Tuesday, October 14, 2008

The Ages of Stars, or My Week in Baltimore

his week I am in Baltimore for a conference entitled "The Ages of Stars." So, as time (and internet) permits, I'll post some of the things I learn during the conference.

Most of the work we do as astronomers involves needing to know how old things are. The ages of things give us tight constraints on the type of physics needed in a certain situation. The classic example of this is our sun.

Throughout history, there have been various guesses as to what powered the sun. Ancient mythology often portrayed the sun as a flaming chariot crossing the sky, while ancient Greek scientists imagined the sun as.

In the late 1800s, we were starting to learn that the Earth was old. At least millions of years old. Physicists by the name of Lord Kelvin and Hermann von Helmholtz came up with the idea that the sun was powered by gravity. If the sun starts as a large cloud of gas, gravity will try and make the gas cloud contract. When a gas contracts, it heats up, causing its pressure to increase. So, the sun would contract under gravity until its internal pressure balances the force of gravity. If you do some basic calculations, you find that the temperature of this gravity-powered sun would be roughly the same as the sun's actual temperature!

As the gravity-powered sun would be as hot as the real sun, it would release heat into space. This heat loss would drop the sun's pressure a bit, and gravity would then cause the sun to contract a little more, causing it to heat back up again, and again stopping its shrinking. This process would continue until the sun finally shrank so small that it would disappear.

If you work out the time it would take the sun to go from a big cloud of gas to an infinitely small ball due to this contraction, it works out to be about 30 million years. For Lord Kelvin & Herrman von Helmholtz, that was great, because the Earth was old, but still younger than the total lifetime of the sun.

Then, early in the 20th century, we learned about radioactive decay. This allowed us to get the true ages of rocks on Earth. And geologists found that the Earth was not millions, but billions of years old. About 4.5 billion years old, to be more exact. Suddenly, Lord Kelvin's contracting sun wouldn't work. The sun needed another energy source, one that could last billions of years.

So, in stepped Sir Arthur Eddington, a British astronomer. Eddington proposed that, since the interior of the sun is so hot, there must be nuclear reactions occurring in the sun. And those nuclear reactions (specifically, the fusion of hydrogen atoms into helium atoms) would release a ton of energy. The sun would have enough energy to last for up to 100 billion years under fusion. It took decades to prove Eddington right (that's a story for another day), but he was right. And his research was spawned by knowing how old the Earth (and therefore the sun) had to be.

In one of those strange twists of nature, it turns out that Kelvin and Helmholtz weren't completely wrong. When stars are formed, they contract from a big cloud of gas to a star, and this contraction follows the basic laws and time scales of Kelvin and Helmholtz.

There are many other examples of the ages of stars and galaxies forcing us to reconsider certain physics. For that reason, we continue to try and determine more accurate ages of stars. And this conference is an attempt to bring everyone who works in this field up to date on the latest research.

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