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Scientists Study Final Moments of Dying Star

May 21, 2008 at 6:50 PM EDT
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The deaths of some stars are marked by a unique event: the supernova. A NASA telescope captured a massive explosion in January, giving scientists clues into the mechanics of our own world. Robert Kirshner of Harvard University explains the significance of the event.
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JEFFREY BROWN: One researcher called it “winning the astronomy lottery.” Back in January, astronomers were looking at the remnants of one star when they detected the death of another and the beginning of a supernova as it was happening.

As seen in this animation, the supernova was created when the core of a massive star collapsed — shown here in blue — and then sent out shockwaves leading to an explosion of X-rays and light.

After it cooled, its remnants formed what’s called a neutron star.

To help us understand more, we turn to one of the researchers involved with the project, Robert Kirshner, a professor of astronomy at Harvard University.

So, first, why is it so exciting to actually see this as it’s happening?

ROBERT KIRSHNER, Harvard University: Well, ordinarily, when we find supernovae, the event has already taken place, because we find them usually by the optical light, the visible light, the ordinary light that our eye is sensitive to.

But what’s unique about this particular supernova is that it was found by the X-rays that it emits. And it turns out the X-rays come first. They’re the signal of the actual energy from inside the star hitting the surface. And this is the first time that a supernova has been discovered from its X-ray emission.

JEFFREY BROWN: All right, tell us a bit more about what you’re actually seeing, which is to say, tell us, what is a supernova? What’s happening?

ROBERT KIRSHNER: A supernova is the death of a star. And in this case, it’s the death of a massive star, maybe 10 or 20 times as massive as our own sun.

Ordinary stars, like the sun, get their energy by nuclear fusion. They change the hydrogen that they’re made of into helium. More massive stars do that, too, but they keep on going and they burn fuels all the way up to iron.

It turns out that there’s something special about iron, and it’s the element out of which you cannot get any more energy by nuclear fusion. So what happens in a star, when it’s made iron in its core, is that it’s poised for disaster.

And this star collapsed on its inside, fell in under the force of gravity with a tremendous release of energy, equivalent more or less to the whole energy output of the sun for its 10-billion-year life, all of that energy coming out in a few seconds.

And a blast wave, a powerful shockwave went out through the star. And when that shock hit the surface of the star, it made the surface very, very hot, so it would emit these X-rays, these very short photons, these very high-energy photons that don’t come through the Earth’s atmosphere.

So you needed a satellite in order to see this first moment when the shockwave slaps the last little bit of gas in the star.

And there was one. There is this satellite that NASA has built called Swift. And it was being used by Alicia Soderberg to study another object in that galaxy. And by the best of good luck, she saw this burst of X-rays, this enemy coming out from the birth of a supernova.

Truly cosmic in scale

JEFFREY BROWN: Everything about this is kind of massive, as you say, and sort of mind-boggling. It's also the case, of course -- if I understand this right -- you're looking back through time, right? The event you're actually looking at took place -- if I understood right -- 88 million years ago?

ROBERT KIRSHNER: Yes. The distances involved are very, very large. The light travels at the speed of light, which is about a foot in a nanosecond, in a billionth of a second.

So you never see the world the way it is; you always see the world the way it was.

Now, for ordinary distances, you see it as it was a few nanoseconds ago. But with astronomy, with telescopes, the telescope is a real, no-nonsense time machine that lets you see light that was emitted in the past.

And that star blew up 88 million years ago. The light traveled at the speed of light and got here on January 9th.

JEFFREY BROWN: January 9th it arrived, huh? Tell us...

ROBERT KIRSHNER: Yes. And if we'd been a little farther away, it would have been gotten here in February. And if we'd been a little closer, it would have been here in December.

And this flash of X-rays is only five minutes long. If the satellite had not been looking at that galaxy at that time, it would have been missed.

These things are quite rare. They only take place about one in 100 years in a galaxy. So Alicia was very, very lucky, and all of us were lucky that this thing got found.

JEFFREY BROWN: And now that you've seen it, does it open up some new areas of research? What happens next?

ROBERT KIRSHNER: Well, the interesting thing, is that, although the X-rays are brand new and novel, the supernova was pretty ordinary of its kind. It's a kind of supernova in a massive star where the star has lost its outer envelope somehow or, anyway, doesn't have it at the time of the explosion.

It's probably pretty compact. And we're really interested in finding out what the stars are like when they explode and by what mechanism they explode.

One of the really interesting things about supernovae is that, when the explosion takes place, the elements that had been formed inside the star -- so the calcium or iron or other elements that are made in the ordinary course of the star's burning -- plus, new elements that are synthesized in the explosion itself, those elements get blasted out into the gas between the stars.

And they become the raw material for the next generation of stars and for planets and for people. So the calcium that's in your bones and the iron that's in your blood -- or if you're wearing gold jewelry -- those atoms, the actual atoms that you're wearing were manufactured in supernova explosions that took place early in the history of our galaxy, before the sun formed and before the Earth formed.

So you are really star material.

JEFFREY BROWN: A good professor ties what happened out there to us right here on Planet Earth. Robert Kirshner, thank you very much for joining us.

ROBERT KIRSHNER: My pleasure. Thank you.