It is not only teenagers who like to congregate in intimate groups and
disturb their neighbours and surroundings.

As Matthew Bate (University of Exeter), will be explaining to the UK National
Astronomy Meeting in Bristol on Friday 12 April, young stars also like to
hang around in crowds and undergo chaotic close encounters with each other
during their formative years.

After performing one of the largest and most complex simulations of star
formation to date, Matthew Bate, Ian Bonnell (University of St Andrews) and
Volker Bromm (Harvard-Smithsonian Center for Astrophysics) have found that
these cosmic furnaces form in a much more chaotic manner than is generally

To perform the calculation, the astronomers used the supercomputer at the
United Kingdom Astrophysical Fluid Facility (UKAFF), a national computing
facility for astronomy sited at the University of Leicester. The calculation
was so enormous that it required 100,000 hours, roughly 10% of the time
available on the supercomputer during 2001.

The simulation followed the collapse of an interstellar gas cloud which was
over one light year across and 50 times the mass of the Sun, eventually
resulting in the formation of a cluster of 50 stars and brown dwarfs.

One of the big surprises found by the astronomers was how chaotic and dynamic
the process of star formation is. The results showed that stars form so close
together that they often interact with each other well before they have grown
to full size.

In the small, new-born stellar groups, the stars compete with each other for
the remaining gas. This process is inherently unfair, with the more massive
stars tending to gather more gas than the lower mass stars, while the lowest
mass stars are kicked out of the group.

About half of the objects are ejected so quickly that they don’t manage to
gather enough gas to become stars at all. Rather, they become brown dwarfs,
objects with less than 1/13 the mass of the Sun. Unable to generate energy by
fusing hydrogen into helium, they cannot continue to shine like the Sun and
quickly fade away.

The new calculation supports recent astronomical surveys suggesting that
there may be as many brown dwarfs as stars in our Galaxy, and indicates that
the high frequency of brown dwarfs is a natural consequence of the
competition between stars during their formation.

Another surprise is that many of the encounters between the stars and brown
dwarfs in such clusters are close enough to strip off the outer parts of the
dusty discs surrounding the young stars. Although many of the discs are
initially very large, by the end of the calculation the majority of them have
been truncated to less than the size of our Solar System.

Since most stars are believed to form in large star clusters, this suggests
that planetary systems like our own may be the exception rather than the rule.

A paper discussing the first analysis of the simulation has been accepted for
publication in the Monthly Notices of the Royal Astronomical Society.

During the UK National Astronomy Meeting, Dr. Bate can be contacted via the
NAM press room (see above) on Tuesday 9 April, Thursday 11 April and Friday
12 April.

Normal contact details:

Dr. Matthew Bate

School of Physics

University of Exeter

Stocker Road



Tel: +44 (0)1392 264126

Fax: +44 (0)1392 264111