Messier 99 (NGC 4254) is a beautiful spiral galaxy situated in the southern part of Coma Berenices, and seen almost face-on.
M 99 was discovered in 1781 by Charles Messier's colleague and friend Pierre Mechain, along with the nearby galaxies M 98 and M 100. Messier confirmed and added these to the final published edition of his catalog later the same year. M 99 was the second "spiral nebula" to be recognized, by Lord Rosse in the spring of 1846, after M 51 in spring 1845.
Amateur Observation
M 99 is one of the fainter Messier objects, at magnitude 9.9. Its disk extends 5.4' x 4.8' E-W, and within it is a bright core. From the north and south sides of the core, the spiral arms curve counterclockwise. The more prominent southern arm arcs toward the west; the fainter, shorter northern arm toward the NE. A conspicuous dark gap separates the galaxy's core from the western curve of the southern arm.
Three supernovae have been recorded in Messier 99: the type II supernova 1967H, which reached mag. 14 in June 1967; the type II supernova 1972Q, mag. 15.6 in December 1972; and 1986I of type I, reaching mag. 14 in May 1986.
Properties and Evolution
Although M 99 appears in Coma Berenices, it is a member of the Virgo Cluster of galaxies, and therefore 50 to 60 million light years distant. It has one of the largest red shifts in the cluster, receding at 2380 km/sec. Its true diameter is at least 95,000 light years; M 99 has an absolute magnitude around -22, or a luminosity of 53 billion suns.
Messier 99 is of type Sc, rotates clockwise (unlike the nearby spiral M 100), and is unusually asymmetric. The galaxy has a normal-looking arm, and an extended arm that is less tightly wound. This asymmetry is now thought be due to a recent encounter with another members of the Virgo Cluster: the dark galaxy VIRGOHI21. The gravity from VIRGOHI21 appears to have distorted M 99 and drawn out a bridge of neutral hydrogen gas that links the two objects. After they go their separate ways, it is expected that M 99's drawn-out arm will relax to match the normal arm.