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GUY RAZ, host:

If you look up in the sky on most nights, even in a big city with lots of light, one of the few stars you can see with your eyes is Epsilon Aurigae. It's big and very bright, except once every 27 years or so, when it dims dramatically and for more than a year.

Epsilon Aurigae is actually two stars spinning around each other, and the latest eclipse began last summer. Now, astronomers have been puzzling over these drawn-out eclipses for centuries. But this time around, using NASA's Spitzer Space Telescope, they might have solved the mystery.

And Don Hoard from the California Institute of Technology has been researching the phenomenon.

Welcome to the program.

Mr. DON HOARD (Research Scientist, California Institute of Technology): Thank you.

RAZ: Before we get to the mystery, because we want to keep our listeners in suspense, tell me about this star. If I wanted to find it, where would I look?

Mr. HOARD: Well, if you were here in Washington, if you were to go out tonight at about 10 o'clock and look almost straight up, a little bit to the north of the exact straight up position above your head, that's where it would be. It's near a very bright star called Capella.

RAZ: So according to your new research, what causes the eclipse?

Mr. HOARD: So about two eclipses ago - so in the mid-1950s, early 1960s - the idea was developed that there's probably a disc formed of gas and dust that surrounds one of the stars. And the reason the eclipse lasts so long is because this disc is passing in front of the other star - the brighter star in the system - and it just takes a long time for it to go past.

And around the time of the last eclipse, 27 years ago, two competing theories were proposed, and both of them involve the eclipsed star, the one that we mainly see the light from, being a very large star. And the main difference in the two competing models was the nature of the primary star.

It was proposed to be either what's called a supergiant star, which is a sort of normal type of star, but it has a mass about 20 times the mass of our sun and will probably end its life as a supernova.

And the other theory is that the eclipsed star, this big bright one, was essentially a dying star...

RAZ: Mm-hmm.

Mr. HOARD: ...something with much lower mass, about two times the mass of our sun, and a star that's on its way out the door.

RAZ: Like a flickering light bulb.

Mr. HOARD: It's - yeah, it's not going to last much longer.

RAZ: So now, based on this new information you have from the Spitzer Space Telescope, what did you determine?

Mr. HOARD: Initially, I and my collaborators, I think, all went into this sort of thinking that the massive star model was probably the right one because it's sort of the least unusual model.

RAZ: Mm-hmm.

Mr. HOARD: So we initially tried reproducing the data that we were seeing using a model where the eclipsed star is very massive. And we kept running into problems. The model kept having to get more and more complicated. And so, we kind of, you know, stopped doing that and looked at the competing theory. And when we start from there, all of a sudden, everything just falls into place.

And so, what we've really done here is just really strongly tipped the scales in favor of one of these two competing models. And this tells us a lot about sort of the big picture of Epsilon Aurigae.

RAZ: That's Don Hoard. He's a research scientist at Caltech. And he led a study investigating the mystery of why the star, Epsilon Aurigae, is eclipsed for so long.

Don Hoard, thanks so much for coming in.

Mr. HOARD: Thank you.

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