The vast Andromeda galaxy¹ appears to have expanded by digesting stars from other galaxies², research has shown.

研究显示，广阔的仙女座星云通过侵蚀其它星系的空间，看起来扩大了。

The Andromeda galaxy is still expanding

When an international team of scientists mapped Andromeda, they discovered stars that they said were "remnants（剩余，残余） of dwarf³ galaxies（矮星系）".

The astronomers⁵ report their findings in the journal Nature.

This consumption of stars has been suggested previously⁶, but the team's ultra-deep survey has provided detailed⁷ images to show that it took place.

This shows the "hierarchical（层系，层次） model" of galaxy formation in action.

The model predicts that large galaxies should be surrounded by relics⁸（遗物，遗迹） of smaller galaxies they have consumed.

The scientists charted the outskirts⁹ of Andromeda in detail for the first time.

They discovered stars that could not have formed within the galaxy itself.

Pauline Barmby, an astronomer⁴ from the University of Western Ontario who was involved in the study, told BBC News the pattern of the stars' orbits revealed their origin.

"Andromeda is so close that we can map out all the stars," she said.

"And when you see a sort of lump（块状，瘤） of stars that far out, and with the same orbit, you know they can't have been there forever."

Andromeda, which is approximately 2.5 million light years away from Earth, is still expanding, say the scientists.

The researchers also saw a "stream of stars" of a nearby galaxy called Triangulum "stretching" towards Andromeda.

Dr Scott Chapman, reader in astrophysics（天体物理学） at the Institute of Astronomy, University of Cambridge, was also involved in the research.

He said: "Ultimately, these two galaxies may end up merging¹⁰ completely.

"Ironically, galaxy formation and galaxy destruction seem to go hand in hand."

Nickolay Gnedin, an astrophysicist from the University of Chicago, who was not involved in this study, described the work as showing "galactic（星系的） archaeology¹¹（考古学） in action".