* Astronomy

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.

"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" - Dr Christian Knigge, who led the study.

"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" - Professor Alison Sills.

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.

"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" - Dr Christian Knigge.

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

Source University of Southampton

Title: Blue Straggler Stars in the Unusual Globular Cluster NGC 6388
Authors: E. Dalessandro, B. Lanzoni, F.R. Ferraro, R.T. Rood, A. Milone, G. Piotto, E. Valenti

We have used multi-band high resolution HST WFPC2 and ACS observations combined with wide field ground-based observations to study the blue straggler star (BSS) population in the galactic globular cluster NGC 6388. As in several other clusters we have studied, the BSS distribution is found to be bimodal: highly peaked in the cluster centre, rapidly decreasing at intermediate radii, and rising again at larger radii. In other clusters the sparsely populated intermediate-radius region (or "zone of avoidance") corresponds well to that part of the cluster where dynamical friction would have caused the more massive BSS or their binary progenitors to settle to the cluster centre. Instead, in NGC 6388, BSS still populate a region that should have been cleaned out by dynamical friction effects, thus suggesting that dynamical friction is somehow less efficient than expected. As by-product of these observations, the peculiar morphology of the horizontal branch (HB) is also confirmed. In particular, within the (very extended) blue portion of the HB we are able to clearly characterize three sub-populations: ordinary blue HB stars, extreme HB stars, and blue hook stars. Each of these populations has a radial distribution which is indistinguishable from normal cluster stars.

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Title: When Stars Collide
Authors: E. Glebbeek O. R. Pols

When two stars collide and merge they form a new star that can stand out against the background population in a starcluster as a blue straggler. In so called collision runaways many stars can merge and may form a very massive star that eventually forms an intermediate mass blackhole. We have performed detailed evolution calculations of merger remnants from collisions between main sequence stars, both for lower mass stars and higher mass stars. These stars can be significantly brighter than ordinary stars of the same mass due to their increased helium abundance. Simplified treatments ignoring this effect give incorrect predictions for the collision product lifetime and evolution in the Hertzsprung-Russell diagram.

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