May 18, 2010 After billions of years of twinkling and shining, some stars in the heavens appear to "dance" as they wind down. Maybe not like Elvis or Michael Jackson, but they definitely have a rhythmic beat, and some may even spin like a top.
For the next two weeks, the Whole Earth Telescope, an international network of cooperating astronomical observatories led by the University of Delaware, will be continuously monitoring three of these stars to try to figure out what's going on inside their luminous masses of cooling plasma.
The primary target is a white dwarf star known as GD358 in the constellation Hercules. It's made of helium and has a surface temperature estimated at around 19,000 Kelvin.
"We recently discovered that this star is pulsating a little strangely, and we are looking for signs that it is spinning like a top," says Judi Provencal, assistant professor of physics and astronomy at the University of Delaware and director of the Delaware Asteroseismic Research Center.
A primary mission of the center, which is sponsored by Mt. Cuba Observatory in Greenville, Del., and UD, is to coordinate the activities of the Whole Earth Telescope.
A white dwarf is a "dead" star that doesn't generate its own energy like the sun does, Provencal says.
"The sun will one day become a white dwarf star, which is why we're interested in knowing more about them and what happens to any planets the original star might have had," Provencal notes.
The Whole Earth Telescope's second target star is the rapidly pulsating PG1325+101 in the constellation Virgo, which is suspected of having one or more planets in orbit around it. The international team will be working to confirm that suspicion, observing the star in collaboration with colleague Roberto Silvotti, leader of the observing group in Italy.
The third target star, WD1524, in the constellation Serpens, was observed during the Whole Earth Telescope's 2009 international campaign. The star was a high-amplitude pulsator until right before the observing run started, when it mysteriously became a small-amplitude pulsator.
"How stars pulsate depends on their structure and composition," says Provencal. "Last year, WD1524 completely changed how it was pulsating. Imagine ringing the Liberty Bell and having it sound like a hand bell. That would be hard to do. We don't understand how this happens with our pulsating stars. We now know that this star has changed yet again, so we are trying to understand how that can happen. Our current theoretical models of white dwarfs don't predict this sort of behavior."
There are thousands of white dwarfs in our galaxy; however, only about 30 percent are bright enough for scientists to study using the science of stellar seismology or asteroseismics, which can determine the age, temperature, and composition of a star from its oscillations and brightness.
A white dwarf star pulsates or quakes as waves of energy travel through it. The star's outer surface sloshes from side to side, like waves on the ocean, Provencal says.
From the shape of these pulses, scientists can measure how the atmosphere is moving around in these pulsating stars and figure out what's going on inside them, and determine whether an external object like a planet is influencing the star.
The scientific goal of the Whole Earth Telescope is to obtain uninterrupted time-series measurements of "variable stars" -- stars whose brightness changes over time -- and then construct theoretical models from which their fundamental astrophysics can be derived. The approach, which has been extremely successful, according to Provencal, has placed the fledgling science of "star quakes" at the forefront of stellar astrophysics.
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