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Galaxies are vast swarms of billions of stars along with huge interstellar clouds of gas and dust. A spiral galaxy has a broad, thin disk shape, with a bulge of stars in its core, Within the disk are winding arms of dark dust lanes and bright star-forming regions, This structure is stable when left alone, but is relatively easily disturbed when another galaxy passes near. Astronomers have studied galaxy interactions for decades, and Hubble's keen vision has been particularly useful for examining new details
