the new infrared camera IRIS, Burton and Allen imaged the infrared radiation from the elements of hydrogen, helium and iron. They demonstrated that the stars found by Krabbe and his colleagues were themselves the brightest members of an even more extensive cluster of massive stars at the very heart of the Galaxy. The radiation from these stars has been travelling for 25,000 years to reach us and comes from hot, bright massive young stars. Some are more than 10 times heavier, and 100,000 times brighter, than our Sun. They have a total light output of some ten million Suns, yet lie within the central light year of our Galaxy (one light year is the distance that light travels in one year, ten million million kilometres). For comparison there are no other stars within 4 light years of our Sun, and none so bright within 600 light years.

These stars have very short lifetimes by astronomical standards, less than 100 million years (the Sun has existed for 5,000 million years, and will last as long again). This means that these stars must have formed very recently, within the last one tenth of one percent of the lifetime of our Galaxy.

Are we observing the centre of our Galaxy at a very special time, or has such massive star formation always been occurring there? We do not know the answer to this question, but the observations are challenging our understanding of the evolution of the Galaxy. For instance, it has led some astronomers to cast doubt on whether there really is a massive black hole in the centre of the Galaxy, as previously believed.

Burton and Allen are continuing their investigations of the Galactic centre, to learn of the range and type of stars that exist there, whether more are forming, and the effect they are having on their environment. Collaborating with scientists from the NASA Ames Research Center and Stanford University in California in this endeavour, they are now combining data from NASA's airborne observatory with that from the Anglo-Australian Telescope, to achieve this end.

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Roger Bell
01 Jan 1998