Monday, October 20, 2008

Gyrochronology, the dating of spinning stars

the gyro
Image source: Delia's Restaurants

I like the English language. And it's not just because I'm a native speaker, but also because you can invent words to describe new inventions or ideas, and if it is a good word, people will use it.

A newer word I heard for the first time last week was "gyrochronology." You might guess from the word that it has to do with figuring out how long the food on display in a Greek restaurant has been sitting there -- from the Greek words chronos, meaning "time", logos, meaning "study", and gyro, meaning a tasty sandwich (pictured above). And you'd be close, but not quite right, because gyro is actually a word meaning "to spin," like the rotating spits of meat from which a gyro's filling are cut. So, gyrochronology is actually a word meaning, roughly, the study of ages determined from spinning. Huh?

At the meeting on the ages of stars I attended last week, I learned quite a bit about stars and how we get ages for them. One technique that was talked about was, in fact, gyrochronology.

Most things in the Universe spin. The Earth spins every 24 hours (actually every 23 hours and 56 minutes, but who's counting?). The sun spins about every 25-36 days, depending on whether you are looking at the equator or the poles. The Milky Way spins every 250 million years. And neutron stars can spin hundreds of times a second.

Due to a law of physics called the "conservation of angular momentum," once an object starts spinning, it will continue to spin unless some other force works to slow it down. Stars don't have a lot of forces acting on them, so they continue to spin for billions of years.

Many decades ago, astronomers noted that very young stars tended to spin very fast, as fast as once a day or more. But the sun takes almost a month to spin. This seemed odd, as there was no obvious reason why the sun should slow down. After a lot of head-scratching and wacky ideas, astronomers finally came to think that it may be the sun's magnetic field that acts as a break. The sun's magnetic field extends well beyond the orbits of the planets of our Solar System, and so may be able to interact with other magnetic fields, which should slow the sun down.

In recent years, a handful of astronomers have studied a lot of clusters of stars (groups of stars born at the same time, and so we can use many methods to get their ages). These astronomers, such as astronomer Sydney Barnes of Lowell Observatory, have determined that stars seem to slow their spinning at a fairly well-defined rate. Not only that, but only stars with magnetic fields slow their spinning in time. This is a nice confirmation of the solar spin-down idea, and it gives us a tool to get the ages of stars, if we can measure how fast they are spinning. Thus, we have a new field of astronomy -- gyrochronology.

There is still a lot of work to be done. Although the hypothesis that magnetic fields seem to slow the spinning seems to work, we don't quite understand how it works. And since we don't yet understand the details of how it works, we can't yet make a predictions using the hypothesis. For the time being, we have to calibrate the spin rates of stars using stars with known ages (like stars in star clusters, or the sun). And we have more calibrations that need to be done; except for the sun, we have few limits on the ages of old stars.

So, astronomy gets a nifty, if as yet somewhat mysterious, new tool to use for calculating how old a star is (as long as the star has a magnetic field). And the English language gets another new word, which you can now use to impress your friends and neighbors.

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