Vampires and thrillseekers rejuvenate dead stars

By Phil Plait | December 23, 2009 11:00 am

I have a tale of death, near death, and undeath to weave for you, but first, gaze upon the jewel-like beauty of the glittering denizens of M30:


This image was taken by the Advanced Camera for Surveys on board the Hubble Space Telescope. I had to cut it and compress it drastically to get it to fit on the blog, so you very much want to click on it to embiggen it massively and see it in its fully resolved glory.

The image is of the insanely beautiful globular cluster M30, an ancient city of a few hundred thousand stars located 28,000 light years away in the constellation of Capricornus. The cluster is ancient, about 13 billion years old, making it as old or even older than the Milky Way itself. The core of the cluster is unusually dense as such things go, which is why it was studied. Where better to find vampires and thrillseekers?

Like people, stars are born, age, and die. Stars born with more mass tend to die off more quickly, consuming their fuel at far higher rates than their lower-mass brethren. These stars tend to be blue, so in an old cluster like M30 you’d expect to see no blue stars at all; they should all be long gone. And yet, there are quite a few — astronomers call them blue stragglers. Where did they come from?

One theory, which has been borne out by observations, is that blue stragglers are in tight binary systems, with a dead star in such close proximity to a normal star that it can siphon off the normal star’s gas, using it to rejuvenate itself. This would make them vampires, of course, sucking the life force of other stars in an attempt to stay young.

But another idea was that dead stars might also physically collide with other stars and merge, forming a single star that would burn blue and bright. In an environment like that near our own Sun this kind of collision can almost literally never happen; even considering the entire Milky Way Galaxy over its entire lifetime a head on collision has probably never happened out here in the stellar suburbs.

But that cluster M30 is pretty densely populated with stars, and collisions are far more likely. What observations like this one of the cluster (and also of an ancient cluster called NGC 188) have shown is that the blue stragglers appear to have two different sub-groups; one that appears to have come from the vampire stars, and another from stars that have collided: thrill-seekers, stars that have physically slammed into each other and merged, their combined mass separating them from the other blue straggler group.

Blue stragglers have been known since the 1950s, and the idea that they were pulling gas off nearby stars was proposed to explain them, too, but it’s only with our modern instrumentation that we can not only show that this is true, but that a second, far-fetched-sounding scenario of collisions also contributes.

I find it wonderful and extremely uplifting that an image as spectacular and gorgeous as the one above — it became my desktop wallpaper as soon as I saw it! — not only satisfies our desire for beauty and art, but can also be tapped to deliver incredible science that boosts our awe of what Nature can do. I love that we can understand such things, but you know what I love even more? The idea that we have only begun to understand the Universe.

CATEGORIZED UNDER: Astronomy, Pretty pictures

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