Messier 91, NGC 4548

Messier 91 (NGC 4548) is a barred spiral galaxy in the southern part of Coma Berenices, a member of the Virgo Galaxy Cluster, and the faintest object in Messier's catalog. For many years, M 91 was one of the "missing" Messier objects, whose true identity was in dispute.

Discovery and Identity

On March 18, 1781, Charles Messier discovered eight nebulous objects near the border of Virgo and Coma Berenices. He catalogued the last of these objects as M 91, but recorded its position erroneously. Thus, for a long time, M 91 was a "missing" Messier object. Although catalogued by William Herschel as H II.120 in 1784, Herschel found nothing there, and instead suspected that M 91 might have been the faint, magnitude 11.3 spiral galaxy NGC 4571 (Herschel's H III.602). Others suggested that M 91 was that was a duplicate observation of some other Messier object (Owen Gingerich suspected M 58), or that M 91 was actually a comet.

Texas amateur William C. Williams finally figured it out in 1969. Messier had recorded M 91's position relative to M 89, rather than M 58, as he had thought; and the true identity of M 91 was finally uncovered as NGC 4548.

Amateur Observation

With a visual magnitude of 10.2, NGC 4548 is one of the most difficult Messier objects for the moderately-equipped amateur to see. It is a barred spiral galaxy of type SBb; suggestions of the bar may be seen at medium power, if the viewing conditions are good enough to see the galaxy at all. Photos show the bar more clearly, and show the spiral arms emanating from the ends of the bar, out to apparent dimensions of 5.4' x 4.4'.

Messier had described M 91 as a "nebula without stars, fainter than M 90". John Herschel described it as "bright, large, little elongated, a little brighter in the middle" in his General Catalog. This is probably due to different viewing conditions: under poor skies, only the bright elongated bar region of this galaxy shows up; while under good conditions the spiral arms show up, and exhibit a slightly elongated shape. This effect can be reproduced to some degree with amateur instruments.

Physical Properties

M 91 is a member of the Virgo Cluster of Galaxies, as confirmed by a distance measurement of 58 million light years. This comes from Hubble Space Telescope observations of Cepheid variables within M 91, after applying the Hipparcos satellite's observational corrections to the Cepheid brightness scale, and coincides well with other Virgo galaxies.

Whether or not NGC 4548 is the original M 91, it is still one of the largest barred spirals in the Coma-Virgo Cluster, with a true diameter of over 100,000 light years. M 91's absolute magnitude is -21.3, a luminosity of 28 billion Suns. The red shift in M 91's spectrum indicates that it is receding at 400 km/sec, so it has a considerable velocity of 700 km/sec relative to the Virgo cluster, whose center is receding at 1100 km/sec.