Andromeda galaxy larger than thought


Monday, May 30, 2005 Posted: 3:22 PM EDT (1922 GMT)

WASHINGTON (Reuters) -- The Andromeda galaxy just got bigger -- three times bigger, astronomers said on Monday.

The galaxy is not actually expanding. But new measurements suggest that the nearest galaxy to our own Milky Way is three times broader than astronomers had thought.

They now believe a thin sprinkling of stars once thought to be a halo is in fact part of Andromeda's main disk.

That makes the spiral galaxy, so close to Earth that it appeared as a fuzzy blob to the ancients, more than 220,000 light-years across -- triple the previous estimate of 70,000 to 80,000 light-years.

It appears that the outer fringes of the disk were made when smaller galaxies slammed together, they told a meeting of the American Astronomical Society in Minneapolis.

The structure is too bumpy to have been formed otherwise, said Rodrigo Ibata of the Observatoire Astronomique de Strasbourg in France.

"This giant disk discovery will be very hard to reconcile with computer simulations of forming galaxies. You just don't get giant rotating disks from the accretion of small galaxy fragments," Ibata said in a statement.

Ibata, Scott Chapman of the California Institute of Technology and colleagues in Britain and Australia worked together using observations from the Keck II telescope in Hawaii.

They studied the motions of about 3,000 stars thought to be a mere halo and not an actual part of the galaxy's disk.

But they are in fact sited in the plane of the Andromeda disk itself and move at a velocity that suggests they are in orbit around the center of the galaxy, Ibata's team said.

Andromeda is 2 million light-years from Earth. A light-year is the distance light travels in a year -- about 6 trillion miles (10 trillion km).