A Galaxy Says: `Hell, No, I Won't Go'

SOME ASTRONOMERS BELIEVE LARGE GALAXIES GROW by swallowing smaller ones. The Milky Way is no exception, says Johns Hopkins University astrophysicist Rosemary Wyse. For several billion years, it has been trying to absorb a dwarf galaxy called Sagittarius. But Sagittarius is resisting, says Wyse. Less than one-hundredth the size of the larger system, it likely has orbited the central regions of the Milky Way a total of 10 or 12 times, plunging deep into our galaxy as it goes.

Along the way, it should have been tugged to pieces by the larger system's gravitational pull. How has Sagittarius escaped this fate? Wyse believes there is more to the dwarf than meets the eye. "It's got a lot of dark matter, so it is able to hold on to its stars," she says. Scientists can't see or characterize "dark matter." But they suspect that it may account for as much as 90% of the total mass of the universe.