PASADENA - Red giants are ancient objects that have exhausted their primary fuel and have little time left in their stellar lives. They are huge and bloated, like a man once fit but now sapped of energy, flopping on the couch and expanding during the second half of his life.
A red giant can swell to 100 times the size it was when it was a normal, or main-sequence star.
Curiously, however, red giants don't just stay fat. They contract and swell again in amazingly rapid cycles.
New images from the Hubble Space Telescope provide the most distant glimpse ever of a host of these red giants in infrared light.
"As they evolve, most old red giants increase in size and brightness for a few hundred million years, up to the same maximum brightness," said Michael Gregg, an astronomer at the University of California, Davis, who led a team that produced the image. "Then they rapidly -- almost instantly -- become fainter again."
Red giants shine more brightly in infrared wavelengths than they do in visible light. And while red giants in our own galaxy can be seen with optical telescopes, spotting them in other galaxies becomes increasingly difficult with distance.
The new images have resolved hundreds of red giants in an elliptically shaped galaxy called NGC 3379, which is about 30 million light-years from Earth.
Gregg presented the images here last month at a meeting of the American Astronomical Society. The new views produced a few surprises.
Gregg told SPACE.com that he and his colleagues found "a smattering of stars which are brighter than old red giants are supposed to get. These stars may be younger red giants, perhaps one-half to one-third as old as the bulk of the stars in NGC 3379."
These younger stars suggest that the galaxy may have experienced an episode of star formation roughly 3 billion to 5 billion years ago. This and other information uncovered in the study might help researchers better understand how elliptical galaxies form, and whether there is just one method or several.
Gregg said the red giants in NGC 3379, on average, have an abundance of heavy elements about somewhere between 30 and 50 percent that of our Sun.
"We are a little surprised that the abundance is so high in the red giants of NGC 3379," he said.
Heavy elements were not present when the universe formed, researchers believe, and are only created in the thermonuclear reactions inside stars. These heavy elements then get flung into space in stellar explosions called supernovae. Red giants also discard heavy elements from their outer layers as they expand and contract.
In the outer parts of galaxy NGC 3379, previous estimates of heavy-element content in red giants have been as low as 10 percent of what Gregg and his colleagues found. Further, a handful of the red giants has two or three times more heavy elements than what's found in the Sun.
All this indicates that the contents of red giants may range more widely than expected.
The Milky Way, a spiral galaxy, is much different from NGC 3379. But the Milky Way has a central bulge loaded with old stars. Several bright stars in our night sky are red giants in our own galaxy.
Comparing the stars in the two galaxies, therefore, provides a means to study each.
"The stars in the center of our own galaxy appear to be very similar in age and abundance to those we have measured in NGC 3379, perhaps indicating that the way you make the central bulge of a spiral galaxy is similar to how you make elliptical galaxies, at least for these two particular objects," Gregg said.
The work also illustrates a short-term variation in the red giants' brightness, a phenomenon that has been observed in other studies but is not well understood.
Two pictures taken three months apart show how some the stars' brightness varies, providing clues to a short-term expansion and contraction. Astronomers call these dynamic objects "variable stars." The study found more variable stars than would be expected in an elliptical galaxy.
The newly released image was captured before Hubble's infrared camera ran out of coolant in 1999 and stopped functioning. Gregg and his colleagues hope that a "cryocooler" scheduled to be installed during a 2002 servicing mission will restore the camera to use.
The researchers would then like to use the refurbished camera, along with an improved optical camera also scheduled to be installed on Hubble, to further study the red giants in NGC 3379.
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