Andromeda just got bigger
The Andromeda galaxy just got three times bigger, astronomers say.
The galaxy isn't actually expanding, but a meeting of the American Astronomical Society in Minneapolis has heard new measurements suggest that the nearest galaxy to our own Milky Way is three times broader than previously thought.
Astronomers from Australia, the United States, Britain and Europe who worked together using observations from the W.M.Keck Observatory in Hawaii 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.
This is triple the previous estimate of 70,000 to 80,000 light-years.
A stellar train wreck
Dr Geraint Lewis, a senior lecturer in astronomy at the University of Sydney, collaborated on the project.
He says astronomers are now scratching their heads about how galaxies like the Milky Way formed.
"The Milky Way and Andromeda are very similar galaxies and nice simple theories said you make a disk and it has this nice edge, but now that we're looking at these outer regions it's just not as simple as we thought it was," he says.
"There seem to be extra stars that shouldn't be there and bits of structure we didn't expect, so it's making us rethink how big galaxies arose."
Lewis says the standard picture relies on theories of galactic cannibalism, or accretion, where large galaxies consume smaller ones.
He says this is still occurring, but in the case of Andromeda it should have resulted in a simple, neat disk.
Instead, astronomers discovered "a train wreck".
"There are clumps of stars and streams of stars that weren't really expected to be there," he says.
"It might mean there's been some sort of big event in the past that's thrown stars in and made a bit of a mess."
It appears that the outer fringes of the disk were made when smaller galaxies slammed together because the structure is too bumpy to have been formed in any other way, says Rodrigo Ibata of the Strasbourg Astronomical Observatory in France.
"This giant disk discovery will be very hard to reconcile with computer simulations of forming galaxies," Ibata says.
You just don't get giant rotating disks from the accretion of small galaxy fragments."
The astronomers studied the motion 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 centre of the galaxy, Ibata's team says.
Andromeda is 2 million light-years from Earth. A light-year is the distance light travels in a year - about 10 trillion kilometres.