The Andromeda galaxy, our nearest major galactic neighbor in the universe, has a rotating nebula of stars three times bigger than previously measured, US astronomers said Monday.

The findings reveal that we know less about our galactic neighbor than thought, according to Scott Chapman, a researcher from the California Institute of Technology who presented the results of a survey of Andromeda's stellar motions at a meeting of the American Astronomical Society.

Chapman is in a team of astronomers using the Keck telescope to measure speeds of 5,000 stars in the outskirts of Andromeda. They were surprised to find that these fringe stars are actually rotating as if they were part of the galaxy's nebula. Their paths had been expected to be more random.

"Finding all these stars in an orderly rotation was the last explanation anyone would think of," Chapman said in a statement released during the meeting held in Minneapolis, Minnesota.

The implication is that the nebula is 220,000 light-years in diameter, instead of the earlier estimate of 70,000 to 80,000 light-years. In the cosmos, that means Andromeda stretches out over the length of 12 full Moons.

This periphery of Andromeda is faint, accounting for about 10 percent of the light from the galaxy. Still, there are millions of stars presumably orbiting in this outer region.

By looking at separate components of a galaxy, one may try to piece together how the galaxy is building up over time. The central area of a spiral galaxy is believed to have formed in the first place, with the rotating nebula coming later. The type and orbit of stars in certain regions provides a kind of fossil record for the evolutionary history.

Andromeda is an "ideal laboratory" because it is so close, and yet it is outside our galaxy. In our own galaxy, it is very hard to study this evolution because the observers are stuck in the middle of it, said Chapman.

Yet this "laboratory" is full of puzzles as to how it came to be. Besides Andromeda's newly found size, researchers are scratching their heads over the fact that the outer rotating stars are arranged into about 20 identifiable clumps.

This would imply that they were formed out of the merger of smaller galaxies with the main galaxy. But rotating nebulae and clumps are not compatible in galaxy formation models, scientists argued.

"This giant nebula discovery will be hard to reconcile with computer simulations of forming galaxies," said Rodrigo Ibata, a teammate from the Observatoire Astronomique de Strasbourg in France. "You just don't get giant rotating nebulae from the accretion of small galaxy fragments."

Chapman said that if a merger is the correct explanation, it could have occurred in a relative recent time -- within the last 200 million years. Otherwise, the clumps should have been "washed out."

"We may, therefore, being viewing our big neighbor at a rare moment in its history, right after it has gobbled up one of its little neighbors," he said.

The Andromeda galaxy is a spiral one similar to our own Milky Way. Although there are smaller dwarf galaxies nearer to us, Andromeda is the closest large galaxy, at about 2 million light- years from Earth and visible to naked eye.

Source: Xinhua