RUSSELL KIGHTLEY / SCIENCE PHOTO LIBRARY RUSSELL KIGHTLEY / SCIENCE PHOTO LIBRARY
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Dwarf galaxies. Computer artwork of dwarf spheroidal galaxies (small multicoloured circles) orbiting a spiral galaxy (blue/yellow, centre). Dwarf galaxies contain a relatively small number of stars (a few million) and can be spheroidal or elliptical in shape. They are fainter than expected for their size. Eight such galaxies orbit our Milky Way. The stars forming the dwarf galaxies seen here are disrupted and drawn out into arcs, due to tidal gravitational forces from the spiral galaxy. Tidal forces arise when the force of gravity acting on one side of an object (in this case, the side of each dwarf galaxy nearer the spiral) is greater than on the other.
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