Hot young stars baffle astronomers

Andromeda head-scratcher

New pictures from Hubble have helped astronomers identify the source of a mysterious blue light that emanates from a supermassive black hole at the centre of the Andromeda galaxy. However, in typical space-science fashion, the discovery poses more questions than it answers.

It turns out that the blue light is coming from a disk of young, hot stars (très Hollywood) that are orbiting the black hole, much like the planets orbit the sun in our solar system. The discovery has baffled astronomers, who are at a loss to explain how the stars could have formed in such a hostile environment.

The blue light in M31 was first observed, with Hubble, in 1995. Further observations three years later suggested that a cluster of stars was responsible for the glow.

These latest observations from the Hubble Space Telescope's Imaging Spectrograph allow astronomers to say that the light is coming from more than 400 stars packed into a disc no more than one light year across. This disc is encircled by a disk of redder, cooler stars that have been seen before.

The stars are orbiting the black hole at around 3.6 million kilometres per hour, fast enough to orbit the Earth in 40 seconds, and to journey from Earth to the moon in six minutes.

But the mystery is not solved. Now astronomers have to explain how the stars came to be in this environment in the first place.

"Seeing these stars is like watching a magician pulling a rabbit out of a hat. You know it happened but you don't know how it happened," said Tod Lauer of the National Optical Astronomy Observatory in Tucson, Arizona.

Lauer goes on to explain that these young stars are so short-lived, the chances of them happening to form just in time for us to see are remote. He suggests that there must be some mechanism that has triggered other similar clusters in the past, and will do so again in the future.

"We still don't know, however, how such a disk could form in the first place. It still remains an enigma," he concludes.

Despite the continuing mystery, the researchers say that this data rules out alternative theories, and confirms that the dark mass at the core of M31 is indeed a supermassive black hole. ®

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