STAR clusters around the Milky Way are being ripped apart by the Galaxy鈥檚 gravity. It鈥檚 bad news for the clusters, but good news for Earthbound astronomers because it helps explain where our Galaxy鈥檚 dark matter came from.
When a small object passes near to a larger one in space it experiences a stretching force because its near side feels the large object鈥檚 gravity more strongly than its far side. Astronomers have often wondered if these 鈥渢idal forces鈥� could affect entire clusters of stars.
Now Michael Odenkirchen and his colleagues at the Max Planck Institute for Astronomy in Heidelberg, Germany, have found the first example of a star cluster that has been stretched like a strand of spaghetti. They鈥檝e spotted a long, thin trail of stars strung out across about 20 times the diameter of the full Moon in the sky. The stars all contain very low levels of heavy elements, which indicates they are part of an ancient globular cluster. But such clusters are normally blob-shaped, and at the distances involved should appear in the sky only about as big as the full Moon. 鈥淭his shows globular clusters get shredded and torn apart by tidal forces,鈥� says Odenkirchen.
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The discovery could explain how stars are drawn into the Milky Way鈥檚 halo 鈥攖he spherical region surrounding the main galactic disc where our Solar System sits. The halo is probably made up of debris from passing clusters and 鈥渄warf galaxies鈥� caught by the Milky Way鈥檚 gravity over the past several billion years.
This could also explain how the Milky Way accumulates dark matter. Dark matter isn鈥檛 visible, but astronomers infer its existence because of its gravitational effect. Passing blobs of dark matter should behave just like passing star clusters, becoming stretched out, torn apart and eventually joining the galactic halo.