Wed, Jan 07, 2009 - Page 7 News List

Milky Way bigger and bulkier than thought

AP , WASHINGTON

Take that, Andromeda! For decades, astronomers thought when it came to the major galaxies in Earth’s cosmic neighborhood, our Milky Way was a weak sister to the larger Andromeda.

Not anymore.

The Milky Way is considerably larger, bulkier and spinning faster than astronomers once thought — Andromeda’s equal.

Scientists mapped the Milky Way in a more detailed, 3D way and found that it is 15 percent larger in breadth. More important, it is denser, with 50 percent more mass. The new findings were presented on Monday at the American Astronomical Society’s convention in Long Beach, California.

That difference means a lot, said study author Mark Reid of the Harvard-Smithsonian Center for Astrophysics.

“Previously we thought Andromeda was dominant and that we were the little sister of Andromeda,” Reid said. “But now it’s more like we’re fraternal twins.”

That’s not necessarily good news. A bigger Milky Way means that it could be crashing violently into the neighboring Andromeda galaxy sooner than predicted — though still billions of years from now.

Reid and his colleagues used a large system of 10 radio telescope antennas to measure the brightest newborn stars in the galaxy at different times in Earth’s orbit around the sun.

They made a map of those stars, not just in the locations where they were first seen, but an additional dimension of time — something Reid said had not been done before.

With that, Reid was able to determine the speed at which the spiral-shaped Milky Way is spinning around its center.

That speed — about 914,0000kph — is faster than the 791,800kph that scientists had been using for decades. That is about a 15 percent jump in spiral speed.

The old number was based on less accurate measurements and this is based on actual observations, Reid said.

Once the speed of the galaxy’s spin was determined, complex formulas that end up cubing the speed determined the mass of all the dark matter in the Milky Way. And the dark matter — the stuff we cannot see — is by far the heaviest stuff in the universe.

So that means the Milky Way is about one-and-a-half times the mass that astronomers previously calculated.

The paper makes sense, but is not the final word on the size of the Milky Way, said Mark Morris, an astrophysicist at the University of California Los Angeles, who was not part of the study.

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