The Milky Way is being invaded by another galaxy.

But don't panic; we've got the size advantage.

Astronomers have known since 1994 that a small galaxy orbiting the Milky Way has actually entered Earth's home galaxy. Now new findings may shed light on the nature of "dark matter" inside the invading galaxy.

A team of scientists made the 1994 discovery unexpectedly while analyzing stars in the concentrated, elliptical bulge at the center of our own galaxy, the Milky Way.

They realized that certain stars, which all had essentially the same velocity, were not moving properly to be in the center of the Milky Way. They were found to be in a dwarf galaxy located along the line of sight to the center of our galaxy, but on the far side of the Milky Way.

Astrophysicist Rosemary Wyse presented findings on the Sagittarius dwarf spheroidal galaxy at the annual meeting of the American Association of Science.

"It's close enough that you can study individual stars in it the same way that you study stars in our galaxy," said Rosemary Wyse, a Hopkins astrophysicist who gave a talk about the galaxy on Feb. 13, during an annual meeting of the American Association for the Advancement of Science, in Philadelphia.

Known as the Sagittarius dwarf spheroidal galaxy--since it is observed in the direction of the constellation Sagittarius--it is roughly one-tenth the diameter of the Milky Way but weighs less than one-thousandth as much as the Milky Way. It is nearly as close to the center of our own galaxy as is the Earth. The galaxy is one of several nearby dwarf galaxies that are considered to be companions to the Milky Way, said Wyse, a professor in the Department of Physics and Astronomy.

"The other galaxies are far enough away that you don't really think of them as invading your space," she said. "You can more or less ignore them. But Sagittarius has come right in."

Wyse and four other astronomers have made new observations of stars inside the dwarf galaxy and have calculated that it makes a complete orbit around the Milky Way in less than 1 billion years. As the small galaxy orbits around the center of the Milky Way, it plunges into the central regions of the larger galaxy. The astronomers have inferred that it has orbited our galaxy at least 10 times.

Findings from that work have led to the conclusion that the small galaxy is surprisingly sturdy; after orbiting the Milky Way that many times, the smaller galaxy should have been pulled apart by our galaxy's strong gravitational forces, unless it harbors more matter than indicated by the number of visible stars it contains.

"It's just got a lot of dark matter, so it's able to hold onto its stars," Wyse said.

The astronomers analyzed spectra from observations they made with the Anglo-Australian Telescope and the Cerro Tololo Interamerican Observatory. The team included Nicholas Suntzeff, from the Cerro Tololo Observatory, and Rodrigo Ibata, from the European Southern Observatory, both located in Chile; Gerard Gilmore, at the Institute of Astronomy, and Mike Irwin, at the Royal Greenwich Observatory, both in the United Kingdom. Ibata, Gilmore and Irwin made the original discovery in 1994.

It is important to study Sagittarius in the overall quest to learn how galaxies form and evolve, Wyse said. Small companion galaxies might have merged to build larger galaxies like the Milky Way.

Astronomers are trying to understand more about a diffuse "halo" of stars that surrounds the central, elliptical bulge and disk of stars in the Milky Way and other galaxies. For example, how does the halo form? Does it represent the shredded bits of small satellites like Sagittarius?

Wyse said her findings indicate that, at most, 10 percent of the stars in the halo came from similar dwarf galaxies, which merged with the Milky Way over the past 8 billion years or so.

Studying Sagittarius may help answer other questions, such as: Does the central bulge of our galaxy also come from merging companion galaxies, but from more dense pieces that were capable of migrating to the center? What types of stars make up other galaxies that can be studied in great detail?

Sagittarius also gives astronomers an opportunity to study the dark matter of another galaxy up close and to clarify its nature. The existence of dark matter in the universe has been inferred for two decades from a number of astronomical findings, such as details about the movement of celestial objects. According to theories and observations, the universe contains more matter than is directly observed using current technology. Astronomers think that at least 90 percent of the mass in the universe is yet to be observed directly. The possible constituents of the dark matter range from stars that are too faint to be detected by standard methods to exotic particles that do not emit visible light.

Recent observations suggest that some of the dark matter in our own galaxy may be in the form of compact objects that are roughly one-tenth the mass of the sun. The objects would be too cold to shine by nuclear fusion like the sun. However, attempts to detect these failed stars, or so-called brown dwarfs, in the Milky Way have not yielded numbers sufficient to account for the amount of dark matter in the Milky Way's halo.

Observations of Sagittarius will yield evidence about the kinds of dark matter that prevail in the galaxy. In the near future, scientists hope to learn whether other companion galaxies also are invading the Milky Way. Because the Sagittarius discovery was made by chance, it is possible that such galaxies have gone undetected, Wyse said.

Picking out the companion galaxies would be difficult because it would involve analyzing the fine color differences of stars in the most concentrated regions of the Milky Way. "Unless you know it's there, you can't find it," Wyse said, referring to Sagittarius.

But a new, advanced telescope expected to be operational within a year promises to expose such companion galaxies. Astronomers will produce the Sloan Digital Sky Survey using an observatory in south-central New Mexico. The 2.5-meter telescope, near Sunspot, N.M., will be extremely sensitive to distant light because its focal plane is lined with light-sensor chips called charge-coupled devices.

"One of the things I want to do with Sloan is look for clumps of very red stars," Wyse said, "since peculiar stars could well be aliens."

The Sloan project is being managed by the non-profit Astrophysical Research Consortium, made up of the University of Chicago, Johns Hopkins, Princeton University and the University of Washington. Also involved in the project are Japanese astronomers, the Institute for Advanced Study, the Fermi National Accelerator Laboratory and the Naval Research Observatory.

Note: An image of the Sagittarius dwarf galaxy entering the Milky Way is available on-line at the following Web address:
( http://tarkus.pha.jhu.edu/~wyse/araa97_fig1.gif).