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Wednesday, March 3, 1999 Published at 18:07 GMT


Hubble reveals stellar traffic jam

Stars are crashing into each other in the Andromeda galaxy

By BBC News Online Science Editor Dr David Whitehouse

A gigantic stellar traffic jam has been spotted by the Hubble Space Telescope.

The stars are piling up on one side of a super-massive black hole, deep in the heart of the nearest galaxy to our own Milky Way.

Some of the stars pass close enough to each other to pull tendrils from each other. Others may actually collide scattering super-hot gas all around local space.

It appears that, in the distant past, a cluster of stars may have wandered too close to the gravitational grip of the monster black hole that lurks at the centre of this galaxy. Forever captured, they are destined to continuously orbit it in swarms, if they survive the collisions of the cosmic jam.

More stars than the Milky Way

The galaxy in question is the Andromeda galaxy, also known as M31. It is the galaxy nearest to ours but it still it takes about 2 million years for light from Andromeda to reach us. It is faintly visible to the naked eye from Earth.

It is a spiral galaxy, like our Milky Way, but it contains more stars. The estimated number is in the hundreds of billions.

But despite its relative closeness, there has been a puzzle about the Andromeda galaxy for many years.

The first observations in the early 1970s, and later views with the Hubble Space Telescope in the 1990s, showed astronomers that Andromeda has a double nucleus - or two points of brightness - at its very centre. Almost all other galaxies have single nuclei.

Answer in black hole

It is a matter that has confounded astronomers for years. But Thomas Statler, at Ohio University, and his colleagues have studied new Hubble images of the centre of Andromeda that may offer a simple explanation for the double core.

Most galaxies, including the Milky Way, are believed to have a core that contains a massive black hole. Black holes have gravitational fields so strong that not even light can escape them.

Even though a black hole is invisible, astronomers can detect them in the centres of galaxies from their gravitational effect on the millions of stars that orbit them. If the stars are orbiting in a circular disk, as astronomers presume, the stars often look like one point of bright light.

Statler and his colleagues used the Hubble's Faint Object Camera to look more closely at Andromeda's two central points of brightness.

The new observations suggest that the stars in Andromeda are orbiting the galaxy's black hole in a lopsided path and are piling up at the part of the orbit that is farthest away from the black hole.

Go slow jams traffic

"When stars swing closer to the centre, they go faster. When they move away from it, they go slower. It's almost like you're getting a traffic jam at the slow section of the orbit," Statler says.

"One of the bright spots in the nucleus would be the area where the stars are piling up, and the other marks where they rush through on their closest approach to the black hole."

The picture being painted of the heart of Andromeda is one of stellar destruction. Many of the stars sweeping around the black hole won't live out their natural lives. Instead, they are destined to collide with each other and then fall into the central black hole and out of our universe completely.

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