Date: February 17, 2012
Title: Runaway Stars
Podcasters: Mel Blake & Deb Bailey
Organization: UNA Planetarium
Link: http://www.una.edu/planetarium
UNA Planetarium Facebook Page
@unaplanetarium
Bios: Dr. Mel Blake. I am from Newfoundland, Canada. I received a PhD from York University in Toronto, Canada, and I am currently director of the University of North Alabama Planetarium and Observatory. I am an Associate professor in the department of Physics and Earth Science. My research interests include binary stars and star clusters.
Deb Bailey is an undergraduate honors student from Alabama who majors in professional writing and English. She plans to pursue graduate studies in the fall.
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Transcript:
Dr. B. – Hello. I am Dr. Mel Blake.
Deb – And, I am Deb Bailey.
Dr. B. – And we’re at the University of North Alabama Planetarium and Observatory, and today we’ll be discussing OB runaway stars.
Dr. B. – So, Deb. I read about something the other day that I thought you might be interested in.
Deb – Oh? What would that be?
Dr. B. – OB Runaway stars. Have you heard of them?
Deb – Actually, I just saw something about them in the news a few weeks ago.
Dr. B. – Well, I don’t know if you’re aware of it, but astronomers have actually known about them for about 50 years.
Deb – Gosh. I wonder what took them so long to make the news.
Dr. B. – They’ve been in the news before, but astronomers have been pretty confused about where they come from.
Deb – I can understand that. Can you imagine finding a star that’s hurtling through space faster than anything else? I’d be confused too.
Dr. B. – That would be something to see wouldn’t it? You know, I read that more than 20% of all the massive stars in the Milky Way travel unusually quickly.
Deb – Me too. I found out that some can travel up to 200 kilometers per second. That’s more than 400,000 mph!
Dr. B. – That’s pretty fast isn’t it? It sure does give new meaning to shooting stars.
Deb – Yeah, it really does. I did a little bit of research to find out why they move so quickly.
Dr. B. – What did you find out?
Deb – Well, I read that some astronomers think that they’re made after a supernova explosion in a binary star system. Most stars, such as the Sun evolve slowly over billions of years. However, the greater the mass a star has, the faster it will evolve. O and B stars are the most massive stars in the galaxy, but they don’t last long. They end their lives in violent explosions called supernovae.
Dr. B. – How exactly does that produce a runaway star?
Deb – Well, most stars are formed in binary systems where they orbit each other. When they orbit each other at high velocities, they are held together by mutual gravitational attraction. When one star ends in a supernova, it loses most of its mass, so the gravitational attraction weakens. The remaining star is “kicked” off into space. It’s like if you were to swing a stone around on a string and then cut it all of a sudden. And it doesn’t necessarily just happen in binary star systems. It can happen in star systems with more than two stars. In those cases, there can be multiple stars sent flying through space.
Dr. B. – That’s really interesting, but it’s not the theory that I read about.
Deb – I didn’t know there was another theory.
Dr. B. – There sure is. The second theory is that they are the result of multiple star systems running in to each other.
Deb – I don’t understand how that could cause runaway stars.
Dr. B. – Most stars aren’t like the sun. Like you said, they’re part of binary systems, and when a binary star is in a dense star cluster, their interactions have interesting results. It’s simple, really. Imagine two binary star systems passing by each other. Now imagine that one binary system is larger than the other. The larger system has more gravity, so it could potentially “steal” a star from the other binary system. The remaining star that is left alone gets slung outward. And sometimes, both systems can collide, sending two stars flying through space.
Deb – So it works a lot like the way that we launch some spacecraft into space.
Dr. B. – Exactly. It’s just like when we use the gravitational slingshot maneuver to increase the speed of a traveling spacecraft. The same thing can happen if a single star gets too close to a binary system made of large stars. When the single star gets close enough, the binary system flings it like a slingshot.
Deb – Wow. Two theories. I wonder which is correct.
Dr. B. – Maybe both. They’re both entirely plausible.
Deb – Did you happen to read anything about how they’re found?
Dr. B. – I did. Apparently astronomers can detect bow shocks around some stars. If a bow shock is detected around a star, it is identified as a runaway star, even if its velocity hasn’t actually been measured.
Deb – I imagine the bow shock would be much like that left by a fighter-jet going through the air.
Dr. B. – That’s one way to think about it. Or you could think about it like a ship going through water. Astronomers can also detect high velocity in a star’s movement through examining pictures taken of the star several years apart. If the distance is far enough, it is assumed that the star is a runaway.
Deb – So if they can see the bow shocks and look at pictures they have taken, can’t they trace the
paths of runaway stars back to their origins?
Dr. B. – Yep. And they all lead to dense star clusters. Astronomers have found that most stars form in dense star clusters. We can trace the path of the runaway star back to its original star cluster and find out how old it is. And all these star clusters are young, which is expected because OB stars usually only last about a million years or so.
Deb – So I guess both theories really are possible. But how does a runaway star end? Do they keep going forever or do they eventually end in supernovas too?
Dr. B. – Well, Deb, stellar evolution theory predicts that all OB stars will end in supernovas. So I would assume that they would keep going until they explode.
Deb – So I guess they’ll end right where they began.
Dr. B. – A fitting end for such a spectacular object.
End of podcast:
365 Days of Astronomy
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