STAR SWARMS COLLIDE IN THE BEEHIVE
One of the most well-known star clusters in the night sky, the 'Beehive' cluster in the constellation Cancer, is probably two distinct clusters colliding with each other according to researchers at the University of Leicester and Queen's University, Belfast. If they are right, it will be the first case of merging star clusters ever to be found by astronomers.
The Beehive cluster is made up of several hundred stars and lies about 500 light years away. Visible to the naked eye as a hazy patch of light in the centre of Cancer (the Crab), it is also known as Praesepe (Latin for 'the manger') or M44. When the Leicester/Belfast astronomers made a careful study of this cluster they found it to be very unusual. Several strands of evidence led Karen Holland, Dr Richard Jameson, and their colleagues to conclude that it is actually two clusters merging. Their results have been accepted by the Monthly Notices of the Royal Astronomical Society and will be published later this year.
One of the first odd things they noticed about the Beehive was that there are two distinct concentrations of stars: it looks like cluster within a cluster. But the Beehive is about 800 million years old and any clumpiness in the distribution of stars when the cluster first formed should long since have evened itself out completely as the stars move around. What's more, the 'sub-cluster' seems to be rather older than the main cluster. The evidence for this is that the emission from its X-ray stars is weaker. Normal stars like the Sun give out X-rays from their very hot outermost layers of gas and, in general, weaker X-rays means older stars.
When the Leicester/Belfast team assessed the energy tied up in the Beehive stars they had another surprise. In a cluster, gravity pulls the stars closer together but the fact that they are moving tends to push the stars apart. It turned out that the Beehive is unstable. Its stars are moving so quickly that they will fly apart over a timescale of only 10 million years. A similar analysis on the 1500 stars belonging to the Pleiades cluster (the 'Seven Sisters' in the constellation Taurus) produced the result that the Pleiades is quite stable as a cluster. So the question is, 'What happened to the Beehive?' An obvious explanation, according to the team, is that two clusters have collided. The energy released by the collision is enough to cause the rapid break-up of both of them.
1. The research team Other members of the research team are Dr Simon Hodgkin and Dr Melvin Davies at the University of Leicester and Dr David Pinfield at Queens University, Belfast. Mrs Karen Holland is a professional scientist but an 'amateur' in astronomy who collaborated with the Astronomy Group at the University of Leicester on this research.
2. Star clusters Most stars form not in isolation but in clusters. Stars formed relatively recently in the history of the Galaxy are found in open (sometimes called 'galactic') clusters. The Beehive and the Pleiades are two of the nearest open clusters to the Sun. As such clusters mature over time, gravitational interactions between the stars transfer kinetic energy from the more massive stars to the less massive ones. This makes it possible for the lower-mass stars to move farther from the centre of the cluster and some escape completely. Ultimately, this process leads to the complete break-up of the cluster. Most stars we see in the night sky were once in clusters that have long ago dispersed.
ISSUED BY: Dr Jacqueline Mitton, RAS Press Officer
CONTACTS FOR THIS RELEASE
Mrs Karen Holland