Exploring a Cluster’s Stragglers


Though stars within the same cluster all typically form around the same time, they don’t all evolve in the same way. A recent study has carefully explored a population of particularly unusual, straggling stars in the old open cluster Collinder 261.

blue stragglers in NGC 6362

This Hubble image of the globular cluster NGC 6362 reveals a number of stars that appear younger and bluer than their companions: so-called blue stragglers. [ESA/Hubble & NASA]

Why So Blue?

A stellar cluster is typically born in a burst of star formation that creates member stars from the same source material. After the stars form, the cluster ages over cosmic time, its individual stars evolving according to their masses. Bright, blue, massive stars have short lifespans, evolving quickly off the main sequence; dim, red, low-mass stars live much longer and evolve slowly. This difference causes clusters to become progressively redder as they age.

For particularly old clusters, we would not expect to see any bright blue stars, as these should have all aged off the main sequence already. And yet, again and again, we find handfuls of these bright blue stars — in the Milky Way’s globular and open clusters, and even in other nearby galaxies. How do these so-called blue stragglers arise?

Oh, to Be Young Again

formation of blue stragglers

Two possible formation channels for blue stragglers: two stars collide (top), or a star gains mass from a binary companion (bottom). [NASA/ESA]

Since blue-straggler stars are more massive and brighter than expected for their host cluster, we think they must have gained that mass more recently. There are two proposed rejuvenation scenarios that could create blue stragglers:

  1. Two stars collide and merge to form one massive star.
  2. A star gains mass from a close-binary companion.

By studying populations of blue stragglers and exploring these possibilities, we have the potential opportunity to learn about about cluster dynamics and histories, and about binary systems. But blue stragglers tend to lie near the very crowded centers of galaxies — which makes it difficult to observe individual stars and be certain of their membership in the cluster.

Led by Maria Rain (University of Padua, Italy), a team of scientists has now met this challenge using the precise stellar measurements of the Gaia mission in conjunction with spectroscopy from an instrument on ESO’s Very Large Telescope in Chile. The team conducted one of the most detailed studies of a blue straggler population, exploring the open cluster Collinder 261.

Stars of a Different Color

proper motions for Collinder 261

Proper motions for Collinder 261. Black circles are cluster members; blue circles are blue straggler candidates, and orange circles are yellow straggler candidates. [Adapted from Rain et al. 2020]

Aged at 7 to 9.3 billion years, Collinder 261 is one of the oldest open clusters of the Milky Way. Rain and collaborators used Gaia data describing the colors and brightnesses, the proper motions, and the parallaxes of stars in Collinder 261’s field to identify 53 blue straggler candidates and one potential yellow straggler — an evolved blue straggler — in the cluster. The authors then followed up 10 of these stars with spectroscopic measurements, determining that at least five of them are members of close binary systems.

While these data are not yet enough to draw firm conclusions about the origin of blue stragglers, it should be possible to spectroscopically follow up the remaining candidate stars to learn more. This study provides a particularly detailed exploration of these odd straggling stars, which we can hope to build on in the near future.


“A study of the blue straggler population of the old open cluster Collinder 261,” M. J. Rain et al 2020 AJ 159 59. doi:10.3847/1538-3881/ab5f0b