Cosmology; knowing a star's age is difficult (Introduction)

by David Turell , Saturday, July 24, 2021, 19:17 (1218 days ago) @ David Turell

There are several methods, with uncertain accuracy:

https://www.sciencenews.org/article/star-age-calculation-astronomy-life-cycle

"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.”

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"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.”

"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) -½.

"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.

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"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.'”

Comment: This only shows we still don't know much about our universe, but we do know it is fine-tuned-for-life