The University of Southampton

Scientists find that ‘stellar cannibalism’ is key to formation of overweight stars

14 January 2009

Astronomical researchers have discovered evidence that blue stragglers in globular clusters, whose existence has long puzzled astronomers, are the result of ‘stellar cannibalism’ in binary stars.

Dr Christian Knigge, Reader in the School of Physics and Astronomy at the University of Southampton, Alison Sills, associate professor in physics and astronomy at McMaster University, and Nathan Leigh, PhD student in physics and astronomy at McMaster, will publish their findings in the journal Nature on Thursday 15 January.

Globular clusters are collections of about 100, 000 stars, tightly bound by gravity, giving them a spherical shape. Blue stragglers are stars within these clusters that are more massive, and appear younger, than the bulk of their counterparts. This violates standard theories of stellar evolution, in which all stars in a cluster are born at the same time. Stars as massive as blue stragglers should have died long ago according to these theories, yet virtually every observed cluster contains some of these overweight stars.

Dr Knigge, who led the study, comments: “The origin of blue stragglers has been a long-standing mystery. The only thing that was clear is that at least two stars must be involved in the creation of every single blue straggler, because isolated stars this massive simply should not exist in these clusters.”

Professor Sills explains further: “We’ve known of these stellar anomalies for 55 years now. Over time two main theories have emerged: that blue stragglers were created through collisions with other stars; or that one star in a binary system was ‘reborn’ by pulling matter off its companion.”

The researchers looked at blue stragglers in 56 globular clusters. They examined the number of stars in each cluster and how that number scales with some key parameters of the cluster.

They found the total number of blue stragglers in a given cluster did not seem to correlate with the predicted collision rate – dispelling theory number one.

They did, however, discover a connection between the total mass contained in the central cores globular cluster and the number of blue stragglers contained in these cores. Since more massive cores also contain more binary stars, they were able to infer a relationship between blue stragglers and binaries in globular clusters. They also showed that this conclusion is supported by preliminary observations that directly measured the abundance of binary stars in cluster cores. All of this points to ‘stellar cannibalism’ as the primary mechanism for blue straggler formation.

Dr Knigge says: “This is the strongest and most direct evidence to date that most blue stragglers, even those found in the cluster cores, are the offspring of binary stars transferring matter. In our future work we will want to determine whether the binary parents of blue stragglers evolve mostly in isolation, or whether dynamical encounters with other stars in the clusters are required somewhere along the line in order to explain our results.”

This discovery comes as the world celebrates the International Year of Astronomy in 2009.

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