NGC 4921

NGC 4921 is a large barred spiral galaxy found in the Coma Cluster, located in the constellation of Coma Berenices. It was discovered by the German-British astronomer William Herschel on 11 April 1785. This galaxy lies about 320 million light-years from Earth.

The galaxy features a nucleus with a bar structure surrounded by a ring of dust containing hot blue stars. Its outer parts have smooth, poorly defined spiral arms. In 1976, the Canadian astronomer Sidney Van den Bergh described NGC 4921 as "anemic" because it forms stars at a very low rate and has a low surface brightness with diffuse spiral arms.

Despite these features, NGC 4921 is the brightest spiral galaxy in the Coma Cluster. It is located near the cluster’s center and moves at a high speed compared to the average cluster velocity. Studies at the 21 cm wavelength show that NGC 4921 has very little hydrogen, likely because interactions with the intergalactic medium have stripped away much of its gas. Some of this gas is now falling back toward the galaxy, making NGC 4921 an important example for understanding how galaxies change in cluster environments.

Supernova

One bright explosion, called a supernova, was seen in NGC 4921. It was found on May 4, 1959, by an astronomer named Milton Humason using a special telescope at the Palomar Observatory. This supernova looked similar to another famous one called SN 1987a.