By Ron Cowen
The Milky Way and its nearest large galactic neighbor, Andromeda, are more alike than earlier evidence had indicated. A new study shows that the two spiral galaxies evolved in a highly similar fashion over the first 3 billion to 4 billion years of their histories.
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The study reveals that the composition of some 1,000 stars in Andromeda’s halo—a vast cloud that includes the outer reaches of the galaxy—are deficient in all elements heavier than hydrogen, just as stars in the halo of the Milky Way are. A halo “is the true fossil relic of the earliest formation of a spiral galaxy,” notes Scott Chapman of the California Institute of Technology in Pasadena. The finding therefore suggests that the Milky Way and Andromeda had similar early histories, he says.