Like cosmic ballet dancers, the stars of the Pleiades cluster are
spinning. But these celestial dancers are all twirling at different
speeds. Astronomers have long wondered what determines the rotation
rates of these stars.
By watching these stellar dancers, NASA's Kepler space telescope
during its K2 mission has helped amass the most complete catalog of
rotation periods for stars in a cluster. This information can help
astronomers gain insight into where and how planets form around these
stars, and how such stars evolve.
"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," said Luisa Rebull, a research
scientist at the Infrared Processing and Analysis Center at Caltech in
Pasadena, California. She is the lead author of two new papers and a
co-author on a third paper about these findings, all being published in
the Astronomical Journal.
The Pleiades star cluster is one of the closest and most easily seen
star clusters, residing just 445 light-years away from Earth, on
average. At about 125 million years old, these stars -- known
individually as Pleiads -- have reached stellar "young adulthood." In
this stage of their lives, the stars are likely spinning the fastest
they ever will.
As a typical star moves further along into adulthood, it loses some
zip due to the copious emission of charged particles known as a stellar
wind (in our solar system, we call this the solar wind). The charged
particles are carried along the star’s magnetic fields, which overall
exerts a braking effect on the rotation rate of the star.
Rebull and colleagues sought to delve deeper into these dynamics of
stellar spin with Kepler. Given its field of view on the sky, Kepler
observed approximately 1,000 stellar members of the Pleiades over the
course of 72 days. The telescope measured the rotation rates of more
than 750 stars in the Pleiades, including about 500 of the lowest-mass,
tiniest, and dimmest cluster members, whose rotations could not
previously be detected from ground-based instruments.
Kepler measurements of starlight infer the spin rate of a star by
picking up small changes in its brightness. These changes result from
"starspots" which, like the more-familiar sunspots on our sun, form when
magnetic field concentrations prevent the normal release of energy at a
star’s surface. The affected regions become cooler than their
surroundings and appear dark in comparison.
As stars rotate, their starspots come in and out of Kepler’s view,
offering a way to determine spin rate. Unlike the tiny, sunspot
blemishes on our middle-aged sun, starspots can be gargantuan in stars
as young as those in the Pleiades because stellar youth is associated
with greater turbulence and magnetic activity. These starspots trigger
larger brightness decreases, and make spin rate measurements easier to
obtain.
During its observations of the Pleiades, a clear pattern emerged in
the data: More massive stars tended to rotate slowly, while less massive
stars tended to rotate rapidly. The big-and-slow stars' periods ranged
from one to as many as 11 Earth-days. Many low-mass stars, however, took
less than a day to complete a pirouette. (For comparison, our sedate
sun revolves fully just once every 26 days.) The population of
slow-rotating stars includes some ranging from a bit larger, hotter and
more massive than our sun, down to other stars that are somewhat
smaller, cooler and less massive. On the far end, the fast-rotating,
fleet-footed, lowest-mass stars possess as little as a tenth of our
sun’s mass.
"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 main source of these differing spin rates is the internal
structure of the stars, Rebull and colleagues suggest. 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. 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.
Thanks to the Pleiades’ proximity, researchers think it should be
possible to untangle the complex relationships between stars’ spin rates
and other stellar properties. Those stellar properties, in turn, can
influence the climates and habitability of a star’s hosted exoplanets.
For instance, as spinning slows, so too does starspot generation, and
the solar storms associated with starspots. Fewer solar storms means
less intense, harmful radiation blasting into space and irradiating
nearby planets and their potentially emerging biospheres.
"The Pleiades star cluster provides an anchor for theoretical models
of stellar rotation going both directions, younger and older," said
Rebull. "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."
Rebull and colleagues are now analyzing K2 mission data from an older star cluster, Praesepe, popularly known as the Beehive Cluster, to further explore this phenomenon in stellar structure and evolution.
"We’re really excited that K2 data of star clusters, such as the
Pleiades, have provided astronomers with a bounty of new information and
helped advance our knowledge of how stars rotate throughout their
lives," said Steve Howell, project scientist for the K2 mission at
NASA’s Ames Research Center in Moffett Field, California.
The K2 mission’s approach to studying stars employs the Kepler
spacecraft's ability to precisely observe miniscule changes in
starlight. Kepler’s primary mission ended in 2013, but more exoplanet
and astrophysics observations continue with the K2 mission, which began
in 2014.
Ames manages the Kepler and K2 missions for NASA's Science Mission
Directorate. NASA's Jet Propulsion Laboratory in Pasadena, California,
managed Kepler mission development. Ball Aerospace & Technologies
Corporation operates the flight system with support from the Laboratory
for Atmospheric and Space Physics at the University of Colorado at
Boulder.
Elizabeth Landau
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6425
elizabeth.landau@jpl.nasa.gov
Michele Johnson
Ames Research Center, Moffett Field, Calif.
650-604-6982
michele.johnson@nasa.gov
Written by Adam Hadhazy
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6425
elizabeth.landau@jpl.nasa.gov
Michele Johnson
Ames Research Center, Moffett Field, Calif.
650-604-6982
michele.johnson@nasa.gov
Written by Adam Hadhazy
Editor: Tony Greicius
Source: NASA/Kepler and K2
