The Sun's Death: Sooner Rather than Later?

Adam Frank for McGraw-Hill

There is an old joke where a guy walks into an astronomy lecture and listens to a professor talking about the fate of the Sun. "In a billion years" the professor says "our star, the Sun, will run out of fuel and die". The guy raises his hand and says, "How long did you say we had?" The professor repeats his billion-year prediction. "Whew," says the guy, "I was getting worried. I thought you said a million years!"

As the joke implies, the time-scales involved in the death of the Sun are so long that there really is no point in losing sleep over it. What's the difference to us paltry humans between a dead Sun in a million years or a billion years? If all you are concerned with is the immediate future (say ten thousand years) then the fate of the Sun is not a big problem, compared with paying off your student loans. From the perspective of homo-sapiens as a species, however, it's a very different story. Remember that the dinosaurs lasted for more than a hundred million years. If we are hoping to be more than an evolutionary flash in the pan then the Sun's fate becomes a palpable issue and, as we learn more about the relationship between stars and their planetary offspring, it appears that we have less time not more.

The Sun is made up of more than a billion billion billion tons of matter. All that stuff produces a lot of gravitational force that squeezes the star ever inward. In order to fight that inward crush stars utilize fusion power to hold themselves up. At the Sun's center, where the crush is most extreme, Hydrogen nuclei are repeatedly squeezed together (fused) to form Helium. A little energy is given up in these fusion reactions (via good old E=mc2). The newly liberated energy flows outward toward the Sun's surface heating the outer layers in the process and puffing the star up to counteract the inward force of gravity. As long as the Hydrogen fusion reactions are running along the Sun is safe from gravity and can serve as a source of heat and light for our happy blue planet.

The Hydrogen fuel can't last forever of course. About five billion years from now, the Hydrogen at the center of the Sun will all be converted to Helium. That is when the Sun begins to die and, traditionally, is when most astronomers think the trouble would begin for the Earth. Without its Hydrogen the Sun has to find a new way to liberate energy and fight gravity. The Helium deposits in its core can be fused into Carbon and Oxygen but only if the conditions in the core become much more extreme. To accomplish this, the Sun changes radically. The core contracts and, more importantly, the outer layers swell. In its old age the Sun will bloat to swallow all the inner planets including possibly the Earth. Even if the Earth is not engulfed, the bloated Sun will expand enough to scorch whatever life might remain on our ill-fated planet's surface. There is little escape from our date with doom five billion years from now.

That is the good news. The bad news is disaster could be waiting for us quite a bit sooner than five billion years. The Sun is not constant now and it never has been. Even though our star seems to be in its comfortable middle age it is, in fact, slowly heating up. Every ton of Hydrogen gas that gets converted into Helium forces the Sun to contract just a little bit and that raises its temperature just a bit. In the next 1.1 billion years the amount of energy the Earth will get from the Sun (which is directly related to the Sun's temperature) will increase by almost 10%. That may not seem like much to you, but to a delicately balanced planet's climate far less then 10% can mean the difference between life and death. When scientists examine computer models for a future climate under the revved up Sun they see a Greenhouse effect gone wild. The polar ice caps will be history, and much of the fertile land will be flooded. As the Sun's output continues to increase so much water is evaporated into the atmosphere that even the stratosphere gets wet. Sunlight can then break apart the water molecules allowing the hydrogen atoms to escape into space. No more water, no more life. The world as we know it will have ended.

One of the greatest revolutions in our understanding of the last thirty years is that biospheres are delicate things. While we don't have to start packing yet it is clear that the Earth does not have a comfortable five billion year tenure as life bearing planet. At some point, somebody's great-great-etc grandkids are going to have to start thinking about a move.


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