00:00Two, one, zero...
00:04Who'd have thought the military would end up doing some astronomy?
00:09Back in the 1960s, the Americans wanted to make sure the Soviets
00:13weren't doing any secret nuclear testing.
00:15So they put a whole bunch of satellites up
00:17to detect gamma radiation coming from the Earth,
00:20a telltale sign that had been a nuclear blast.
00:22Well, incredibly, they found many gamma ray blips.
00:25After a while, it became very clear that there were a lot more blips
00:30than they were expecting.
00:32You know, the first one, they were scared.
00:34About number 12, they were like,
00:35I don't think the Soviet Union's blowing up that many nuclear bombs.
00:41And it was a big mystery.
00:42Suddenly, we discovered around the whole sky,
00:44there are bursts of energetic gamma rays coming at us
00:47that maybe last a second or less.
00:49And no one had the slightest idea what they would do.
00:52A few decades of study and a dedicated space telescope
00:57have revealed the blips are due to massive detonations
01:00on the far side of the cosmos.
01:05These are the most powerful explosions
01:08that we have ever seen in the universe.
01:11And the fact that we can see them means
01:13more energy is being emitted in those few seconds
01:16than essentially our sun is emitting in its lifetime.
01:19A single gamma ray burst for a second or two
01:24can outshine the rest of the visible universe.
01:28Rather than put its energy in all directions,
01:32it puts it out as a jet.
01:35When that jet happens to be pointed directly at us,
01:38we see a very bright burst of gamma rays.
01:43What's the explanation?
01:47Well, one possibility is neutron stars.
01:50These are the corpses of dead suns.
01:53Their matter is incredibly dense.
01:56If you could grab a handful, it'd weigh as much as a mountain.
02:00Now, sometimes neutron stars find themselves orbiting each other.
02:06But it's an unstable relationship.
02:08That causes them to get closer and closer under each orbit.
02:12When they get really close together, they go...
02:14They merge and create a gamma ray burst.
02:23Another explanation for a GRB, as they're known,
02:26is an exploding star, a supernova, but one on steroids.
02:31I like to call them super-dupernovae.
02:33They are also known as hypernovae.
02:36But we think these things occur
02:38when stars much, much bigger than the sun die.
02:41And when they run out of nuclear fuel, they have a very exciting ending
02:44because they're going to form a black hole.
02:48As matter is thrust onto the forming black hole,
02:51a prodigious amount of energy is released.
02:55We always think of nuclear energy
02:57being the strongest thing in the universe.
02:58Uh-uh. Gravity is much, much stronger.
03:01And that gravity can take the material
03:04and throw it down onto the black hole.
03:06And so you end up with a nuclear bomb
03:08that is much, much stronger than a nuclear bomb
03:11the size of the sun going on.
03:15So, how likely is a gamma ray burst in our neck of the galaxy?
03:18Well, something like three or four of these
03:20happen every day, somewhere in the universe.
03:22But that translates to about one in every hundred million years
03:26in our Milky Way galaxy.
03:28And for that to happen close to the sun,
03:30well, we'd have to be pretty unlucky.
03:33But you don't know when it's going to be your bad day, I guess,
03:36and that would be a bad day.
03:38The good news is we wouldn't have much notice.
03:40We would just go,
03:41Oh, okay, wonder what that is,
03:43and then that would probably be the end of things.
