The stars of the Pleiades cluster have fascinated and interested humanity for as long as we know. They are mentioned in Japanese documents, in Hinduism, in Islam, in the Bible, and they are even in myths from the Celts, the Aztecs, the Maori, and the Sioux.
Now, after millennia, we know how fast they are spinning. This information will help astronomers better understand how planetary systems and their host stars form and evolve.
The Pleiades are more than the seven sisters in the Greek myth. They are a populous star cluster with almost 1,000 members. These stars formed at the same time about 125 million years ago, which make the population of the Pleiades a great example of stars entering young adulthood. During this age, planets tend to form and stars tend to spin faster than when they are mature.
"The Pleiades star cluster provides an anchor for theoretical models of stellar rotation going both directions, younger and older," said Luisa Rebull, research scientist at the Infrared Processing and Analysis Center at Caltech, in a statement. "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."
The research will be published in three papers in the Astrophysical Journal and they're currently available online at arXiv.
The team has used the planet-hunter telescope Kepler to study the light emitted by the Pleiades. Kepler focused on dips in the luminosity produced by star spots, tracking them as they moved around the stars.
The cluster was tracked for 72 days, with the rotation of 750 cluster members measured, including 500 of the lowest-mass, dimmest, and smallest stars in the Pleiades. The team discovered that big stars tend to rotate more slowly, with a period between one and 11 Earth days, while many smaller objects rotate in less than a day. By comparison, sunspots near the equator of the Sun rotate in about 26 days.
"In the 'ballet' of the Pleiades, we see that slow rotators tend to be more massive, whereas the fastest rotators tend to be very light stars," said Rebull.
The Pleiades are located 450 light-years from Earth, and have a rich range of types and masses, with bright blue stars a few times the mass of the Sun to stellar dwarfs 10 percent of our yellow companion.
"We hope that by comparing our results to other star clusters, we will learn more about the relationship between a star's mass, its age, and even the history of its solar system," Rebull concluded.
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