Sagittarius is 10,000 times less massive than the Milky Way.

"It's clear who's the bully in the interaction," said Steven Majewski, a University of Virginia professor of astronomy.

Theorists have long said that small galaxies like Sagittarius are the building blocks of large galaxies. Several clumps and streams of stars in the Milky Way are thought to be remnants of other small galaxies long ago consumed.

About a dozen minor star swirls are gravitationally bound to the Milky Way and face the possibility of being absorbed. Sagittarius is the smallest of these. It was not discovered until 1994, because its stars are obscured from view in visible light.

The new study used infrared observations from the Two-Micron All Sky Survey (2MASS).

"If people had infrared-sensitive eyes, the entrails of Sagittarius would be a prominent fixture sweeping across our sky," Majewski said. "But at human, visual wavelengths, they become buried among countless intervening stars and obscuring dust. The great expanse of the Sagittarius system has been hidden from view."

With the 2MASS catalogue, the astronomers filtered away millions of foreground stars to focus on a type of star called an M giant. These large, infrared-bright stars are populous in the Sagittarius galaxy but uncommon in the outer Milky Way.

Previous work had detected scattered sections of Sagittarius. The astronomers describe the new view as resembling a spaghetti noodle winding through our galaxy.

"This first full-sky map of Sagittarius shows its extensive interaction with the Milky Way," Majewski said. "Both stars and star clusters now in the outer parts of the Milky Way have been 'stolen' from Sagittarius as the gravitational forces of the Milky Way nibbled away at its dwarf companion. This one vivid example shows that the Milky Way grows by eating its smaller neighbors."

Majewski is the lead author of a paper on the results that will be published Dec. 20 in the Astrophysical Journal.

David Spergel, a professor of astrophysics at Princeton University, said the observations "reinforce the idea that galaxy formation is not an event, but an ongoing process."

The study suggests that the feeding has been going on for 2 billion years and has reached a critical phase.

"After slow, continuous gnawing by the Milky Way, Sagittarius has been whittled down to the point that it cannot hold itself together much longer," said 2MASS science team member and study co-author Martin Weinberg of the University of Massachusetts. "We are seeing Sagittarius at the very end of its life as an intact system."

The event is seen as one of many in the course of the galaxy's history. But there is a special aspect to it.

"For only a few percent of its 240 million-year orbit around the Milky Way Galaxy does our solar system pass through the path of Sagittarius debris," Majewski said. "Remarkably, stars from Sagittarius are now raining down onto our present position in the Milky Way. Stars from an alien galaxy are relatively near us."

The alien invasion could help astronomers learn more about mysterious dark matter, invisible stuff that dominates the universe.

"The shape of the Sagittarius debris trail shows us that the Milky Way's unseen dark matter is in a spherical distribution, a result that is quite unexpected," Weinberg said.

• Gallery of 2MASS Images
• More Deep Space News
• Space Mailbag