Is our galaxy merging on the sly?

WASHINGTON — A previously unrecognized galaxy appears to be merging with the Milky Way, bringing hundreds of thousands of stars into our home galaxy that no one has noticed until now, astronomers announced Monday.

A survey of the northern sky has detected a huge but diffuse structure within the confines of the Milky Way that does not seem to fit in with other parts of the galaxy that contains our solar system.

Robert H. Lupton of Princeton University told a meeting of the American Astronomical Society that the large but faint collection of stars rises almost perpendicular to the flat, spiral disk of the Milky Way. The most likely interpretation of the structure, the astronomer said, is that it is a dwarf galaxy that has been merging with our galaxy.

The dwarf galaxy lies in the direction of the constellation Virgo at an estimated distance of 30,000 light years from Earth, researchers reported. While some of the stars of the companion galaxy may have been observed with telescopes for centuries, they said, no one realized they belonged to another body because they are so close and commingled with Milky Way stars.

The Milky Way is a flat, pinwheel galaxy measuring more than 100,000 light years across and containing an estimated 200 billion stars, including the sun, along its long, spiral arms and around the bulge at its center. A light year is the distance light travels in a vacuum in a year's time, about 6 trillion miles.

Astronomers found the merging galaxy through the Sloan Digital Sky Survey, a project that for more than five years has been mapping the distance and characteristics of millions of objects in space. The project is operated by a consortium of universities and other institutions and uses a telescope at Apache Point Observatory in New Mexico.

The latest study, which has been submitted to The Astrophysical Journal for publication, shows that the Milky Way still is changing and evolving, said Mario Juric, a Princeton graduate student who is the principal author of the report. "It looks as though the Milky Way is still growing, by cannibalizing smaller galaxies that fall into it," Juric said in a statement.

Reporting at the same meeting, another group of researchers said they had an explanation for a mysterious warp in the disk of the Milky Way that has baffled scientists for decades.