By Keith Johnson
We have had two months in a row containing a Friday the Thirteenth: perhaps this would be a good time to discuss the end of the world. Astronomers have discovered several ways in which our planet will be come uninhabitable, and even totally destroyed.
The Sun is the culprit in most of these stories (and what follows here is a simplified version). Our home star has been around for about 4.6 billion years. Down deep in its core, hydrogen atoms are fusing together into helium atoms, releasing enormous amounts of energy, keeping the Sun hot and fairly stable. About half of the original supply of hydrogen has been "burned" by now. (The quotation marks indicate that this process is much more intense than the burning of a campfire: these nuclear fusion reactions are more like millions of hydrogen bombs thrown together each second, continuously exploding).
However, something strange happens to the upper layers while all this is going on: they balloon outwards into space. The surface of the Sun expands dramatically, perhaps reaching the current orbit of the Earth. If the Earth were still there, it might be totally engulfed by the Sun, or it might barely escape. Even if the Sun's surface doesn't quite reach the Earth, it's close enough to melt the rocks, evaporate the oceans, make the air oven-hot, and generally make our climate decidedly uncomfortable. The Sun becomes a red giant star.
Fortunately, there is a compensating process going on that may help us out a little bit. While the Sun is expanding, a process that may take a billion years, nearly half of its gas will be gently streaming into space, lost to the emptiness between the stars. This "super solar wind" will make the Sun less massive: it will "weigh" less.
As a result of this solar weight-loss program, the Earth's orbit will slowly drift outward, away from the Sun. Eventually our planet will find itself circling its home star nearly twice as far out as it is now.
Unfortunately, that won't help enough. The Sun will also be getting much brighter as it expands, because it will have a much larger surface to shove its light through. It will eventually become more than 5,000 times brighter than it is today, and no piddling orbit-shift by the Earth will allow it to survive that level of heating.
So can we expect to live on Earth for another 5 billion years? No, that figure is far too optimistic. Long before it approaches red giant-hood, the Sun is slowly increasing its luminosity: it's getting brighter even as we watch. The rate is slow, but our star will be 10% more luminous in about a billion years, enough to cause a "moist greenhouse" effect on Earth. In 3.5 billion years, the solar luminosity will be up by 40%, and we'll be experiencing a "runaway greenhouse" effect. This is already true for Venus, and her temperatures are over 800 degrees Fahrenheit! Three billion years from now, our oceans will be gone. And we'll be gone far before then.
For the sake of argument, let's imagine we can solve the heating problem. There is one final catastrophe we probably can't deal with. Once it gets hot enough to start fusing helium, the Sun will suddenly eject some of its outer layers into space. Well, astronomers call it an "ejection"; the rest of us should probably use the word "explosion."
Only about 5% of the Sun's gas will get blasted into space, but that will be enough to create a beautiful cosmic spectacle: a "planetary nebula." Our solar system will become a prime target for alien stargazers, like the Ring Nebula and Helix Nebula are for us. But that shock wave, speeding through the solar system, will likely be violent enough to wipe out life in the system.
This might be a good time to write to our congresspersons, encouraging them to support the development of warp drive so that we can all to move to some other, younger star. Remember, we only have about a billion years to get it done!
Keith Johnson can show you the Sun becoming a red giant at Rowan University's Edelman Planetarium. He's happy to answer questions about the end of the world and other happy topics at firstname.lastname@example.org or (856) 256-4389. The planetarium presents public star shows on Saturday nights: visit http://www.rowan.edu/planetarium/ for information.