Milky Way puts on weight: new estimate makes galaxy comparable to Andromeda.

LONG BEACH, CALIF. -- Hey Andromeda, you'd better watch out. Your little brother, the Milky Way galaxy, isn't so little after all. In fact, the Milky Way is just as massive, weighing in at about 3 trillion suns, according to a new study.

That means that the Milky Way and Andromeda--the largest members of the Local Group of galaxies--might smash into each other earlier than astronomers had predicted.

To map the Milky Way, the study used the Very Long Baseline Array of 10 radio telescopes stretched out over thousands of kilometers. Unlike visible-light observations, which are obscured by interstellar dust, radio studies enable astronomers to penetrate through dusty byways.

Mark Reid of Harvard University and colleagues based their findings on observations of nearly 20 regions of intense star formation across the galaxy--many of them traced by methanol masers, concentrations of methanol that act as amplifiers for radio waves. Reid reported the study's results January 5 at the American Astronomical Society meeting.

Masers require the high temperatures and densities of star-forming regions. By repeatedly observing the masers when Earth is at opposite sides of its orbit around the sun, Reid and collaborators could measure the parallax of these regions--the slight, apparent shift of each maser's position due to Earth's motion.

Those apparent shifts accurately revealed the distances to the masers and their two-dimensional motion across the sky as they orbit the galactic center. Astronomers determined the full three-dimensional motion of the masers--which includes the velocity along the line of sight to Earth--by measuring tiny shifts in the frequency of the masers' radio emissions.

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From these velocities and distances, Reid and collaborators found that the Milky Way spins at about 254 kilometers per second, about 15 percent faster than previously calculated. Applying Newton's law of gravity to the faster spin speed shows that the Milky Way is about 50 percent heavier than had been calculated, putting it on par with Andromeda.

"The more mass in the big galaxies--the Milky Way and Andromeda--... the sooner they will collide," says Reid. Researchers had previously thought that the two galaxies would merge in about 5 billion years. Reid says he is not certain exactly how much sooner the two giants would collide because the full three-dimensional motion of each galaxy has not yet been determined. A heavier Milky Way might also have a greater retinue of satellite galaxies--tiny galaxies, like the Large and Small Magellanic clouds--that orbit it, he adds.

"Mass is the most fundamental parameter for our galaxy, and there are a lot of important implications if the claimed revisions are correct, including the orbits of satellite galaxies around us," comments Robert Benjamin of the University of Wisconsin--Whitewater, who has studied the Milky Way using NASA's Spitzer Space Telescope.