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NGC 4921
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NGC 4921 (also MCG 5-31-98, UGC 8134, and PGC 44899) is a magnitude +13.0 face-on barred spiral galaxy located 320 million light-years away in the constellation Corona Borealis.
In 1976, Canadian astronomer Sidney Van den Bergh categorised this galaxy as "anaemic" because of the low rate at which stars are being formed. He noted that it has "an unusually low surface brightness and exhibits remarkably diffuse spiral arms". Nonetheless, it is the brightest spiral galaxy in the coma cluster.

The galaxy was discovered by German-British astronomer William Herschel using a 47.5 cm (18.7 Inch) speculum reflecting telescope at Datchet, Berkshire, on the 11th April 1785.

Right Ascension 13h 01m 26.1s, Declination +27° 53' 09"

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A spectacular new image of an unusual spiral galaxy in the Coma Galaxy Cluster has been created from data taken by the Advanced Camera for Surveys on the NASA/ESA Hubble Space Telescope. It reveals lots of new details of the galaxy, NGC 4921, as well as an extraordinary rich background of more remote galaxies stretching back to the early Universe.

Credit: ESA/Hubble (M. Kornmesser & L. L. Christensen)

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A spectacular new image of an unusual spiral galaxy in the Coma Galaxy Cluster has been created from data taken by the Advanced Camera for Surveys on the NASA/ESA Hubble Space Telescope. It reveals fine details of the galaxy, NGC 4921, as well as an extraordinary rich background of more remote galaxies stretching back to the early Universe.

The Coma Galaxy Cluster, in the northern constellation of Coma Berenices, the hair of Queen Berenice, is one of the closest very rich collections of galaxies in the nearby Universe. The cluster, also known as Abell 1656, is about 320 million light-years from Earth and contains more than 1000 members.
The brightest galaxies, including NGC 4921 shown here, were discovered back in the late 18th century by William Herschel.
The galaxies in rich clusters undergo many interactions and mergers that tend to gradually turn gas-rich spirals into elliptical systems without much active star formation. As a result there are far more ellipticals and fewer spirals in the Coma Cluster than are found in quieter corners of the Universe.
NGC 4921 is one of the rare spirals in Coma, and a rather unusual one - it is an example of an "anaemic spiral" where the normal vigorous star formation that creates a spiral galaxy's familiar bright arms is much less intense. As a result there is just a delicate swirl of dust in a ring around the galaxy, accompanied by some bright young blue stars that are clearly separated out by Hubble's sharp vision. Much of the pale spiral structure in the outer parts of the galaxy is unusually smooth and gives the whole galaxy the ghostly look of a vast translucent jellyfish.
The long exposure times and sharp vision of Hubble also allowed it to not just image NGC 4921 in exquisite detail but also to see far beyond into the distant Universe. All around, and even through the galaxy itself, thousands of much more remote galaxies of all shapes, sizes and colours are visible. Many have the spotty and ragged appearance of galaxies at a time before the familiar division into spirals and ellipticals had become established.

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