Catching up with Blue Stragglers

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Every once in a while, I learn about something that sticks with me just because of the unique mental image. Take blue stragglers, for example.

When I first heard of them in my “Stellar Interiors” class, I pictured a blue star, with a look of determination on its face, struggling up a hill behind a pack of other stars. I know, that doesn’t help at all with the physical explanation, but you are going to remember that.

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What is a blue straggler anyway, and how is it being “left behind?” Astronomers have devised a clever way of determining the age of a group of stars that have all been born together, such as those in an open cluster like NGC 188, shown above.

When a cloud of gas fragments and collapses to form a new cluster, stars of various sizes are born within a relatively short time period. If you measure the color and brightness of each of the stars in the cluster, you get a familiar shape of the famous Hertzsprung-Russell diagram.

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An example Hertzsprung-Russell, or color-magnitude, diagram. Each dot is a star! 

The line of stars that go across the plot from top left to bottom right are called the “main sequence” stars. At the top right are the blue giants that are many times more massive than our sun. As you go down the main sequence, the stars get progressively smaller and less massive as they get cooler and fainter and redder.

Large, hot stars live fast and die young. That means, the stars in the uppermost left corner of the diagram will run our of fuel for stellar fusion first, going supernova, followed by the smaller ones after those, and the smaller ones after those. Stars that are smaller still (eight times the mass of our sun and less) die more peacefully, but they, too, will disappear from their place in the main sequence (and likely show up elsewhere, but that’s a different story).

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The color-magnitude diagrams of two open clusters, M67 (yellow, younger) and NGC 188 (blue, older). 

This makes the color-magnitude diagram a useful tool for finding the ages of stellar clusters. The further down the main sequence that stars begin to show up, the older the cluster is, since all the stars were born at around the same time. It is as if the main sequence is candle that burns down with the passage of time. We say that stars “move off” the main sequence, though they really haven’t physically moved in space at all.

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Now, you might be able to imagine what the blue stragglers are. When you make a color-magnitude diagram of the cluster NGC188, the main sequence cutoff is at a certain point that indicates the cluster’s age.

However, some bright blue stars appear above and to the left this cutoff, thus appearing younger than the rest of the cluster. (See the sprinkling of blue dots in the previous diagram?) But how can that be when star formation happens all at once?

Several hypotheses have been proposed for how these stars “keep their youth.” My favorite involved two smaller stars smashing together to create one bigger star that suddenly burned hotter and more brightly. It turns out, this probably isn’t the case. Instead, astronomers using the 3.5-meter WIYN Telescope on Kitt Peak in Arizona determined that these blue stars aren’t the products of collisions but that they are getting extra material from a companion star.

Though the companion stars were too small and faint to see, the researchers detected a wobble in the larger blue star that indicated that it was in a binary system.

Artist’s conception of the blue straggler system, actually drawn by one of the astronomers!

It’s not too surprising that the collision and merger hypotheses are unlikely for blue stragglers, though they paint a more interesting mental picture.

Even in star clusters, space is woefully empty, so it is extremely unlikely that stars will ever just happen to collide. Instead of spiraling in towards each other in a violent collision, the stars tend to distort each others shape so that a stream of material can be passed from one to the other in a process called accretion.

The end result is a bit tragic as both stars are sped along to a faster death. The larger one becomes a hot blue star that burns through its fuel at a faster rate than before, and the companion gets stripped until all that remains is the leftover core, usually a white dwarf. So, the stars aren’t catastrophically smashing as I always pictured. But as my friend Gail reassured me, “Bah! Smashing is involved on some level. Stars don’t accrete gently.”

I have the best friends.

Images: 1st – Open cluster NGC 188as seen by the 3.5-meter WIYN telescope with the blue stragglers circled. Credit - K. Garmany, F. Haas NOAO/AURA; 2nd Credit – Richard Powell and here; 3rd Credit - Maeder and here; 4th Credit – Aaron M. Geller. Gail Zasowski is an astronomer and my officemate, and one of the coolest people I know.

Tags Space Stars