Pleiades Star Cluster Could Reveal Factors Affecting Stellar Rotation

The Pleiades star cluster is one of the closest star clusters to Earth and is also the most easily observable one. The stars in this cluster, located roughly 445 light-years away, are all young adults — each about 125 million years old.

Three new studies to be published in the Astronomical Journal have now provided a detailed look into the stars that make up this cluster. The studies, based on data collected by NASA's Kepler space telescope during its K2 mission, aim to understand the complex relationship between the spin rate of stars and their internal structure.

“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,” Luisa Rebull, a research scientist at the Infrared Processing and Analysis Center at Caltech in Pasadena, California, who is the lead author of two papers and co-author of the third one, said in a statement released Friday. “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.”

Using the Kepler telescope, scientists were able to observe approximately 1,000 stars and measure the rotation rates of over 750 stars in the cluster over a period of 72 days. They found that many low-mass stars in Pleiades — some of which possessed as little as a tenth of our sun’s mass — completed a rotation in less than one day. The most massive stars — slightly hotter and more massive than the sun — took up to 11 days to complete one rotation.

“The main source of these differing spin rates is the internal structure of the stars. ... Larger stars have a huge core enveloped in a thin layer of stellar material undergoing a process called convection, familiar to us from the circular motion of boiling water. Small stars, on the other hand, consist almost entirely of convective, roiling regions,” NASA said in the statement. “As stars mature, the braking mechanism from magnetic fields more easily slows the spin rate of the thin, outermost layer of big stars than the comparatively thick, turbulent bulk of small stars.”

By comparison, our sun has a rotational period of roughly 26 days. This slow rotation speed is what allows, to a certain extent, life on Earth to exist. As a star’s spinning slows, so does the generation of starspots and solar storms associated with starspots. This means less harmful radiation is blasted into space, giving life on the planets orbiting the star an opportunity to arise and evolve.

Therefore, a deeper understanding of the factors affecting a star’s spin rate can also help scientists narrow down their search for habitable exoplanets.

“The Pleiades star cluster provides an anchor for theoretical models of stellar rotation going both directions, younger and older,” Rebull said in the 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.”