Cornell University 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 today (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."

The team originally was attempting to map the dark matter in spiral galaxies. For this study -- Jore's Ph.D. thesis -- they attempted to examine only galaxies that had not been involved in mergers or interactions with other galaxies that might have peculiar characteristics. Her thesis was jokingly referred to as the "boring galaxy project." NGC 4138 was the first galaxy they observed. They used the Very Large Array radio telescope of the National Radio Astronomy Observatory in New Mexico for eight hours to map the hydrogen cloud. It was a year later, when they observed the galaxy for two hours with the 200-inch Hale telescope at Mt. Palomar that they discovered -- to their complete surprise -- the counter-rotation. Cornell astronomers have 25 percent of the observing time at the Hale telescope.

NGC 4138 -- just south of Ursa Major, the Big Dipper -- is about 16 megaparsecs away, or about 50 million light years. The galaxy is about 50,000 light years in diameter, but the hydrogen cloud that surrounds it is about 2.5 times as big, or 125,000 light years in diameter.

Haynes speculates that perhaps this kind of peculiar rotation is common and points out that "if we had stopped after we made the VLA observations, we would not have even found the real rotation of the galaxy." Most astronomers do not observe both the stars and the gas when they study how galaxies rotate. The Cornell researchers have found two other examples of counter-rotating stars where they are not expected, but those results are pending.

"This is not what we were looking for. Probably it has nothing to do with the dark matter problem," Haynes said. "But NGC 4138 is not a boring galaxy after all." The studies were funded by the National Science Foundation (NSF). Broeils' postdoctoral position was funded by a special NSF Faculty Award for Women to Haynes, designed to help women faculty develop research groups.