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Did Giant Stars Feed Blue Stragglers?

IRA FLATOW, host: This is SCIENCE FRIDAY. I'm Ira Flatow. If you take a telescope, and you trained it on a cluster of stars, you're looking at a group of stars that formed at about the same time, but they may not all look the same. In some clusters, astronomers have caught a glimpse of some stars that stand out from the others: They burn bluer and brighter, they seem to be younger than they should be. They're called Blue Stragglers.

There are a couple of theories about how Blue Stragglers form. Are they the result of collisions between two stars? Or maybe they came from a merger of stars. Writing this week in the journal Nature, my next guest offers his theory on the origin of Blue Stragglers. Aaron Geller is Postdoctoral Fellow at the Center for Interdisciplinary Exploration and Research in Astrophysics at Northwestern University in Evanston, Illinois. Thanks for being with us today, Dr. Geller.

Dr. AARON GELLER: My pleasure, thanks for having me.

FLATOW: Tell us about - what is a Blue Straggler, and why should we care about it?

GELLER: Well, like you say, Blue Stragglers are stars that we observe to be bluer and brighter than normal stars of similar mass and age. But let me give you a little context for that definition. And normal stars - well, normal stars for instance like our sun - they spend most of their lives having a very similar size and temperature.

And then at a certain point, toward the end of their lives, they become much bigger, up to 100 times the size, and become what we call giant stars. Even later on, they die and become stellar remnants, and for a star like our sun, that would be a white dwarf.

Now, we can predict how long it should take for a normal star to go from, you know, being this normal, kind of steady state, to becoming a giant. And when we look at Blue Stragglers, we see that they are - they have somehow maintained this normal state for longer than we would have expected.

Now physically, we think that this is because Blue Stragglers have gained some extra material from another star, and this extra mass has allowed the Blue Straggler to become bluer and brighter, and therefore it looks younger, if you will, than we should expect.

And this is basically why we call them stragglers, because they're in a sense straggling behind all the other stars that they grew up with in terms of star evolution.

FLATOW: And so what's the mystery about them?

GELLER: Well, they were discovered some 60 years ago, and you mentioned a few of the theories about where they come from. There have been a lot of other theories along the way, as well. And the three that have really survived scientific scrutiny is collisions, like you said; mergers; and also mass transfer.

And so the mystery is really which one of these is really the dominant way of making these Blue Stragglers in certain different environments. And we studied a star cluster, NGC188, and we were trying to determine where these Blue Stragglers came from.

FLATOW: And you discovered - which one of your theories is correct, then, that it sucks up other stuff?

GELLER: So yes, the theory that we think is the - explains most of the Blue Stragglers in our cluster is called mass transfer. And let me explain what that is. You start out with two stars that are in a binary system, so they're orbiting each other. One of the stars evolves to become a giant, like I was explaining before, and they are close enough together that the outer material from this giant star gets, like you were saying, sucked on to the other star, and this other star eventually consumes all of the outer material from the giant and becomes a Blue Straggler.

And in doing so, it - all that's left of the giant is the core of that star, which is a white dwarf. So you're left with a Blue Straggler that has a binary companion that is a white dwarf, and this is what we think we observe in NGC188.

FLATOW: Can we actually see any - if we had our, you know, our backyard telescope out or our binoculars?

GELLER: Well, you could see the Blue Straggler stars. These would be some of the bluest stars and brightest stars in the cluster. Now, you could not see with your telescope the white dwarf companion. It turns out that white dwarfs don't emit very brightly in optical light, in visible, that we can see.

In fact, the Blue Straggler is much brighter than the white dwarf. But if you were to switch to a different wavelength range, for instance the ultraviolet, then the white dwarf is actually quite bright in comparison to the Blue Straggler, and that is when you can actually potentially see these white dwarfs. And this is a project that my collaborator Bob Matthew(ph) and a few others are involved with to get ultraviolet images of these Blue Stragglers with the Hubble Space Telescope.

FLATOW: And what more would you like to learn about all this?

GELLER: Well, there is still a few - so we've looked at many of the Blue Stragglers in NGC188 and said that most of these look like they come from mass transfer, but that's not the whole story. There are still some other Blue Stragglers in the cluster that we don't know very much about, and of course I would love to get the confirmation on these Blue Stragglers that we have studied with the Hubble data. I would love to see the white dwarfs myself.

FLATOW: Why do you call them Blue Stragglers? That's such a demotion it sounds to me of the victorious star.


GELLER: Well, it's true. I guess when they were first discovered, they weren't called Blue Stragglers. It wasn't until a few years later that someone coined that term. Well, they're blue.

FLATOW: Yeah, that we can buy.

GELLER: They're blue stars, and they seem like they are straggling behind normal stars. They're just not evolving as quickly as we would expect. So they look younger, and they're straggling.

FLATOW: And they're better well-fed, is what it is, from the other companion stars.

GELLER: They are indeed, and this actually brings up a funny kind of analogy that people have been throwing around with the Blue Stragglers formed by mass transfer. They're calling them stellar vampires because they feed off of their companions to make them look younger, which is, I guess, kind of timely since we're getting so close to Halloween.

FLATOW: And I certainly can't top that. So I'm going to let you go.


FLATOW: And thank you for taking time to be with us today, Dr. Geller.

GELLER: Thanks for having me.

FLATOW: Aaron Geller is the Lindheimer Postdoctoral Fellow at the Center for Interdisciplinary Exploration and Research in Astrophysics at Northwestern University in Evanston, Illinois. Transcript provided by NPR, Copyright NPR.