Cornell astronomers, observing what they call "the most boring, average galaxy" they could find, have discovered some unusual mechanics: counter- rotating stars in a spiral galaxy.

About 80 percent of the stars in the galaxy NGC 4138 -- mostly older stars -- are rotating in a direction opposite to the younger stars and a huge cloud of hydrogen gas encircling the galaxy, according to findings by Martha Haynes, Cornell professor of astronomy, Katherine P. Jore, doctoral student, and Adrick H. Broeils, research associate at Cornell's Center for Radiophysics and Space Research. They presented their findings Jan. 18 at the American Astronomical Society meeting in San Antonio, Texas.

"This galaxy appears to be completely normal," said Haynes, who directed the studies. "This is a relatively featureless, smooth-armed spiral galaxy. But what we found was a big surprise."

The researchers found that a huge cloud of atomic hydrogen gas surrounds the stars that comprise the galaxy visible in optical images. But the hydrogen gas is rotating in the opposite direction from the stars in the visible galaxy. A closer look revealed that they were not all traveling together in their orbits: 20 percent of them are traveling along with the atomic gas while the other 80 percent of the stars are going around the opposite way.

"It's like being in a car on a traffic rotary with 80 percent of the cars going in the opposite direction," Haynes said.

The concept of two intermingled disks of stars and gas, traveling in opposite directions, appears to contradict astronomers' understanding of how spiral galaxies, like the Milky Way, form. "Counter-rotation can be understood in rare instances if two galaxies merge with each other," Haynes said. "However, in such cases, we would expect to see some disturbance in the galaxy's appearance or some nearby companion."

Mergers appear to have been more common at high redshift and frequently are seen in Hubble Space Telescope images of distant objects. Counter-rotating disks, believed to be the result of the swallowing of a small companion, have been seen in the centers of some elliptical galaxies and a few other spiral galaxies, such as the peculiar galaxy known as the "Evil Eye."

The researchers said they did not know why this was occurring in NGC 4138. Said Jore: "Our review of all of the evidence suggests that the counter-rotating disk may come from the accretion of a gas-rich dwarf companion, or might just be the result of the continual infall of material with an opposite spin onto NGC 4138 from far outside."

The scientists will have more observing time at the Kitt Peak National Observatory and the Palomar Observatory in March and April to look more closely at the two stellar components. "It is possible," Jore said, "we may be able to separate them and to better develop the picture of how the two came to coexist."