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The Question

(Submitted March 19, 1998)

How can scientists figure out how old a specific star is?

The Answer

Can you look at a stranger on the street, and guess how old that person is? And how do you do that?

You can do it because you have a good idea of how a person grows and changes with age, and you look for certain physical characteristics of that person.

Astronomers can do something similar with stars. When a star like the Sun is born, it is shrouded in a cloud of dust and gas (planets can form from this cloud). Then it reaches the 'main sequence', where it spends most of its life. The Sun is probably halfway through the main sequence. Then it will turn itself into a red giant.

So if an astronomer sees a star in a cloud of dust and gas, she will guess that it's a young star. When she sees a red giant, she knows that the star is approaching the end of its life. You can tell between a main sequence and a giant, for example, by measuring the spectrum of the star.

There is one important complication: We know that, more massive a star is, the faster it burns up its fuel and the faster it grows. If there is a star 25 times as massive as the Sun, it can't be very old (which may still mean that the star is 3 million years old!).

This is the kind of thinking an astronomer has to use to estimate the age of a star. Of course, this isn't perfect. For one thing, our model of how a star ages may be wrong (it's not likely to be completely wrong, but it does need occasional fine tuning), just like your idea of how a person looks at age 30 could be a little bit off.

Hope this helps.

Best wishes,
Koji Mukai
for Ask an Astrophysicist

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