AUDIE CORNISH, HOST:
About five years ago, one of the brightest objects in the sky suddenly went dark. It was an astronomical who-done-it and perfect candidate for NPR science correspondent Joe Palca's series Joe's Big Idea. He's been reporting on scientific mysteries and how scientists solve them - today, the case of the disappearing quasar.
JOE PALCA, BYLINE: I'm sure everyone knows what a quasar is, but just in case...
JESSIE RUNNOE: A quasar is the very luminous result of lots of gas and material falling onto a supermassive black hole.
PALCA: That's Jessie Runnoe, an astronomer at Penn State University. Supermassive black holes aren't massive in size. They're massive in mass, very dense mass, so they have a lot of gravity. Anything that gets too close to a supermassive black hole falls in, including clouds of interstellar gas. And when that gas falls in, Runnoe says it doesn't go down quietly.
RUNNOE: Because it can't just fall straight in like when water goes down your bathtub drain. It was to swirl around. When the gas does that, there's a lot of friction, and it heats up. And then it lights up like a light bulb.
PALCA: Like an extremely bright light bulb. Now, quasars are extremely bright, but they're also very, very, very far away, so you need a good-sized telescope to see them. Runnoe and her colleagues were part of a project to study the nature of the light from quasars, something called the quasar's spectral signature. They were particularly interested in how that spectral signature changes over time. For this one particular quasar, they had taken its spectrum in 2003 and decided to take another look last year.
RUNNOE: And when we went back to look, all of the spectral signatures that we normally associate with a quasar were gone.
PALCA: Poof - it seemed the quasar just disappeared. Did they maybe point the telescope at the wrong patch of sky?
RUNNOE: No. We asked that first.
PALCA: Yep, they were looking at the right place, all right. The light from the quasar was just gone. What in the world was going on?
RUNNOE: One possibility is that something just passed between us, you know, that there was a cloud of dust or something in the galaxy and it got between us and the quasar, and so we can't see it anymore.
PALCA: Runnoe said astronomers have seen dust clouds pass in front of bright objects before, but the object's spectrum usually gives a clue when that happens - and in this case, nothing. Besides, another telescope caught a brief glimpse of the quasar in 2009, and it was still shining brightly. No - what Runnoe and her colleagues think happened is the quasar just ran out of gas, literally. All the nearby gas fell into the black hole, and there was not enough left to produce the brilliance quasars are known for. And as crazy as it sounds, the end came quickly from shining brightly to virtually nothing in a celestial blink of an eye, maybe just a couple of years.
RUNNOE: This thing is tens of millions of times the mass of the sun. And so for it to do anything on timescales that we can experience is kind of nuts (laughter).
PALCA: Runnoe says it's a reminder that even though the night sky appears static, things are actually happening all the time. So there are probably more quasars winking off out there.
RUNNOE: And if we can find more and we can study them, we can learn about how gas falls onto the black hole and stuff like that.
PALCA: And that will help solve the biggest mystery of all - how the universe works. Joe Palca, NPR News.
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