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

研究发现，巨大的仙女座星云通过侵蚀其他星系的星体来扩张。

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

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（层次，层系） 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（郊区） 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, Canada, 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 from Earth, is still expanding, say the scientists.

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

Dr Scott Chapman, reader in astrophysics（天体物理学） at the Institute of Astronomy, University of Cambridge, UK, 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, US, who was not involved in this study, described the work as showing "galactic（星系的） archaeology¹¹（考古学） in action".