The skies of stars might
experience weather like that on planets, researchers now find.
The drifting clouds scientists
have seen are wispy, "just like cirrus clouds on Earth"—except these
are made of mercury, explained astrophysicist Oleg Kochukhov at Uppsala
University in Sweden.
Investigating these metal clouds
might shed light on how elements form inside stars.
Kochukhov and his colleagues spent
seven years peering at alpha Andromedae, the brightest star in the Andromeda
constellation. The bluish-white star lies about 100 light-years from Earth and
is more than twice as hot as the Sun and roughly three times its mass and
diameter.
Stars can develop spots on their
surfaces, which are largely thought due to magnetic fields. These are how
sunspots on the Sun are created, for instance. However, alpha Andromedae is not
magnetic, so the spots Kochukhov and his colleagues discovered on it five years
ago were an enigma.
Now the researchers find these
spots are clouds in that form and disperse in alpha Andromedae's skies,
following dynamics similar to weather patterns on planets, findings detailed
online June 24 in the journal Nature Physics.
Such weather could be seen on
stars that, like alpha Andromedae, are hot and massive. They also have to spin
relatively slowly—alpha Andromedae completely rotates about once every 60
hours—as spinning too fast might destroy any clouds. Kochukhov noted a
half-dozen or so other candidates have recently been seen already, including
the star AR Aurigae, which might have clouds of strontium, yttrium and platinum
as well.
Mystery process
What makes these metal clouds form
remains uncertain. Kochukhov suggested random disturbances in alpha
Andromedae's atmosphere could generate clouds, "a process similar to what
is quite often seen in the atmosphere of our own planet," or perhaps the
gravitational pull of alpha Andromedae's companion star helps perturb its sky.
The discovery of weather on stars
could help solve a mystery concerning why startling discrepancies were seen in
the levels of mercury and other heavy elements in stars like alpha Andromedae.
"These are all the same type
of star, so you'd expect them all to have roughly the same abundances of
mercury, but they could vary by a factor of 100 from one star to the
next," explained George Preston, astronomer emeritus at the Carnegie
Observatories, who did not participate in this study. "Now it turns out
this variation could be due to weather. You might see different amounts of
mercury at different times."
Alpha Andromedae and similar stars
"are critical to understanding the origin of elements," Kochukhov
said, as heavy elements are concentrated in them that are too faint to detect
in sun-like stars. Stellar weather could shed light on how elements mix around
in these stars, he added.
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