How do scientists calculate the age of a star?


We know a lot about the stars. After centuries of telescopes pointed at the night sky, astronomers and enthusiasts can discover the key attributes of any star, such as its mass or composition.

To calculate the mass of a star, just look at its orbital period and do a little algebra. To determine what it is made of, examine the spectrum of light emitted by the star. But the only variable that scientists haven’t quite deciphered is time.

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

Even well-studied stars surprise scientists from time to time. In 2019, when the red supergiant star Betelgeuse died out, astronomers weren’t sure if it was just going through a phase or if a supernova explosion was imminent. (Turns out it was only a phase.) The sun also shook things up when scientists noticed that it didn’t behave like other middle-aged stars. It is not as magnetically active as other stars of the same age and mass. This suggests that astronomers might not fully understand the chronology of the middle age.

Physics-based calculations and indirect measurements of a star’s age can give astronomers rough estimates. And some methods work best for different types of stars. Here are three ways astronomers calculate the age of a star.

Hertzsprung-Russell diagrams

Scientists have a pretty good idea of ​​how stars are born, how they live, and how they die. For example, stars burn their hydrogen fuel, swell, and eventually expel their gases into space, whether with a bang or a whimper. But when exactly every stage of a star’s life cycle occurs, this is where things get tricky. Depending on their mass, some stars reach these 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 of ​​plotting the temperature of stars as a function of their brightness. The patterns on these Hertzsprung-Russell, or HR, diagrams corresponded to the location of the different stars in this lifecycle. Today, scientists are using these models to determine the age of star clusters, which are believed to have all formed at the same time.

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

Measuring the age of a star is not as easy as you might think. Here’s how scientists get their rough estimates.

Turnover rate

In the 1970s, astrophysicists noticed a trend: stars in younger clusters rotate faster than stars in older clusters. In 1972, astronomer Andrew Skumanich used the rate of rotation and the surface activity of a star to come up with a simple equation to estimate the age of a star: Rate of rotation = (Age).

It was the method of choice for individual stars for decades, but new data has dug holes in its usefulness. It turns out that some stars don’t slow down when they reach a certain age. Instead, they keep the same rotational speed for the rest of their lives.

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

Stellar seismology

The new data that confirmed that 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 a boon to exoplanet research, Kepler has brought stellar seismology to the fore by simply staring at the same stars for a very long time.

Watching a star twinkle can give clues as to its age. Scientists consider changes in a star’s brightness as an indicator of what is happening below the surface and, through modeling, roughly calculate the star’s age. To do that, you need a very large set of data on the star’s brightness – which the Kepler telescope could provide.

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

This approach has revealed the sun’s magnetic quarantine crisis and has recently provided some clues to the evolution of the Milky Way. About 10 billion years ago, our galaxy collided with a dwarf galaxy. Scientists have discovered that the stars left by this dwarf galaxy are younger or the same age as the original stars in the Milky Way. So, the Milky Way may have evolved faster than previously thought.

As space telescopes like NASA’s TESS and the European Space Agency’s CHEOPS study 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.

Besides curiosity about the stars in our own backyard, the Star Ages have implications beyond our solar system, from the formation of planets to the evolution of galaxies – and even the search for alien life.

“One of these days – it will probably take a while – someone will claim to see signs of life on a planet around another star. The first question people will ask themselves is, “How old is this star?” “,” Soderblom says. “It’s going to be a tough question to answer.”

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