ROBERT GENDLER / SCIENCE PHOTO LIBRARY ROBERT GENDLER / SCIENCE PHOTO LIBRARY
Open star cluster M103, optical image. This is an old and loose cluster of stars that lies over 8000 light years from Earth in the constellation Cassiopeia. Several massive bright blue stars are seen, along with one prominent red giant at centre. Red giants are stars that swell up and cool due to changes in their internal structure.
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