Scientists Can Tell How Old A Star Is Based On How Fast It's Spinning

NASA, ESA, and L. Bedin (STScI)

Unlike humans, stars don’t have to put any effort into concealing their age—they are notoriously, and frustratingly, good at it. But we do share one thing in common: Stars slow down as they get older, a feature that scientists have been taking advantage of for some years now. However, with a limited data set, scientists have struggled to make reliable calculations.

Now, thanks to Kepler observations, scientists have finally demonstrated that they can accurately determine the age of sun-like stars from how fast they are spinning. Their work, which has been presented at a meeting of the American Astronomical Society, represents a giant leap towards the ultimate goal of building a clock that can precisely measure the ages of stars from their spins.

Learning the age of a star is crucial for many astronomical studies, in particular for the search of planets outside our solar system (exoplanets), and of course extraterrestrial life. Given that stars and planets form together, if we know the age of a star, we can determine the age of its planets. And the older the planet, the greater the possibility of finding life as it has had more time to get started.

Scientists are particularly interested in stars like our own, or “cool stars.” These are the most abundant stars in our galaxy and are also very bright. These galactic lamp posts also host the majority of Earth-like planets spotted so far. Unfortunately, these stars are tricky to age because their size and brightness don’t change much throughout most of their lives. But scientists have identified something that does change as a star grows old: its rotation, which gradually gets slower.

According to the new work, there is a close relationship between a star’s mass, spin and age, and if the first two can be measured, the third can be calculated. To measure a star’s spin, scientists look at dark patches, called star spots, which travel across the surface as it rotates. When astronomers look at distant stars, they can’t directly see these spots, so instead they look for dips in brightness that occur when the spot appears.

Typically, these spots only dim a star’s brightness by less than 1%, meaning the changes are very difficult to measure. This is where NASA’s Kepler spacecraft came to the rescue, which has provided precise measurements of stellar brightness since 2009.

In order to calibrate their stellar clock, scientists needed to measure the spin periods of stars with known ages and masses. Prior to this study, this had only been achieved for stars within a 1-billion-year-old cluster NGC 6811, which rotated about once every 10 days, and of course for our own 4.6 billion year old sun, which had a spin period of 26 days. Now, scientists have added to this data set by measuring the spin of 30 sun-like stars in a cluster known to be 2.5 billion years old, closing a “four-billion-year-gap.” As described in Nature, the stars in this cluster, NGC 6819, sat beautifully in this gap, rotating around every 18 days.

Prior to this work, the ages of cool stars came with a margin of error as large as 100%; now, this has been reduced to around 10%.

[Via Harvard CfA, Nature and BBC News]

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