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The Milky Weigh Galaxy

LONG BEACH, CALIF.—One of the unnerving aspects of astronomy as a science is how astronomers continue to argue over measurements you’d have thought they settled long ago. A good recent example is the mass of our own Milky Way galaxy. Estimates keep swinging back and forth, and our galaxy and the Andromeda galaxy periodically switch places as the alpha galaxy of the local cosmos. A new study announced at the American Astronomical Society's conference suggests that astronomers may finally be starting to converge on a consensus—and, in a counterexample to the usual trend of relegating humanity to the cosmic backwaters, our Milky Way looks like the bigger one after all. (My colleague Steve Mirsky also describes the study on today’s podcast.)

Artist's conception of Milky WayTo weigh the Milky Way, Mark Reid of the Harvard-Smithsonian Center for Astrophysics and his colleagues tracked the motion of bright gaseous clouds in 12 star-forming regions scattered over the galaxy. They observed the clouds with the Very Long Baseline Array, a network of radio telescopes stretching from Hawaii to St. Croix which work in unison as a single planet-sized telescope. The network is so sharp-eyed that it can see clouds on the other side of the galaxy inching across the sky. The team combined these observations with measurements of the Doppler effect to deduce the clouds’ full three-dimensional orbital velocity: 254 +/– 16 kilometers per second.

As other studies have also shown, the speed does not vary with distance from the center of the galaxy. That is different from orbital speeds in, say, the solar system, which fall off with distance from the sun. The near-constancy of galactic speeds is one piece of evidence that the galaxy is ruled by some unseen dark matter.

This velocity is 15 percent faster than the textbook value of 220 kilometers per second. According to Newtonian physics, the mass interior to an orbit is proportional to the orbital velocity squared. In addition, the overall size of a galaxy—defined by where its density falls to a certain reference value—scales up with orbital velocity. Thus the 15-percent increase in velocity amounts to a 50-percent increase in the total galactic mass, the equivalent of adding another trillion or so suns. The study does not revise the estimate of the number of stars in the galaxy, so the additional mass is all dark matter, perhaps suggesting that our galaxy and Andromeda are endowed with different amounts of this stuff, whatever it may be.

Although a 50-percent change sounds pretty substantial, such measurements are hard to make, and astronomers consider anything within a factor of 10 close enough for government work. The flip side of being so uncertain about seemingly basic numbers is that simple observations still have the power to overturn our ideas about the universe.

Image credit: Robert Hurt, IPAC; Mark Reid, CfA, NRAO/AUI/NSF

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