Friday, May 11, 2007

How old is it?

Just like stars in Hollywood, stars in the heavens don't like you to know how old they really are. There are a few clues -- we know that the biggest stars don't live more than a few billion years, and when a star is getting ready to die, we know because it begins to swell up into a red giant star. But for most stars, we can only make educated guesses.

But sometimes we get lucky. Anna Frebel, a postdoc here at the University of Texas, announced yesterday that she has discovered a star nearly as old as the Universe itself. Frebel's luck was in finding a star where she could detect radioactive elements she could use as clocks to measure the age of the star. Her skill comes in recognizing the utility of those elements and being able to make precise measurements, which is very difficult to do.

The heaviest elements in the Universe, elements like uranium, lead, gold, and mercury, are made in the death throes of dying stars. Some are made by slow processes in the atmospheres of red giants, while others are made quickly during supernova explosions. Some elements can be made both ways, while others are only made by one process or the other. But the amazing thing is that the relative amounts of each element produced one way or another doesn't change from one star to the next. For example, for every three atoms of the element europium in a star, you will find one atom of barium.

So, if you can find and measure radioactive elements in a star and can predict how much of that material the star must have started with, you can determine how old the star is. This is just like radioisotope dating on the planet Earth. It's not used that much in stars because the radioactive elements are hard to find -- their "fingerprints" in the light from the star are usually buried by fingerprints of other metals, especially iron. But in some of the first stars, not much iron had been created in the lifetime of the Universe, so those radioactive lines are buried.

What Frebel did was measure the amount of uranium and thorium in her star. These are both radioactive elements, and they decay at different rates. She then compared the amount of those two elements with three elements that are not radioactive: europium, osmium, and iridium. She then calculated how much time had to pass for the "missing" amounts of uranium and thorium to have decayed. And the answer, which gives the age of the star, is: 13.2 billion years.

How accurate is this measurement? There are ways to make mistakes in measuring how much of each element, and there are some uncertainties in the exact ratios of each element that the star would have started with. For any single element, the error is pretty large. But, because Frebel had six measurements (two radioactive elements to compare to each of three stable elements), those errors get reduced in size. So, the age of the star is very likely within a billion years of being correct.

This measurement is an important one. Since this star is in the Universe, we know that it has to be younger than the age of the entire Universe. But because the star has so little iron and other metals, we think it must have been made early in the Universe. Once our galaxy started producing iron, it produced it at a very fast rate.

From studying the echoes of the Big Bang, astronomers have estimated the age of the Universe to be 13.7 billion years. This agrees well with Frebel's age for her star of 13.2 billion years. This is comforting to us as astronomers. Two completely different lines of reasoning give roughly the same answer for the age of the Universe. This makes astronomers more confident that our understanding of the physics behind the formation and early history of the Universe is correct. And there are other even more different observations that give a similar age to the Universe, giving us yet more confidence in this age.

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