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Although Abell 21, or Sh2-274, has a large apparent diameter of 10 arcminutes, its surface brightness is so low, with features ranging from magnitude 16-25, that it was not discovered until 1955. It is a planetary nebula lying at a distance of 1,500 light-years in the constellation Gemini, which gives it an estimated diameter of 4 light-years. The parent star, now a white dwarf, is thought to be the deep blue star near the center of the nebula.