Friday, May 28, 2010

The Ages of Stars

Astronomers have used many tricks to try and measure the ages of stars, but most methods are not very precise. A star like the Sun has a lifetime of around 10 billion years, so knowing the age to within a few billion years is about the best we can expect from conventional techniques. Using new observations from the Kepler space telescope, this week I finished a project that measured the age of a star to within just 50 million years, a precision that has only been achieved for one other star: the Sun.

We can set a lower limit on the age of the Sun just by measuring the age of the Earth. This is usually established by finding rocks containing elements (like Uranium) that radioactively decay, and determining what fraction of the material has been converted into the stable byproducts. The oldest rocks on the Earth are found to have an age around 4.4 billion years, while similar methods applied to meteorites give an age just over 4.5 billion years. This is a good first guess for the age of the Sun, but there are also more direct methods. As a star gets older, it converts hydrogen into helium and energy through nuclear fusion -- this is what makes a star shine. Sound waves generated by the boiling motions of the hot gas near the surface travel through the center of the Sun and reveal the composition deep inside. This technique, known as helio-seismology, implies an age for the Sun of 4.68 billion years.

The Kepler space telescope is observing thousands of other stars like the Sun, and we can also use seismic techniques to measure their ages. For a typical star, astero-seismology can determine the age to within about 1 billion years. However, during certain phases of a star's life we can see an interaction between the sound waves generated at the surface and buoyancy waves (similar to the waves in the ocean) that are trapped deep inside. Essentially the two types of waves couple to one another briefly and influence the tiny changes in brightness that we can measure at the surface. The net effect is like a very precise clock -- by measuring the frequency of the brightness variations, we can tell the age of the star to better than 100 million years. To make such measurements, we rely on computer models of the star -- so any imperfections in our models carry over into uncertainties about the absolute age. It's as if we have a watch that keeps very good time, but we still don't know whether it is set to the correct time zone.

It is difficult to measure the ages of individual stars, since the visible changes throughout their lifetimes are fairly subtle. Whatever the limitations of seismology for determining the absolute ages of stars, this technique can certainly place stars into a precise chronological sequence. By applying the method to many other stars like the Sun we can get a better understanding of what our own star was like in the past, and how it will be in the future.

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