November 10, 2006 (ScienceNOW) – If a castle is built with nearby rocks, you’d expect the stone walls to be made of the same stuff as any remaining boulders in the area. Likewise, if our Milky Way galaxy grew through the merging of smaller dwarf galaxies, you’d expect it to contain the same types of stars as the remaining dwarfs that have not been incorporated. But detailed observations of four of these dwarf galaxies show that this is not the case, indicating that theories about how galaxies form are incomplete.

“It’s a very interesting find,” says astronomer Leo Blitz of the University of California, Berkeley. “The accretion hypothesis is now firmly entrenched in the minds of people studying the evolution of galaxies, and this result calls the simplest version of the model into question.” But don’t go changing the models just yet, says theoretical astrophysicist Andrey Kravtsov of the University of Chicago. Although he finds the results “somewhat surprising,” he notes that the stars that make up the Milky Way’s halo came from dwarf galaxies that were accreted early in the Milky Way’s history. As a result, they may have had less time to evolve than current outlying dwarf galaxies, and this may explain their different composition.

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