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ROBERT SIEGEL, host: Who could resist the headline: Mysterious, Green Blob in Space Explained? That comes from a presentation at the annual meeting of the American Astronomical Society.

And we've asked NPR's science correspondent Joe Palca to share the explanation with us. Hi.

JOE PALCA: Hi.

SIEGEL: And before the explanation, when did astronomers first start investigating this mysterious blob, and is blob actually a technical term among astronomers?

PALCA: Some. But I think it's, no, it's probably not, but it's close enough. Anyway, they found this thing in 2007 and it was actually found by a school teacher in the Netherlands named Hanny van Arkel. And she told at the time she knew almost nothing about astronomy, but she was looking around on Brian May's website.

SIEGEL: Brian May?

PALCA: Yes, of course, you know Brian May.

SIEGEL: Who's Brian May?

PALCA: The guitar player for Queen.

SIEGEL: Ah-ha.

PALCA: Who also happens to have a PhD in astrophysics. Anyway, he had a link on his website to something called the Galaxy Zoo（星系動物園）. And this was a project where people were asked to help astronomers identify and characterize galaxies and they were just supposed to look at these things.

So, van Arkel sees this bluish greenish smudge on one of the pictures on the Galaxy Zoo website and she writes to the people, and she says, what's the smudge? And they go, we don't know. So they decided to investigate it.

SIEGEL: Smudge being another term.

PALCA: That's another - yeah, it's one of those.

SIEGEL: Well, what did they find? What did they think it was?

PALCA: Well, they were pretty sure that it was a cloud of gas - a huge cloud of gas almost the size the Milky Way that was glowing from some extremely bright object shining behind it. You can kind of think of it as a fog bank being lit up by a floodlight off to the side.

SIEGEL: They could see the bright object behind or they just inferred everything?

PALCA: No. They couldn't see a bright object. And that's the thing, when you see something being lit up by a bright object, you want to find the bright object. So that was the mystery in the mysterious green blob. So they started looking around and they thought, well, a quasar(類星體) could light this up. That's that powerful thing at the center of galaxies, but they're bright enough, but there wasn't any quasar around.

And they looked for it and looked for it and finally, they think they've come up with the answer, which was there was a quasar around, but it turned off about 200,000 years ago.

SIEGEL: An extinct quasar.

PALCA: It's an extinct quasar. But in the time that it took for the light to get from the quasar to the cloud, it still had enough light even though it's now dead. So in about 200,000 years this cloud will go dark as well.

SIEGEL: So the explanation of the green blob is a former quasar behind a lot of stuff.

PALCA: Behind, or actually, they tell me it's off to the side because the other thing they see are these jets of sort of a jet wind, they call it, coming out of the quasar that happens when a quasar gets sucked into a black hole or ends its life near a black hole and these jets come out and they are actually pushing the cloud around and causing them to actually start to form planets inside this cloud of gas.

SIEGEL: And all this came about thanks to the Dutch schoolteacher.

PALCA: That's right. And just - by the way, if you want to talk to an astronomer about this green blob, you can call it Hanny's Voorwerp.

SIEGEL: Voorwerp.

PALCA: Voorwerp. That's the Dutch word for object. So, Hanny's object, for Hanny van Arkel.

SIEGEL: Live and learn.

PALCA: You bet.

SIEGEL: NPR's Joe Palca. Thank you.

PALCA: You're welcome.

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