Blue stragglers are the weird grandparents of the galaxy

Blue straggler stars are the weird grandparents of the galaxy: They should be old, but they act young. Finding and studying these strange stars helps us understand the complicated life cycles of normal, more well-behaved stars.

All stars follow a particular path in life, known as the main sequence. The moment they begin fusing hydrogen in their cores, they maintain a strict relationship between their brightness and temperature. Different stars will have different combinations of brightness and temperature, but they all obey the same relationship. For example, smaller stars, like red dwarfs, will be relatively dim but also cool, with their surfaces turning a characteristic shade of red. Medium stars, like the sun, will be both hotter and brighter, turning white. The largest stars will be both incredibly bright and extremely hot, making them appear blue.

All stars get progressively hotter and brighter as they age, because the fusion of hydrogen leaves behind helium. That helium gets in the way of all the fusion fun, forcing the star to burn hydrogen at a faster rate to maintain equilibrium. So astronomers say stars "move along the main sequence" as they age.

Related: How do stars die?

Eventually, though, the party stops. Once the star runs out of available hydrogen in its core, it turns to fusing helium — or even heavier elements, if the star is massive enough — at the end of its life. When this happens, the star moves away from the main sequence, coming up with strange new combinations of temperature and brightness. For example, red giants are incredibly bright but have relatively cool surfaces — something that can't happen to a hydrogen-burning main sequence star.

Astronomers can use the main sequence to estimate the ages of stars, especially if those stars belong to a cluster. Stars within a cluster tend to form around the same time, which means that stars begin fusing hydrogen, and enter the main sequence, at roughly the same time.

After enough time, the largest stars begin to die, leaving the main sequence as they do. As more time passes, smaller and smaller stars follow suit. If astronomers build a diagram of the temperatures and brightnesses of all the stars in a cluster, they can see the turning point where stars with masses above that threshold have left the main sequence. Because all the stars in the cluster have the same age, astronomers can use this observation to calculate that age.

That is, except for the blue stragglers. Blue stragglers are stars in a cluster that are very bright and very blue but stubbornly persist on the main sequence. Unlike their more well-behaved siblings, they burn hydrogen long after they should have run out of fuel.

A new chance at life

For some time, astronomers argued that blue stragglers simply weren't members of the star cluster family, either because they just happened to lie in the line of sight of the cluster or because they were captured after the cluster formed. Then their trend-defying nature wouldn't be a mystery, because they simply have different lifetimes than the other stars.

But almost every cluster observed contains blue stragglers, and those stars tend to sit within the centers of those clusters — something that can't be explained by random alignments or capturing. While astronomers don't yet have the perfect explanation for what causes blue straggler stars, they do have a rough guess, and it has to do with collisions.

If two low-mass stars collide, the remnants can sometimes survive as a star on its own. At first, that newly merged star will be both massive and large, with its outer surface flung far away from the core due to the enormous rotation after the collision. But eventually, some astrophysical process (perhaps strong magnetic fields) drags down the rotation rate of the star, causing it to slow down and settle into equilibrium. In this new state, the star will appear massive and incredibly hot: a blue straggler.

Or perhaps the process unfolds more sedately, with one star cannibalizing a neighbor that wandered too close.

In either case, blue straggler stars get a second chance at life. Usually, large stars have relatively short lives. And in a cluster, those stars would have left the main sequence long ago. But because of the dense environment of a cluster, stars tend to merge more frequently than they do in an average galaxy. Those mergers transform small stars into big stars, and only then do they enjoy their hydrogen-burning main sequence lives.

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