The Science of Discworld Revised Edition

radiation, and put them into satellites, they discovered that two or three times per day the Earth is illuminated by an intense burst of gamma-rays coming from somewhere out in space. These gamma ray bursts seem to be extremely energetic: there is good evidence that the source of one of them was 12 billion light years away. Even a supernova would not be visible from that distance, so gamma-ray bursts have to be caused by something really serious.
    What? That’s a mystery – perhaps the biggest mystery in today’s astronomy. The best bet is a collision between neutron stars. Imagine a binary star – two stars, orbiting their common centre of mass. Suppose they are both neutron stars. As time passes, they lose energy and fall in towards each other. If you wait long enough, they will come so close together that they collide. This, by the way, is likely to be a very messy business, not at all as simple as two tennis balls sticking together and rounding off. They probably break up and reform. So far, all the gamma-ray bursters we’ve seen are a long, long distance away. But one could light up anywhere. If a pair of neutron stars collapsed on to each other within a hundred light years of Earth, life might survive in the deep seas and the deepest rocks, but the rest of our planet would be dead.
    And we wouldn’t even see it coming.
    Asteroids and comets give you a bit of notice. We have the capability, given a year’s run-up time, to tackle small Earth-crossing asteroids now. We can see them coming and plot their arrival. But gamma-rays are electromagnetic: they travel at the speed of light. They could be on their way now: we couldn’t know. As soon as we did know, we and our technology would be dead.
    Even our own Sun is not trustworthy. The nuclear reactions that make stars burn also make them change, as elements are created or used up, or just reach some critical level that triggers new kinds of reactions. Most stars follow the same series of changes, called the main sequence.
    When the Sun first arrives on the main sequence, it is just like our Sun, with a surface temperature of about 6,000 degrees Kelvin, a light output of about 400 septillion watts, and a composition of 73% hydrogen, 25% helium, and 2% everything else. It stays on the main sequence for ten billion years, until nearly all of its hydrogen has been fused into helium. At that point, its core starts to contract, and becomes degenerate – consisting of closely packed neutrons. Outside the core there remains a shell of hydrogen, which continues to undergo nuclear reactions, which cause the outer layers of the star to expand and cool. The star becomes a red giant, between 10 and 100 times as big.
    The radius of the Sun now is roughly 450,000 miles (700,000 km). At this stage its surface will probably be somewhere between the orbits of Mercury and Venus, and the Earth will already be in serious trouble. But there is more to come. As the core heats, it ignites a nuclear reaction that turns helium into carbon – the very reaction that allegedly is responsible for the existence of carbon-based lifeforms like us. This ‘helium flash’ happens very quickly, on astronomical timescales, and it destroys the degeneracy of the core. Now the core can once more sustain nuclear reactions, but now it burns helium. The outer layers of the star shrink, and become hotter.
    When the helium in the core is used up, the star again burns its nuclear material in two shells: an inner one that burns helium to make carbon, and an outer one that converts hydrogen to helium. The outer layers expand again, and the star becomes a red giant for the second time. Now the outer layers start to blow away, exposing the hot core. The star loses layer upon layer of its material, and shrinks. Finally, the outer layers are all gone, and the core once more becomes degenerate. The star has become a white dwarf.
    Our Sun has about 5.7 billion years left on the main sequence; then:
kerblooie
! Red Giant, and Earth becomes a cinder, or even gets gobbled up completely. But don’t lose any sleep over it. The typical lifetime of a species is 5 million years. We’ll be long gone.
    Planets are not comfortable. Even when life has made its own bed (nice oxygenated atmosphere with an ozone layer to keep out the nasty ultra-violet, nice ooze in the bottom of the oceans, nice long relaxation times for the thermal atmospheric oscillators), there are still plenty of things the Universe can throw at a