Apocalypse When?

Today marks the end of the Mayan long count calendar. So will the world come to an end today? Probably not. But the world will come to an end eventually, and there are a number of scientists looking into various possible ways it might happen.

All life on Earth depends on the Sun. Without its energy plants couldn't grow, and everything would freeze before you could say "it's a bit chilly today". So it is perhaps fitting that the Sun may also be the doom of the planet, in about 5 billion years. Or maybe 6 billion. Our Sun isn't big enough to explode as a supernova, that most spectacular firework of nature, but it will start to get bigger. Much as we fear running out of oil on Earth today, sooner or later the Sun will run low on hydrogen, and will start to change.

Our Sun is performing a balancing act. Its gravity is trying to make it collapse to a tiny point, but the energy from nuclear fusion counteracts it. You can think of it a bit like a balloon, the elastic skin is trying to squeeze it all together, but the pressure of the air inside is keeping it at a steady size. Take away the gravity (elastic) and it would fly apart - but take away the air pressure, and it contracts. When the Sun runs out of hydrogen, this energy will reduce, and gravity starts winning, making the star smaller (our Sun is a star). But it won't make it to a tiny point, because as it gets smaller the pressure gets higher (try compacting a tin can into the size of a pea) and nuclear fusion starts up again - this time, burning helium.

What would our Sun look like as a Red Giant?
Image credit: Hiro Sheridan (Creative Commons)

But as the core of the Sun collapses in this way it also gets hotter, so the outer layers of the Sun will expand and turn the Sun into a red giant - about 250 times bigger than it is now. There has been quite some debate about exactly how large the Sun will get, and what will happen to the Earth. Mercury and Venus will be swallowed, but the Earth may be left intact. As well as being scorched to a crisp, the Earth's gravity may create a 'tidal bulge' on the surface of the Sun, which could eventually drag the Earth down to fiery doom. Either way, you wouldn't want to be around.

Whether or not humanity will survive to see this happen is another question entirely. Or rather, several questions. There are so many possible ways for humanity to meet its grisly end, that it might feel we should be asking "which one will get us first?"

The future is bleak. Image by Julian Povey (Creative Commons)

From nuclear war, to the threat of asteroid impact or the emergence of a drug-resistant flu, scientists around the world are investigating what might happen, how bad it would be, how we might be able to prevent or mitigate it, and how likely it is to occur. Even the UK's prestigious University of Cambridge has formed a Centre for the Study of Existential Risk (CSER) to delve deeper into the issue. According to their own webpage, "Many scientists are concerned that developments in human technology may soon pose new, extinction-level risks to our species as a whole. Such dangers have been suggested from progress in AI, from developments in biotechnology and artificial life, from nanotechnology, and from possible extreme effects of anthropogenic climate change. The seriousness of these risks is difficult to assess, but that in itself seems a cause for concern, given how much is at stake."

Going into each of these topics in detail would be too much for a single blog post. So we'll run a series of posts instead - check back soon for our next installment: "Death from the skies".