Several years of observation have revealed how a fast-spinning star moves more quickly at its equator than at its poles, but with a twist.
The research, which examined "starspots," also showed that material at the equator speeds up in relation to the poles for several years, then the cycle reverses and stuff at the poles speeds up while the rest slows down.
Starspots are like sunspots, dark and cool regions of intense magnetic activity. Galileo first observed sunspots in the early 1600s. They showed that the Sun spins on its axis roughly once a month and that the equator moves more rapidly than polar regions.
Every 3 months, a spot at the Sun's equator can catch up to and pass one in a polar region, though individual sunspots don't necessarily last that long.
Torrid pace
In the new study, a young Sun-like star called AB Doradus was seen to rotate 50 times faster than the Sun. The star spins on its axis roughly once every 12.4 hours.
The research team, led by Andrew Collier Cameron of the University of St. Andrews and Jean-Francois Donati of the Observatoire Midi-Pyrenees, studied the star for a few days each December over a period of 8 years.
They tracked individual starspots at different latitudes. Starspots and sunspots are both thought to be the locations where strong magnetic fields erupt and loop out into space, hurling charged particles away from the surface. The spots block energy from rising from the star's interior, hence their dark and cool nature.
"Stars that spin more rapidly than the Sun display greater coverage of starspots -- so much, in some cases, that the stars concerned grow brighter and dimmer by 10 to 20 percent as the star rotates," Cameron explained.
Cameron and Donati first computed the star's rotation pattern four years ago. Their earliest observations showed that a spot on the equator took about 100 to 120 days to "lap" a spot near one of the poles.
Changing paces
But after making new observations from this past year and reanalyzing the old ones, the astronomers were surprised to find that the spin rates changed from one year to the next. They found that the lapping time took anywhere from 70 to 140 days.
"This is the first time we've seen a star whose equator not only spins faster that its poles as on the Sun, but whose spin rate speeds up and slows down with time," Cameron told SPACE.com. "It's as though the star's magnetic field is reacting back on the very gas flow that generates it."
The study showed that the changing spin rates are tied directly to the number of starspots. Magnetic activity acts as a sort of glue, the researchers figure, altering the circulation of gas inside the star.
Magnetic activity is triggered in the first place by the differing rotation rates, and the magnetic field must be strong enough to eventually force this differing rotation to reverse its trend at some upper limit, Cameron said.
A similar throttling effect may occur on the Sun, he said, but it is not noticeable on the surface because the phenomenon is less intense overall.
Changing shape
The researchers suggest that the finding could help solve a long-standing mystery involving pairs of stars that orbit around each other.
Astronomers have found that the orbits of such binary stars, as they are called, sometimes speed up and slow down. The change could be related to the stars' shapes, which when they change would exert slightly different gravitational pulls on each other.
The new finding implies that a star's shape likely changes as its rotation rates change. A faster-spinning star would bulge out
, for example.The results will be published in the journal Monthly Notices of the Royal Astronomical Society.
About the star
AB Doradus sits 50 light-years from Earth in the southern constellation of the Swordfish. It is about the same size as the Sun, though slightly less massive and cooler, Cameron said.
While the Sun is middle-aged at about 4.6 billion years old, AB Doradus is estimated to be just 30 million years old. Like many other young stars, its rapid rotation and high magnetic activity are residual effects of its birth.
The star was studied with the 3.9-metre Anglo-Australian Telescope in New South Wales, Australia.
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