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FOUR-ARMED MONSTERRadio observations reveal the Milky Way galaxy must have four spiral arms: two dominant ones and two others that contain only newborn stars, as shown here in this artist's conception. Most star formation in our galaxy takes place in these spiral arms.Robert Hurt, IPAC; Mark Reid, CfA, NRAO/AUI/NSF

LONG BEACH, Calif. — Hey Andromeda, you’d better watch out. Turns 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 the new estimate of 3 trillion suns, according to a new study.

That means that the two galaxies — 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 new study uses the Very Long Baseline Array of 10 radio telescopes stretched out over thousands of miles. 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 his 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 that act as amplifiers for radio waves. Reid reported the study’s results January 5 during the winter meeting of the American Astronomical Society.

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 measurements revealed highly accurate measurements not only of the distances to the masers but also of their two-dimensional motion across the sky as they orbit the galactic center. Astronomers were able to determine the full three-dimensional motion of the masers — the velocity along the line of sight to Earth — by measuring tiny shifts in the frequency of the masers’ radio emissions.

From these velocity and distance measurements, Reid and his collaborators determined that the Milky Way spins at about 254 kilometers/second, about 15 percent faster than previously calculated. Applying Newton’s law of gravity to the faster spin speed, the team finds that the Milky Way is about 50 percent heavier than calculated, bringing it up to 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 calculated that the two galaxies will 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 infrared Spitzer Space Telescope.

The radio observations yielded another surprise. Most star formation in our galaxy takes place in the Milky Way’s spiral arms — where gas is compressed, triggering the birth of stars. By measuring the distances to masers that appeared to reside within the same spiral arm, the researchers were able to determine how tightly wound each spiral arm is and how many times it wraps around the Milky Way. From these observations, the team deduced that the Milky Way must have four spiral arms: two dominant ones that include both new stars traced by the VLBA observations along with old stars seen by Spitzer; and two other arms that contain only newborn stars.   

Benjamin’s team, using Spitzer, came to a similar conclusion in summer of 2007, when Spitzer saw evidence for only two dominant spiral arms. 

“There has been decades of debate about the spin speed, mass

and number of spiral arms,” says Reid. “It is a notoriously difficult problem to measure these quantities from inside the Milky Way, since we are rotating with most of the material,” he adds. “The advantage of our data is that we have for the first time full three-dimensional information, both location and velocity.” 

Benjamin cautions that although the observations by Reid’s team “are the gold standard for distance measurements,” the researchers are only “just beginning to map out the structure of the arms.… Twenty star-forming regions are enough to give you a hint, but not enough to nail down the entire structure of the arms.”

Leo Blitz of the University of California, Berkeley says he agrees that Reid’s team hasn’t yet covered enough of the galaxy to fully determine the overall structure.

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