Friday, February 29, 2008

It's the end of the world as we know it

A red giant star threatens to swallow a planet
Image Credit: Dirk Terrell

There is a Robert Frost poem quoted far too often by astronomers, Fire and Ice, that begins:

Some say the world will end in fire, Some say in ice...

The poem very naturally leads into a discussion of the fate of the Earth when our sun dies. And, since a recent news story picked up on some new research regarding the Earth's fate, I thought I might as well use the poem myself for an easy-to-write blog post.

The sun is the key to all life on Earth. It provides us with energy, with warmth, and even with protection from some dangerous cosmic radiation. Every day we see the sun rise, every day we see it set, and for eons, it seems that the sun has not changed.

Astronomically speaking, this view is almost correct. The sun does change slightly from year to year due to its sunspot cycle, but its brightness is amazingly constant. This is good for life on Earth -- if the sun were to get much brighter or much fainter, we would experience drastic climate changes.

The source of the sun's constancy is its nuclear furnace, buried deep in the core of the sun. Here nuclear fusion is going on, as the sun burns its giant supply of hydrogen into helium. (We've actually detected subatomic particles from these fusion reactions called neutrinos; in 2002 Raymond Davis and Masatoshi Koshiba were awarded the Nobel Prize in Physics for discovering neutrinos coming from the sun). The sun has enough hydrogen to power itself for nearly ten billion years, and the sun has only been shining for about 4.6 billion years, so there's a long way for the sun to go!

However, when the sun starts to run out of fuel, it begins to realize that there is a problem. Gravity will pull the helium ash from the sun's nuclear reaction inward, heating it up. And this extra heat will cause the sun's outer layers to swell up, past the orbit of Mercury and toward the orbit of Venus. As the sun's outermost layers swell outwards, they will cool, turning the sun from a soft yellow to a robust red color. The sun has become a red giant. The Earth will still be around, but the extreme brightness of the red giant sun will boil our oceans away and perhaps even melt the Earth's crust -- not a pleasant place to be!

Eventually, gravity squeezes the helium enough that it ignites nuclear fusion, with three helium atoms burning to form one carbon atom. This new source of energy will keep the red giant sun (which actually will shrink a bit at this point) going for another billion years or so, but when it runs out of helium, the sun will swell up again, this time bloating out to Earth's orbit. And after a short time, the sun will blow off its outer layers, spewing half of its mass out into space, and leaving behind a white dwarf star. If the Earth survives, it will go from being hellishly hot to bitterly cold, as the feeble light of the white dwarf sun slowly fades away.

So, what will happen to the Earth? The red giant sun will reach out beyond Earth's orbit, but the sun will also be losing a lot of mass. This reduces its gravitational pull, allowing the Earth to move a little further away from the sun. Our best calculations put the Earth right on the edge of the red giant sun, with some people saying the Earth will be swallowed by the sun, and some saying the Earth will escape. Fire or ice?

So, back to the news article. Two astronomers, Robert Smith of the University of Sussex and Klaus-Peter Schroeder of the University of Guanajuata, modeled the red giant Sun using some of the most up-to-date physics available. They also included some physics that they had previously neglected, such as the aerodynamic drag of the sun's atmosphere on the Earth. The result of their new calculations: the Earth will be swallowed by the red giant sun and vaporized. The Earth will end in fire.

So, is this the final word on the subject? I don't think so. Although these models are some of the most sophisticated yet used to determine the fate of the Earth, there are some things we still don't fully understand about the death throes of stars. If the sun loses more mass than these models predict, the Earth may move far enough away from the red giant sun to avoid being swallowed. And we don't really know exactly how much material the sun will lose and when -- these details could make all the difference!

I don't know about you, but I don't want the Earth to be swallowed by the sun, even though all life on Earth will have been extinct for a couple billion years. Call me sentimental. But, like any science, it doesn't matter how badly I want something, we have to let the facts speak for themselves. And, at least for now, the facts spell doom for our planet.

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