NGC 4151 is an intermediate spiralSeyfert galaxy with weak inner ring structure located 15.8 megaparsecs (52 million light-years) from Earth[4] in the constellation Canes Venatici. The galaxy was first mentioned by William Herschel on March 17, 1787; it was one of the six Seyfert galaxies described in the paper [5] which defined the term. It is one of the nearest galaxies to Earth to contain an actively growing supermassive black hole.[6] The black hole would have a mass on the order of 2.5 million to 30 million solar masses.[7] It was speculated that the nucleus may host a binary black hole, with about 40 million and about 10 million solar masses respectively, orbiting with a 15.8-year period.[8] This is, however, still a matter of active debate.
Some astronomers nickname it the "Eye of Sauron" from its appearance.[9]