Young stars in an old neigh­bour­hood

Some star clus­ters are much bluer than ex­pected

Sky at Night Magazine - - BULLETIN -

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’”

M30 con­tains a sur­pris­ing num­ber of young, blue stars for such an an­cient clus­ter. Could a process known as core col­lapse be the rea­son?

CHRIS LINTOTT is an as­tro­physi­cist and co-pre­sen­ter of The Skyat Night on BBC TV. He is also the direc­tor of the Zooni­verse project

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