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What's going on around Andromeda? Curious structure puzzles scientists.

Scientists have found 13 dwarf galaxies orbiting the Andromeda galaxy in what appears to be a fairly narrow ring. That makes no sense according to current models of galaxy formation.

Image

New research shows a massive structure of dwarf galaxies orbiting the Andromeda galaxy (pictured).

University of Utah/AP

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Thirteen dwarf galaxies are playing a cosmic-scale game of Ring Around Andromeda, forming an enormous structure astronomers have never seen before and are hard-pressed to explain with current theories of how galaxies form and evolve.

According to current theories, the small galaxies, which contain as many as a few tens of billions of stars each, should be randomly arranged around the Andromeda galaxy.

Instead, they orbit Andromeda within a plane more than 1 million light-years across and about 30,000 light-years thick. For comparison, the latest estimates of Andromeda's girth put its diameter at more than 220,000 light-years.

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The ring, if it can be called that, represents "the largest organized structure in what we call the local group of galaxies," says Michael Rich, a research astronomer at the University of California at Los Angeles and a member of the team reporting the results in the Jan. 3 issue of the journal Nature. The local group consists of more than 54 galaxies, including dwarfs, about 10 million light-years across.

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