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Thursday, 29 March, 2001, 09:17 GMT 10:17 UK
Black hole excites astronomers
Black hole R Hynes
The black hole is pulling material off its companion star
(Impression by R. Hynes)

Astronomers have discovered a relatively small black hole in the halo that envelops the Milky Way.

Scientists have already detected a black hole at the very heart of our galaxy, but this is the first time that one of these weird objects has been found outside the main disc of material that sits on the galactic plane.

The black hole is approximately 6,000 light-years from Earth and is being circled every four hours by a companion star. Indeed, it was the rapid movement of the companion that finally led astronomers to pinpoint the presence of the hole.

The discovery should provide scientists with a remarkable opportunity for study. "Because this is so far above the galactic plane, there is almost no interstellar medium between us and the object," said co-researcher Professor Phil Charles, at the University of Southampton, UK. "So, we can study it in detail like no other object in its class."

The discovery was made from observations using the newly refurbished telescope at the MMT (Multiple Mirror Telescope) Observatory in Arizona, US, and will be reported shortly in an issue of Astrophysical Journal Letters.

The companion star is slowly being consumed by the black hole; its gas is being ripped off and pulled over the hole's event horizon by huge gravitational forces.

This generates a powerful burst of X-rays that would normally "blind" telescopes to the presence of the companion. It was in a period of inactivity, or quiescence, that astronomers were able to get a good look at the companion and use its motion to measure the mass of the black hole.

The researchers calculate the hole to be about six times more massive than our Sun. In contrast, the supermassive black hole at the centre of our galaxy is probably about 2.6 million times as massive as our Sun.

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19 Dec 00 | Sci/Tech
Journey to the galactic core
01 Dec 00 | Sci/Tech
Black hole takes 'light snack'
17 Oct 00 | Sci/Tech
Heart of the Milky Way
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