# How do scientists calculate the age of a star? It’s not as easy as you think

We know a lot about stars. After centuries of aiming telescopes at the night sky, astronomers and amateurs alike can figure out the key attributes of any star, such as its mass or composition.

“The sun is the only star whose age we know,” says astronomer David Soderblom of the Baltimore Space Telescope Science Institute. “Everything else is calculated from there.”

Calculations based on physics and indirect measurements of a star’s age can provide astronomers with estimates. And some methods work better for different types of stars. Here are three ways in which astronomers calculate the age of a star.

### Diagramele Hertzsprung-Russell

At the beginning of the twentieth century, two astronomers – Ejnar Hertzsprung and Henry Norris Russell – came up with the idea of ​​representing the temperature of stars according to their brightness. The patterns on these Hertzsprung-Russell, or HR, charts corresponded to where different stars were in that life cycle. Today, scientists use these models to determine the age of star clusters, whose stars are thought to have all formed at the same time.

### Rotation rate

By the 1970s, astrophysicists had observed a trend: stars in younger groups rotate faster than stars in older groups. In 1972, astronomer Andrew Skumanich used the star’s rate of rotation and surface activity to propose a simple equation for estimating a star’s age: The rate of rotation = (Age) -½.

This has been the method used by individual stars for decades. But it seems that some stars do not slow down when they reach a certain age. Instead, they maintain the same rotational speed for the rest of their lives.

“Rotation is the best thing to use for stars younger than the sun,” say astronomers.

For stars older than the sun, other methods are better.

### Stellar seismology

The new data that confirmed the rate of rotation was not the best way to estimate the age of an individual star came from an unlikely source: the Kepler space telescope, which hunts exoplanets. Not just an advantage for exoplanet research, Kepler has brought stellar seismology to the forefront, simply looking at the same stars for a very long time.

Observing the flickering of a star can give clues about its age. Scientists view changes in a star’s brightness as an indicator of what is happening beneath the surface and, by modeling, calculate the approximate age of the star. To do this, you need a very large set of data on the brightness of the star – which the Kepler telescope could provide.