People in the Cosmoknowledge Youtube channel have asked about how astronomers determine the age of stars!

First of all, it’s very hard for astronomers to determine the age of a single star. What they do is look at groups of stars instead. The general estimation is much easier when a star is in a cluster.

This is possible because all of the stars in a cluster typically form at around the same time. But keep in mind that they don’t necessarily all have the same properties.

So now, astronomers determine the mass and luminosity of a range of stars in one cluster because it’s easier to determine their overall age.

Astronomers often use a plot called the Hertzprung-Russell (H-R) diagram to measure the temperature or color of a star versus its luminosity.

Hertzsprung–Russell diagram. Credit: Richard Powell

More massive stars are bluer meaning they are hotter and have a higher luminosity. The smallest stars, on the other hand, are redder, so they are cooler and have low luminosity.

Most stars spend the majority of their lives in the “main sequence” phase, and how long they remain in that phase is directly correlated to the mass of the star. When stars run out of fuel they enter a much less stable period, where they can expand, collapse, go supernova, form neutron stars or black holes.

By knowing the mass of the star we know how much fuel it has. Meanwhile, the brightness tells us how fast it is burning that fuel.

Now that we have all this information we can roughly calculate the age of the star, and then the age of other stars in the cluster.

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But what about lone stars?

Determining the age of stars that aren’t in clusters is much more difficult. But the best way to determine their age is to measure their spin rate.

Stars slow down as they get older — sort of similar to a spinning top, which slows down as it continues to rotate.

To measure a star’s spin, astronomers use incredibly powerful telescopes, like the Kepler spacecraft, to look at dark patches, called starspots, which travel across the surface as the star rotates.

Because of the vast distances, astronomers can’t directly see these spots, so instead, they look for dips in brightness that occur when the spot appears.

Any time a spot crosses the star’s face, it dims slightly. Once the spot rotates out of view, the star’s light brightens again.

Typically, these spots only dim a star’s brightness by less than 1%, meaning the changes are very difficult to measure, but Kepler can handle the task.

The speed with which these dark spots reappear on the surface of the star can tell how fast the star is spinning.

Even though stars slow down with time, scientists still aren’t sure exactly how much.

Now, astronomers measure the spin rates of stars with a known age. What they do is they measure the spin of a star in a star cluster, and then compare that measurement to the spin rate of a similarly-sized lone star. Therefore, we can now determine the age of the star.

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Our 4.6 billion-year-old Sun, for example, has a spin period of 26 days.

But the spin of a star is more complicated than that. Not all parts of the sun, for example, rotate at the same rate.

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