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Last Updated: Tuesday, 5 April, 2005, 11:04 GMT 12:04 UK
Galaxy has mystery star clusters
By Jonathan Amos
BBC News science reporter, in Birmingham

The giant Andromeda spiral galaxy, WASP Consortium
Andromeda: The clusters are sited in its diffuse halo
Unusual clusters of ancient stars have been found in the Andromeda Galaxy, which neighbours our own Milky Way.

UK scientists say the groupings contain hundreds of thousands of stars but are spread out over far greater distances than presently can be explained.

They told the National Astronomy Meeting these extended clusters were not seen in the Milky Way.

"It could go to their different formation histories," Dr Nial Tanvir explained to the BBC News website.

The discovery was made during the course of an unprecedentedly broad and detailed survey of the Andromeda Galaxy, often referred to by the catalogue number M31.

The UK-led team hopes now to answer some of its outstanding questions with time that has been allocated to it on the Hubble Space Telescope.

Similar but different

Astronomers have long been familiar with the so-called globular clusters in the Milky Way. These objects, which sit in the diffuse halo of our galaxy, contain stars that are about 12 billion years old.

These stars famously had scientists scratching their heads for many years because they appeared to be older than the age of the Universe according to some initial cosmological measures.

The Andromeda's clusters occupy similar halo regions, also contain very old stars, but are far less compact - being spread out over a few hundred light-years.

Researchers have the idea they may actually be the remains of small so-called dwarf galaxies, which have been consumed some time in Andromeda's past and are now being slowly pulled apart.

"At first, this sounds quite convincing but the fact is we have dwarf galaxies being pulled apart around the Milky Way and they haven't seemingly produced these extended clusters," said Dr Tanvir, who is based at the University of Hertfordshire.

"Nonetheless, this may be our best bet at the moment."

Growing pains

Whilst also being a spiral galaxy, Andromeda is different from the Milky Way in a number of areas. Along with these clusters which are spread out, there are also very tenuously spread out stars within M31's halo.

The stars are a lot more enriched with heavy metals, too, which means they have incorporated material "cooked" in previous generations of stars; and are therefore relatively youthful.

"Is all this telling us something distinctly different about formation mechanisms? One galaxy may have been involved in a much more major merger of galaxies in the past," Dr Tanvir speculated.

The data for the survey was acquired with the 2.5m Isaac Newton Telescope in La Palma, Canary Islands, and the 3.6m Canada-France-Hawaii Telescope in Hawaii.

A first paper announcing the discovery has been submitted to the Monthly Notices of the Royal Astronomical Society (RAS). Observations of the clusters with Hubble are scheduled for later this year.

The RAS National Astronomy Meeting is being held at the University of Birmingham.

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