The nearby Andromeda galaxy is surrounded by relatively young stars, new observations show, suggesting that our neighbor has stirred one or more other galaxies into its stellar mix more recently or more vigorously than has been the case with our own Milky Way.

Andromeda is a lot like the Milky Way. Both are large spiral galaxies anchored by supermassive black holes. At just 2.5 million light-years away, Andromeda is the largest nearby galaxy in a universe that spans some 13.7 billion light-years.

Both galaxies are surrounded by sparsely populated halos, in each case a spherical collection of stars that envelopes a relatively flat main disk, where most stars reside. In the Milky Way, the halo stars tend to be nearly as old as the universe itself, generally between 11 billion and 13 billion years.

Brown says the relatively young stars indicate Andromeda either went through a colossal merger or consumed several smaller galaxies billions of years ago. He can't say which.

Brown and his colleagues used the Hubble Space Telescope to examine faint stars -- common ones like our Sun -- in Andromeda's halo, where only giant, bright stars had previously been studied. They analyzed the stars for chemical composition, knowing that stars are chemical factories, and the universe's earliest stars -- like those in the Milky Way's halo -- are bereft of heavy elements. Only later generations of stars produce and contain heavier elements.

Andromeda halo stars are rich in heavy elements, which reveals their relative youth.

One of three things happened, Brown will tell a meeting of researchers today at the STScI, which operates Hubble for NASA:

• Collisions destroyed Andromeda's disk when it was young and dispersed many of its stars into the halo;
• or a single collision destroyed a relatively massive invading galaxy and dispersed its stars and some of Andromeda's disk stars into the halo;
• and/or many stars formed during the collision itself.