How do scientists calculate the age of a star?

It’s not as easy as you’d think

telescope image of a star cluster, which is surrounded by a blue haze

Clusters of stars like this one, called NGC6405 or the Butterfly Cluster, formed all of their stars around the same time. That fact has helped astronomers figure out how old star clusters are. But finding the age of individual stars is much harder.

N.A. Sharp, Mark Hanna, REU program/NOIRLab/NSF, AURA

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

To calculate a star’s mass, just look it its orbital period and do a bit of algebra. To determine what it’s made of, look to the spectrum of light the star emits. But the one variable scientists haven’t quite cracked yet is time.

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

Even well-studied stars surprise scientists every now and then. In 2019 when the red supergiant star Betelgeuse dimmed, astronomers weren’t sure if it was just going through a phase or if a supernova explosion was imminent. (Turns out it was just a phase.) The sun also shook things up when scientists noticed that it wasn’t behaving like other middle-aged stars. It’s not as magnetically active compared with other stars of the same age and mass. That suggests that astronomers might not fully understand the timeline of middle age.

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

Hertzsprung-Russell diagrams

Scientists do have a pretty good handle on how stars are born, how they live and how they die. For instance, stars burn through their hydrogen fuel, puff up and eventually expel their gases into space, whether with a bang or a whimper. But when exactly each stage of a star’s life cycle happens is where things get complicated. Depending on their mass, certain stars hit those points after a different number of years. More massive stars die young, while less massive stars can burn for billions of years.

At the turn of the 20th century, two astronomers — Ejnar Hertzsprung and Henry Norris Russell — independently came up with the idea to plot stars’ temperature against their brightness. The patterns on these Hertzsprung-Russell, or H-R, diagrams corresponded to where different stars were in that life cycle. Today, scientists use these patterns to determine the age of star clusters, whose stars are thought to have all formed at the same time.

The caveat is that, unless you do a lot of math and modeling, this method can be used only for stars in clusters, or by comparing a single star’s color and brightness with theoretical H-R diagrams. “It’s not very precise,” says astronomer Travis Metcalfe of the Space Science Institute in Boulder, Colo. “Nevertheless, it’s the best thing we’ve got.”

Measuring a star’s age isn’t as easy as you’d think. Here’s how scientists get their ballpark estimates.

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Rotation rate

By the 1970s, astrophysicists had noticed a trend: Stars in younger clusters spin faster than stars in older clusters. In 1972, astronomer Andrew Skumanich used a star’s rotation rate and surface activity to propose a simple equation to estimate a star’s age: Rotation rate = (Age).

This was the go-to method for individual stars for decades, but new data have poked holes in its utility. It turns out that some stars don’t slow down when they hit a certain age. Instead they keep the same rotation speed for the rest of their lives.

“Rotation is the best thing to use for stars younger than the sun,” Metcalfe says. For stars older than the sun, other methods are better.

Stellar seismology

The new data that confirmed rotation rate wasn’t the best way to estimate an individual star’s age came from an unlikely source: the exoplanet-hunting Kepler space telescope. Not just a boon for exoplanet research, Kepler pushed stellar seismology to the forefront by simply staring at the same stars for a really long time.

Watching a star flicker can give clues to its age. Scientists look at changes in a star’s brightness as an indicator of what’s happening beneath the surface and, through modeling, roughly calculate the star’s age. To do this, one needs a really big dataset on the star’s brightness — which the Kepler telescope could provide.

“Everybody thinks it was all about finding planets, which was true,” Soderblom says. “But I like to say that the Kepler mission was a stealth stellar physics mission.”

This approach helped reveal the sun’s magnetic midlife crisis and recently provided some clues about the evolution of the Milky Way. Around 10 billion years ago, our galaxy collided with a dwarf galaxy. Scientists have found that stars left behind by that dwarf galaxy are younger or about the same age as stars original to the Milky Way. Thus, the Milky Way may have evolved more quickly than previously thought.

As space telescopes like NASA’s TESS and the European Space Agency’s CHEOPS survey new patches of sky, astrophysicists will be able to learn more about the stellar life cycle and come up with new estimates for more stars.

Aside from curiosity about the stars in our own backyard, star ages have implications beyond our solar system, from planet formation to galaxy evolution — and even the search for extraterrestrial life.

“One of these days — it’ll probably be a while — somebody’s going to claim they see signs of life on a planet around another star. The first question people will ask is, ‘How old is that star?’” Soderblom says. “That’s going to be a tough question to answer.”

Lisa Grossman is the astronomy writer. She has a degree in astronomy from Cornell University and a graduate certificate in science writing from University of California, Santa Cruz. She lives near Boston.

Helen Thompson is the multimedia editor. She has undergraduate degrees in biology and English from Trinity University and a master’s degree in science writing from Johns Hopkins University.

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