Messier 91, also known as NGC 4548, is one of the faintest objects in the Messier catalog and is notable for its prominent central bar structure. The galaxy is a member of the Virgo Cluster and is classified as an anemic galaxy, meaning it has a relatively low rate of star formation and little interstellar gas. Its bar and spiral arms are best revealed in long-exposure astrophotography rather than through visual observation, especially in smaller telescopes. M91 was discovered by Charles Messier in 1781, but its true identity was lost for nearly 200 years due to a cataloging error, only being correctly identified as NGC 4548 in 1969.
Names and Catalog numbers
- Messier 91 (M91)
- NGC 4548
- UGC 7753
- PGC 41934
Position and the cosmic neighborhood
Messier 91 is situated in the southern part of Coma Berenices, just north of the border with Virgo and close to the line connecting the galaxies M89 and M90. It lies about two degrees north of the slightly brighter elliptical galaxy M89, making it accessible for wide-field astrophotography that captures multiple Virgo Cluster galaxies in one frame. The region is densely populated with other notable Messier galaxies, providing a rich field for astrophotographers interested in galaxy clusters. M91’s location within the Virgo Cluster means it is surrounded by numerous other spiral and elliptical galaxies, all gravitationally bound within the supercluster.
Nice to Know
- M91 is the faintest object in the Messier catalog, making it a challenging but rewarding target for astrophotographers.
- The galaxy’s bar structure is much more apparent in photographs than in visual observations, especially with exposures of several minutes.
- Its identity was a mystery for almost two centuries due to a bookkeeping error by Messier, only being matched with NGC 4548 in 1969.
- M91 is classified as an anemic galaxy, with very little gas and a low rate of star formation compared to typical barred spirals.
Brightnes, distance and size
Messier 91 has an apparent magnitude of about 10.2 to 11.0, making it one of the faintest Messier objects and best observed with medium to large telescopes under dark skies. The galaxy is approximately 60 to 63 million light-years away from Earth and spans about 100,000 light-years in diameter. Its apparent size in the sky is roughly 5.4 by 4.3 arcminutes, so long-exposure astrophotography is recommended to reveal its structure and surroundings within the Virgo Cluster.