'Stellar Cannibalism' Is Key To Formation Of Overweight 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,  have published their findings in the journal Nature.

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 with the mass of the cluster core, and inferred a connection to the number of binary stars in a cluster core. This connection is supported by preliminary observations of binary stars in clusters, and 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."