Young stars in an old neighbourhood
Some star clusters are much bluer than expected
Not all star clusters are the same. Look at an open cluster like the Pleiades, for example, and you will see a host of brilliant blue stars. On the other hand, if you turn your attention to a globular cluster, such as M30 in the constellation of Capricornus, you’ll be looking at some of the oldest stars the sky has to offer; young, blue stars are conspicuous by their absence, and these sprawling stellar cities are populated only by redder stars.
Things would once have been different. We think all the stars in a cluster form at the same time, so if you’d looked at M30 some 12 billion years ago, you’d have seen both blue and red stars. The blue stars are more massive, and so the enormous temperatures and pressures at their cores mean that they run through their supply of hydrogen fuel more quickly than their smaller siblings. In turn, that means they leave the main, hydrogen-burning sequence more quickly and reach the end of their lives much sooner. By looking at the most massive stars that still survive within a given cluster, astronomers can work out its age.
Or at least, they should be able to. Nothing is ever quite that simple. Look closely at any large globular cluster, and you’ll find there’s a small population of blue stars scattered in amongst the main, red population. These stars, which seem to be behind the pace with which the rest of the population is evolving, are called ‘blue stragglers’, and a recent paper by Simon Portegies Zwart in Leiden tries to explain where they come from.
There are two main ideas, both of which suggest that the stragglers are the products of violent mergers between two stars. If two smaller stars are in a tight enough binary they will, over time, spiral in toward each other and eventually merge. The result will be the apparently sudden appearance of a massive, and therefore blue, star, seemingly from out of nowhere.
The other route to a merger is even more dramatic. There are thousands of stars in a cluster like M30, and direct collisions between them are not unheard of, especially when the cluster has undergone a process known as core collapse. During this process, which can happen billions of years into the life of a cluster, close encounters between stars lead some to migrate to the cluster’s outskirts while the core becomes denser. A denser core means more stellar collisions, and hence more blue stragglers.
Portegies Zwart built himself a computer version of M30, and set it loose to see what would happen. In the model, a binary merger happens about once every 350,000 years, a process which produces about half the blue stragglers we see. The rest form about 9.5 billion years into the cluster’s life, the result of sudden core collapse making collisions – for a short time – likely.
In the model, both processes are more efficient than we might expect: they produce 10 per cent more blue stragglers than we see in reality. As a result, the author reckons that there are more stragglers hiding in the cluster, and finding them is a direct challenge for observers.
Chris Lintott was reading… “The origin of the two populations of blue stragglers in M30” by Simon Portegies Zwart (Leiden Observatory). Read it online at: arxiv.org/abs/1811.00058
“Look closely at any large globular cluster and you'll find there's a small population of blue stars called ‘blue stragglers’”