The star cluster Praesepe, also known as the 'Beehive' or M44 is familiar to amateur astronomers as a pretty object in the spring sky. Astronomers at Leicester University and Queen's University Belfast now have evidence that it may be two separate clusters in collision.
Praesepe is at a distance of 500 light years from the Earth. With the naked eye it looks like a hazy patch, but even a small pair of binoculars resolves it into hundreds of stars.
When the Leicester and Belfast teams looked at the cluster they noticed that it had two distinct concentrations of stars or 'sub-clusters'. The ages of the stars in the Beehive indicate that it is 800 million years old – the distribution of stars should have smoothed out completely over such a long period of time.
Also, the stars in one of the 'sub-clusters' give off weaker X-ray emission than those in the other. This is another key indicator that there are two separate groups of stars, one much older than the other. The team then analysed the motion of the stars in Praesepe and to their surprise found that it will fly apart completely in just 10 million years – a relatively small amount of time compared with the cluster's age.
The scientists believe that the most consistent explanation for all this is that M44 really consists of two separate clusters which collided at some point – the energy released by this event is causing both of them to break up.