Astronomers have for decades puzzled over the Milky Way’s warped shape — like a bowl, a saddle or the brim of a fedora hat, depending on when you look.
The warp is most clearly visible in a thin disk of hydrogen gas that extends across the entire 200,000-light-year diameter of the Milky Way.
Scientists have known about the warp for half a century, but they never knew the cause.

Leo Blitz, professor of astronomy at the University of California, and his colleagues Evan Levine and Carl Heiles charted the warp and found evidence that it is a ripple or vibration set up by two small galaxies that circle the Milky Way. These satellite galaxies, called the Magellanic Clouds, cause vibrations at certain frequencies as they pass through the edges of the Milky Way, the researchers said.

Besides being warped, the Milky Way turns out to be growing, pulling in a galaxy that was detected with the Sloan Digital Sky Survey, which studies huge swaths of sky.

A survey of the northern sky has detected a huge and 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 Lufton of Princeton University, whose team discovered the big, dim feature, called it a “large pathetic galaxy” with a mass comparable to a cluster of stars, much less than the Milky Way. The 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 latest study, which has been submitted to The Astrophysical Journal for publication, shows that the Milky Way is still changing and evolving, said Mario Juric, a Princeton graduate student who is involved in the study. “It looks as though the Milky Way is still growing, by cannibalizing smaller galaxies that fall into it,” he said.