Pulsing Giant Star Dissected
The varying sizes of S Orionis compared to the inner Solar System. The red giant pulsates from the yellow disk out to the inner red disk. In 5 billion years the Sun will evolve to this stage before cooling down as a white dwarf. All the distances are to scale, while the diameters of the Sun, planets and maser spots (in red and green) are not.
Credit: ESO

Using the largest telescopes available, astronomers have dissected the dusty, gassy layers of the red giant S Orionis--a star that pulsates in size from a diameter roughly equal to the orbit of Mars to halfway between Mars and Jupiter every 410 days.

The information provides a glimpse at the future of our own sun, which will puff into a red giant like S Orionis in about 5 billion years, said astronomer David Boboltz of the U.S. Naval Observatory.

"No study of a red giant has been done to this level, looking at infrared and radio-wave views simultaneously," Boboltz said. "This really show us where the layers are."

Giant mystery

Red giants are older versions of the sun that, once they have burned off most of their hydrogen fuel, begin to burn helium. This creates intense "flashes" of radiation that puff the star up to more than 100 times its original size as it pushes stellar gas and dust out into space. S Orionis sheds about the mass of Earth each year.

"A lot of material escapes from the star's gravity, and begins to form beautiful planetary nebulas," Boboltz said. "But gravity overcomes a lot of gas and dust that gets pulled back into the star, starting the cycle all over again and forming a kind of pulse."

Where those layers are located and exactly what they're made of, however, was a mystery until Boboltz and his colleagues' investigation, which is detailed in a July issue of the journal Astronomy & Astrophysics.

The team measured the shells of gas and dust surrounding the star to most detailed level to date, discovering that the star's dusty shell of corundum--a compound used in sandpaper--was twice as large as previously thought. They also showed dusty corundum mixing heavily with gaseous silicon monoxide, a compound astrophysicists thought existed as a dust outside of red giants.

"We've essentially mapped the envelopes of material around these stars, which has never really been done before," Boboltz said.

Big investigation

The researchers aimed two of Earth's biggest interferometer telescopes at the star to peer at its layers: the Very Long Baseline Array (VLBA), a series of 10 telescopes spread over 5,350 miles that can see radio waves, and the infrared-seeing Very Large Telescope Interferometer (VLTI) in Chile.

If the telescopes were in New York, Boboltz noted, they would allow someone to read a newspaper in California. But regular sources of radio and infrared waves would make S Orionis look more like "a blob of emissions," so his team recorded its "masers," or naturally occurring lasers.

Astrophysicists aren't entirely sure how they form, but the basic principles of a laser apply: Some process evenly energizes one kind of molecule to produce a "synchronized" wave of light.

"Stuff like corundum and silicon monoxide, which we detected in S Orionis, emit their own unique masers," Boboltz said. By watching the masers move over the period of a few months, they recorded an extremely detailed picture of the red giant's pulsating behavior.

A video exists of another pulsating red giant star, call TX Cam, but Boboltz expects to surpass it's a visualization of only radio images.

"Soon we'll be able to create even better views of the pulsating cocoon around S Orionis by looking at water masers," Boboltz said, which exist at the farthest reaches of the cocoon. "We also hope to explain how a planetary nebula forms from a red giant near the end of its life as a white dwarf star."