The magnificent spiral NGC 4921 is about 320 million light-years from Earth located near the center of the Coma cluster . The galaxy has a nucleus with a bar structure that is surrounded by a distinct ring of dust that contains recently formed, hot blue stars. The outer part consists of unusually smooth, poorly distinguished spiral arms. Ihas been informally dubbed anemic because of its low rate of star formation and low surface brightness. Visible in the image are, from the center, a bright nucleus, a bright central bar, a prominent ring of dark dust, blue clusters of recently formed stars, several smaller companion galaxies, unrelated galaxies in the far distant universe, and unrelated stars in our Milky Way Galaxy.
In May 4, 1959, a supernova explosion was observed in this galaxy by M. L. Humason at the Palomar Observatory. It appeared "quite far from the center" of the galaxy,and reached an estimated peak magnitude of 18.5. The light curve proved similar to supernova SN 1987a in the Large Magellanic Cloud, and it displayed "unusual photometric behavior".
Located more than five times farther away from the Sun than the prominent Virgo Cluster of galaxies, astronomers have estimated that more than 3,000 galaxies within a diameter of 20 million light years are part of the Coma Supercluster (below), making it extremely rich and dense. Nearly spherical, Coma is composed of mostly elliptical and lenticular galaxies within a one-megaparsec diameter, with some spiral galaxies towards the outer reaches. Most of these galaxies have been estimated to be as old as the Cosmos, as much as 13+ billion years old. About 90% of the mass of the Coma cluster is believed to be dark matter.
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