Artist impression of a gamma ray burst exploding near the Earth. Image credit: NASA. Click to enlarge.
We live in a dangerous Universe. Our tiny home planet is at risk from many extraterrestrial threats: asteroid strikes, solar flares, rogue black holes, supernovae. Now add gamma ray bursts to the list – those most powerful explosions in the Universe. Even 10 seconds of radiation from one of these events would be a deadly setback to life on Earth. Before you start looking for another planet to live on, Dr. Andrew Levan from the University of Hertforshire is here to explain the probilities of a nearby explosion. It looks like the odds are in our favour.
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Fraser Cain: Now, I want to learn how safe I am from gamma ray bursts, but first can you give the explainer on what these explosions are?
Dr. Andrew Levan: Gamma ray bursts were really a mystery for much of the last 30 years. They were first discovered in 1967 by satellites which were launched to search for evidence of nuclear tests going on in space. So in the 1960s there was worry on both sides – the Russians and the Americans – we’re worried that the opposing side might be testing nuclear weapons somewhere in space. And so there was a test ban treaty that banned this and then various satellites were launched to be able to detect the signature of these tests. And these tests would have given a signature that would have been a bursts of gamma rays. And so the satellites were launched to search for this. They never actually saw any gamma rays from nuclear tests, but what they did find were these very bright explosions that were happening nowhere in the Solar System. Not associated with anything that was happening that was obvious; not really the Moon or any of the planets or anything like that. And so these were the first discovered gamma ray bursts.
For most the next 20 or 30 years, that was really all that we knew about them; these strange unexplained flashes of high energy radiation. This is light with wavelengths much shorter than X-rays that medical images use. And they were very difficult because of that to pinpoint them. So we really didn’t know where they were, whether they were anywhere near us or whether they were a long way away. And then in the late 1990s, finally we succeeded in pinpointing their origin by optical emissions, by normal light, and that showed that they were incredibly bright explosions which happen in the distant Universe, so you’re talking about looking right back to only a few hundred million years after the Big Bang – 95% of the way back through the age of the Universe.
And so, that was sort of the first breakthrough. And then over the next few years, it was realized that these gamma ray bursts were actually caused by the collapse of a very massive star. So when you’re talking very massive, you’re actually talking about 20-30 times as heavy as the Sun. And what happens with these stars is that they burn, or fuse, hydrogen into heavier elements at their cores. And eventually that process stops, they fall into themselves, form a black hole, and it’s that process which creates a gamma ray burst.
Fraser: That sounds very similar to the process of a supernova explosion. So, what’s the difference?
Dr. Levan: Well indeed, many gamma ray bursts are supernova explosions. So they are just a subset of supernova. Supernova happen when stars more massive that 8 times the mass of the Sun run out of nuclear fuel and collapse, but most of the time they form a neutron star rather than a black hole. Now a neutron star is just slightly less extreme an object, but it’s still very extreme. And so it is more or less the mass of the Sun, but collapsed into a region only 10 miles across. But what happens there is that you actually get a lot less energy out. And so when you have these very massive stars that become gamma ray bursts, the energy from these gamma rays is launched in a jet. So it’s like a hosepipe being pointed straight at you, and it goes basically out the poles of the star at either end. It illuminates the sky as a very bright source. But it only illuminates perhaps a few percent of the sky. And that is where the gamma rays are emitted, and that’s what makes a gamma ray burst. And only a few types of supernova are those which create both the black holes and the necessary conditions to create a jet are those that create the gamma ray burst. And then the gamma ray bursts are much much brighter than the normal supernovae that we see.
Fraser: And being nearby these is a pretty dangerous place to be. How risky is it, and how far out is the sphere of destruction?
Dr. Levan: People talk about supernovae and they talk about gamma ray bursts as being dangerous to the Earth. For a supernova, it really has to be very close; it has to be within about 10 parsecs of us (or 30 light-years). There really aren’t very many stars in that. Now with gamma ray bursts is so much more luminous that it could be 30 or 40,000 light years away from us. So that’s halfway across the galaxy. If one went off in the centre of the galaxy and it hit the Earth, then that would be an incredibly dangerous thing for us. Because what would happen is the high energy radiation would hit us would ionize the high atmosphere and create lots of new, quite nasty, nitrogen oxides which would create acid rain. It would destroy the ozone layer, and at the same time, it would shower the side of the Earth facing it with an incredibly high dose of ultraviolet radiation.
Fraser: If one of these goes off in your galaxy, that’s a huge setback for life. I can’t imagine much that could withstand that, apart from the microbial life underground.
Dr. Levan: Yes, absolutely, it really does. The impact for us is that you would have the rather paradoxical situation that the nitrogen oxides that were created in the atmosphere could actually block the optical light, so you’d have global cooling. You’d have problems with plants photosynthesizing and stuff like that. But at the same time because you have the ozone layer being destroyed, you’d have a high flux of ultraviolet light that would really be damaging to any life that encountered it. And so it would drastically affect the process of evolution. Whether it would be possible for us to evolve sufficiently to live through that is very unlikely.
Fraser: Do scientists think that’s responsible for some extinction events in the past?
Dr. Levan: There’s been a lot of discussion about this. Obviously the most talked about extinction is that of the dinosaurs and a lot of people now believe that it was probably an asteroid hit from outside the Earth or something like that. There certainly was an extinction event about 400 million years ago which people have talked about perhaps being due to a gamma ray burst. Obviously it’s very uncertain when you look back and you’re trying to look through the fossil record, but certainly gamma ray bursts have been talked about because of the fact that they’re less common than supernova, they can affect you over such a big volume the Earth that people have talked about past extinctions being due to gamma ray bursts.
Fraser: Okay, now I’ve been promised some good news. Lay it on me.
Dr. Levan: What we’ve done is study a lot of these bursts, about 40 of them. Now these are gamma ray bursts that you can relax, they’re so far away that they’re actually difficult to see with even the biggest telescopes in the world. But what we can study from them is the type of galaxy in which they happen. And so the Milky Way, which is our galaxy, is called a grand design spiral. It’s a great big, very massive galaxy. Now when you look at the types of galaxies these tend to occur in, you find that they’re always in these small, messy, very irregular galaxies which have a very low mass, which are very unlike the Milky Way. And the reason for this is that the Milky Way has lots of what we call metals. Now when astronomers talk about metals, we don’t really mean things like aluminum or iron, or things like that. We really mean anything heavier than hydrogen or helium. And so in order to have life, you have to have carbon and oxygen and things like that which are very rare in the little galaxies that have gamma ray bursts going off. And so what you realize when you look at it is that little galaxies are vital to creating gamma ray bursts because what you need basically is very massive stars that form black holes, and it’s much easier to do that in these little galaxies that have very few metals. And what that essentially means is that although we’ve had that in the past, gamma ray bursts just don’t happen in galaxies like our own.
Fraser: I know that some recent research shows us some star forming regions in nearby satellite galaxies to the Milky Way that are building up stars that are 50-80 times the mass of the Sun, so are those good candidates or is there something about the heavier elements?
Dr. Levan: Yes, so there’s something very specific about the heavier elements. When you have heavier elements in a star, it actually effects the evolution of the star very fundamentally. And so what happens is that these heavy elements have what we call stellar winds; quite strong stellar winds. And what this means is that they push off all of the material that’s outside them. So although they start their lives as very massive stars, by the time they end their life, they’ve actually lost much of that mass that they’re no longer massive enough to form black holes. And so they actually form these neutron stars as normal supernovae. So there’s very little doubt that these massive stars that you see and the massive star forming regions that you see are going to form supernovae, because they’re much further away, they’re no threat to us. And because of their stellar winds, they will lose so much of their mass that they can’t make black holes and so they can’t make gamma ray bursts.
Fraser: Since all of the gamma ray bursts have been seen across the Universe, is it almost like a function of age – as you look further away, you’re looking back in time. We used to have gamma ray bursts, but they just don’t happen anymore.
Dr. Levan: Yes, very much so. Obviously, as stars evolve, you make your first generation of stars. All of the metals, all of the atoms that you see around you, in your body, in the building, and everything like that, are made from supernova explosions in the past. They enrich everything around them, and then there’s another generation of stars that are made from that, and so on. And so when you look back into the Universe, there were less of these metals around, and less of these heavy elements, and so the early Universe is a much more promising place to look for gamma ray bursts than the Universe as we see it now where only gamma ray bursts occur in little galaxies where there hasn’t been so much star formation for so long as there has been in the Milky Way.