Kepler snaps spinning stars in the Seven Sisters

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The Pleiades cluster of stars seen through the eyes of NASA’s Wide-field Infrared Survey Explorer.
NASA / JPL-Caltech / UCLA

The Kepler space telescope has catalogued the spin rate of more than 750 stars in the Pleiades cluster – the most complete record of rotation periods for stars in a cluster yet.

By linking spin rate to mass and age, the work, to be published in The Astronomical Journal, will let astronomers explore stellar evolution and where and how planets form around the stars. 

Also known as the Seven Sisters, the Pleiades star cluster is only 445 light-years from Earth (on average) and at 125 million years old, has just reached the cosmic equivalent of “young adulthood”. At this age, stars spin about as fast as they ever will.

But they lose a bit of speed over time as, for instance, they blow off particles in the form of a stellar wind (the sun does this too – we call it the solar wind).

To unpick the intricate relationship between stellar spin, age and mass, the Kepler space telescope trained on the star cluster for 72 days. It observed around 1,000 stars and clocked the spin of around 750.

Kepler did this by measuring tiny variations in a star’s brightness. Stars aren’t uniform bundles of hot gas. They have cool, dark spots – like the sun has sunspots – that dim the star’s luminosity a smidge from our perspective.

The incredibly sensitive instruments on Kepler were able to detect these minuscule blips – even for around 500 tiny, dim stars in the cluster.

From these, a pattern emerged: more mass meant slower spin. 

A star’s spin has knock on effects for any planets in its orbit. A slower rotating star tends to generate fewer spots, and the vast plumes of material that spew forth and irradiate nearby planets become rarer.

“The Pleiades star cluster provides an anchor for theoretical models of stellar rotation going both directions, younger and older,” says Luisa Rebull, co-author of the papers and scientist at NASA’s Jet Propulsion Laboratory.

“We still have a lot we want to learn about how, when and why stars slow their spin rates and hang up their ‘dance shoes’, so to speak.”

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