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Friday, 27 October, 2000, 10:46 GMT 11:46 UK
Hubble solves cosmic mystery
Stephan's Quintet Esa & NOAO
Part of Stephan's Quintet: Spectacular and intriguing
By BBC News Online science editor Dr David Whitehouse

The Hubble Space Telescope has zoomed in on one of most exciting star forming regions in the Universe and solved a cosmic mystery.

It has imaged a group of galaxies known as Stephan's Quintet, which has been a puzzle for decades.

Some astronomers have suggested that the group questions the idea that the Universe is expanding because one of its galaxies is moving away from Earth at a much lower velocity than the others.

Hubble has resolved this dilemma by showing that most of the galaxies of Stephan's Quintet are not connected.

Distant and distorted

Stephan's Quintet is a group of five galaxies about 270 million light-years away in the constellation of Pegasus.

Stephan's Quintet Esa & NOAO
Not a problem anymore: Galaxy NGC 7320
The galaxy group was the first of its kind to be discovered when it was seen by French astronomer Edouard Stephan in 1877. Today, we know of hundreds of similar groupings, but few are as spectacular as Stephan's Quintet.

Hubble has taken a close-up image of the central part of Stephan's Quintet, giving a magnificent view of a gigantic cosmic collision.

The image shows highly distorted features, dust-lanes crossing between galaxies and long filaments of stars and gas extending far beyond the central regions. The galaxies have been twisted by the violent jostling that has occurred between some of them.

The galaxies float through space with distorted shapes caused by tidal interactions.

Cosmic doubt

Observations of Stephan's Quintet had thrown doubt on the long-established connection between an object's distance and its redshift, the degree to which its light has been "stretched" by the expansion of the Universe.

In 1961, it was discovered that all but one of the galaxies was receding from the Earth at about the same velocity. The discordant galaxy, called NGC 7320, is receding much less rapidly. Some astronomers saw this as evidence that redshift is unrelated to distance, opposing the idea that the Universe is expanding.

Other astronomers countered that NGC 7320 was merely a foreground galaxy, 35 million light-years away, projected on to the more distant (270 million light-years) compact group by chance. And Hubble's data confirms this view.

Mariano Moles, from Instituto de Matematicas y Fisica Fundamental in Madrid, has studied Stephan's Quintet for many years. He said: "It is a personal pleasure for me to see this magnificent image from Hubble. Stephan's Quintet has been a very puzzling object for many years because of the discordant redshift of NGC 7320.

Huge bursts

"Just by looking at this splendid image, it is clear that the redshift discordance of NGC 7320 is now finally resolved. Hubble's resolution is so high that individual stars can be discerned in NGC 7320, showing that it is definitely closer than the more remote, compact group of galaxies."

Other measurements have also shown that two other galaxies in the collection are also just passing and are not bound to the group.

Astronomers have located huge bursts of star formation in the star tails streaming away from the galaxies.

Pierre-Alain Duc, from the Service d'Astrophysique in Saclay, France, said: "The quintet is a prime site for the study of the formation of giant star clusters and even dwarf galaxies. Stars form in bursts triggered by the interaction between the galaxies far from the galactic centres."

See also:

24 Jan 00 | Science/Nature
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