Cosmic death beams: Understanding gamma ray bursts
Gigantic explosions of light are reverberating across the universe.
Dr. Michelle Thaller is an astronomer who studies binary stars and the life cycles of stars. She is Assistant Director of Science Communication at NASA. She went to college at Harvard University, completed a post-doctoral research fellowship at the California Institute of Technology (Caltech) in Pasadena, Calif. then started working for the Jet Propulsion Laboratory's (JPL) Spitzer Space Telescope. After a hugely successful mission, she moved on to NASA's Goddard Space Flight Center (GSFC), in the Washington D.C. area. In her off-hours often puts on about 30lbs of Elizabethan garb and performs intricate Renaissance dances. For more information, visit NASA.
MICHELLE THALLER: Walter, you've asked a question about how explosions propagate through space. And of course, the amazing thing is that there are incredibly violent explosions going on all around us. I remember I spent one night at Mount Palomar. And there is a specific telescope up there it was called the Palomar Transient Factory that scans the sky for supernova explosions. And the amazing thing was that in a single night-- just one night being up on top of this mountain in Southern California they detected about 20 supernova explosions. And that's an entire solar system ripping itself apart. In a single night, you see 20 of them. And there are actually more violent explosions still, more energetic explosions called gamma ray bursts. And gamma ray bursts are almost sort of unbelievably violent. In a single flash of radiation, one little object can outshine literally the rest of the known universe, billions of galaxies. It becomes so bright that, quite honestly, we had a lot of trouble explaining where all that energy could possibly come from.
Now, a gamma ray burst, you think, wow, something outshines the entire universe, that must not happen very often. Well, incredibly, with our satellites, we detect about one a day. So all around us, there are these mind-blowingly violent events. By and large, they don't affect us much. And that's because of the distance. You see, the main amount of radiation that comes out of these is in the form of light. And I don't mean just visible light. Gamma ray bursts, as the name suggests, have a lot of gamma rays. Now, gamma rays are very high-energy form of light. And gamma rays are produced when conditions the gas around the object reach the many, many billions of degrees. When you're a billion degrees hot, you actually shine, naturally, in gamma rays. Things like supernova explosions, you get a lot of x-rays, things that are millions of degrees hot. But it's still light gamma rays, X-rays, ultraviolet light, the light that we see, visible light, and then there's lower-energy light, too, like microwaves and radio.
The amazing thing is that there's so many different kinds of light, and we are really blind almost all of them. We see a tiny little bit of light that's available. But life is made of photons, and light travels through space. But there's a wonderful thing called the inverse square law, and that describes how the intensity of light drops as you move farther and farther away. And really what it has to do with is the area of a sphere. If you have a light source, and light is coming away, say, from the sun. The sun is shining in all directions. There's a sphere of photons coming away from the light. As those photons move out into space, they're covering a larger and larger area as they move away. And the area of a sphere is related to the square of the distance away that you are from the object. So the square of the radius, the inverse square law. So if you move twice as far away from the sun as we are now, the light from the sun would drop by a factor of four. We've gone twice as far away, it's four times as faint. These objects are so far away from us that that light is spread over an incredibly large area, and it's really lost any sort of intensity it had. In fact, the challenge is really to detect them at all. All of the radiation that has actually arrived here at the surface of the Earth from space-- there are these giant explosions that are putting off very-high-energy radiation.
But that radiation is expanding out in a sphere, away from that source. By the time it gets here, it's so faint that every bit of energy we've ever collected is about the equivalent to a snowflake. It's very hard to even detect that high-energy radiation at such a great distance. Explosions are not very dramatic. And one of the things that kind of blows my mind is, a couple of years ago, one of our satellites picked up a gamma ray burst from a source about 7 billion light years away. And that means that the light has taken 7 billion years to travel to us. But that light, that explosion, happened before the sun even formed. And that explosion was so bright not just in gamma rays, there was also some visible light that we could see associated with it. And there was a tiny little burst of light you would have seen in the Southern Hemisphere. If you were looking at exactly the right part of the sky at the right time, you would have seen a tiny little faint star turn on and off. There was something that was visible to the naked eye that was 7 billion light years away. That's amazing. Think about the power of that explosion. And one of the things that we know is that if you were anywhere close to a gamma ray burst, things could get very bad, indeed. Supernova explosions don't really seem very violent in comparison.
Even if there are stars in the sky that are on the order of 10, 20, 50 light years away, if one of those went supernova, it would be a beautiful show, there'd be a really bright star in the sky for a while at night, probably bright enough to read by but the radiation wouldn't be damaging to us. It really would probably be hardly measurable. In the case of a gamma ray burst, however, if a gamma burst that violent of an explosion went off anywhere in our galaxy. And one of the things that we now know from observations of gamma ray bursts is that they appear to actually go off in beams. So there must be very intense magnetic fields during this event. And whatever is collapsing the star that's exploding, there's a magnetic pole, and radiation gets beamed up the magnetic poles. So the good news is that if you're not in the way of one of those beams, you're probably safe. So even if a gamma ray burst goes off fairly near to us but we're not in direct line of sight of one of those beams, we're OK. Of course what that means, that we have actually identified stars around the sky that may go gamma ray burst someday, very massive stars that will probably explode in extremely violent ways. One of them is Eta Carinae, in the south. But it looks like the pole of Eta Carinae is not pointing toward us.
There's another star, called Wolf-Rayet 104. And for a while, as we were observing the star, it looked like the pole of the star was actually pointed pretty close to us. But now we have better measurements and we actually think it's not very close. Now, all of these things are not really worth worrying about. But the truth is, we don't understand these very well at all. We don't know the exact angle of the beam. We don't know how wide it is or how narrow it is. So there's no guarantee that, someday, we won't be hit by a gamma ray burst. And the amazing thing, like I said, is we actually detect one of these once a day. But pretty much all of those are in very distant galaxies, galaxies that are really far away, millions or billions of light years away. And yes, we were in the way of the beam, and we saw the gamma rays, but it's so far away that radiation is very, very dispersed by the time it reaches us. So, explosions propagate in the form of light and as long as you're far enough away, the inverse-square law will actually drop down that radiation to a very, very low level. In the case of a beamed event, it may not follow the inverse-square law, because it's a beam of light coming right at you. But they're still very, very faint that far away, and we're safe.
- In a single night, astronomers are able to detect about 20 supernova explosions.
- Gamma ray bursts are even more energetic — in a single flash of radiation they can outshine the rest of the known universe.
- If a gamma burst went off anywhere in our galaxy, it could be disastrous. The radiation would be beamed far and wide. However, as long as you're not in the way of one of those beams — or very far the site of the initial explosion — then you're probably safe.
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