Many close-up images of globular star clusters reveal pin-prick blue stars in places where no blue stars have a right to be. These interlopers are called “blue stragglers”, and they’ve fascinated astronomers for decades. That’s because every theory of how stars evolve shows that blue stars in old globular clusters should have disappeared billions of years ago. So where did these blue stars come from? And does their existence prove that astronomers are wrong about how stars work?
First, a bit of perspective.
Just before 1940, astronomers applied the newly-discovered principles of nuclear physics to determine the secret of how stars shine. This work led to a flurry of research over the 1940’s and early 1950’s about how stars are born, how they evolve as they burn through their fuel, and how they might come to an end.
One thing became clear from this work: the life span of a star depends almost entirely on its initial mass. Heavier stars burn much hotter, bluer, and faster than lighter stars. And after a few hundred million years, not long for a star, the big blue stars stop burning fuel and eventually wink out as dim white dwarfs or explode as supernovae and disappear forever. Yellow and red stars, which are much lighter and cooler, last for billions of years. So when clusters get old, that’s the only kind of stars that should remain.
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But in 1953, the great astronomer Allan Sandage discovered hot blue stars in the ancient globular star cluster M3, a cluster we now know to be at least 10 billion years old. Such stars have since been seen in many other globulars, as well as in ancient open clusters like NGC 188. Because they seem to have stayed around much longer than theories predict, these stars got the name “blue stragglers”.
Did these blue stars simply form much later than other stars of the cluster? Astronomers think not. Nor do they believe these are simply foreground stars that do not belong to the cluster
Perhaps a blue straggler is formed when two old, smaller red stars collide or closely interact with each other to create a single higher mass star that shines blue? Possibly. This might explain why there are more stragglers in the dense core of a cluster, where collisions are more likely. But the chances of two lone stars colliding are too small to explain the full population of these stragglers.
Another likely explanation, as it turns out, is that material transfers from one star to another in a close binary star system. The bigger star in the system evolves into a red giant, swells, and loses its outer layers to a smaller red companion star which absorbs the extra mass, squeezes itself tighter, and burns hotter and brighter as a blue star. If this theory is true, many blue stragglers should have small a tiny white dwarf companion, which is all that would remain of the red giant “donor” star. Studies are underway with the Hubble Space Telescope to test this theory.
You won’t see blue stragglers in your backyard telescope. But enjoy images of them from the big scopes, and keep in mind the unexpected dynamical processes that created them.