Messier 91: Barred Spiral Galaxy in Coma Berenices
- Catalog
- M91 · NGC 4548
- Type
- Barred spiral galaxy
- Constellation
- Coma Berenices
- Best season
- Spring evenings
- Magnitude
- 10.2
- Apparent size
- 5.4′ × 4.4′
- Distance
- 60,000,000 light-years
- Position (J2000)
- 12h 35.4m, +14° 30′
- Best seen with
- Larger telescope
- Discovered
- Charles Messier, 1781
M91 is the faintest of the Messier galaxies in the Virgo Cluster, and for nearly two centuries nobody was sure which galaxy it was. Today it is identified with NGC 4548, a barred spiral just east of M88.
What it is
M91 is a barred spiral galaxy: a straight bar of stars crosses its center, and its arms start from the ends of the bar. It lies about 60 million light-years away, and its visible disk spans roughly 95,000 light-years, similar in size to our Milky Way. We see it fairly face-on, so the bar shows well in photographs.
It is one of several spirals in this part of the Virgo Cluster, along with M88 and M90. Like them, it seems to have lost some of its gas to the cluster's hot surroundings, and its outer arms are faint and smooth, with relatively little new star formation.
When to see it
From about 40°N, M91 is highest around midnight in early April and around 10 p.m. standard time (11 p.m. daylight time) in early May, reaching about 64° above the southern horizon. It can be seen from most of the Southern Hemisphere as well, in the northern sky in autumn.
How to find it
Find M84 and M86 at the midpoint of the line from Denebola, the star at Leo's tail, to Vindemiatrix (Epsilon Virginis). Hop about 2.3° northeast to M88, the brighter, elongated galaxy. M91 is about 0.8° east of M88, and about 1.4° north of M90. Use a chart, because the field has no bright stars and M91 is easy to sweep past.
What you will see
M91 is not a binocular object. In a 70–100 mm telescope under a dark sky it is a small, faint, round glow with a slightly brighter center, and from the suburbs it is usually invisible. A 200 mm telescope shows a soft round haze with a bright core, and on a good night the bar can appear as a slight elongation across the middle. Filters do not help.
History
Charles Messier recorded M91 on March 18, 1781, but at the position he gave there is no galaxy, so for a long time M91 was counted as a "missing" Messier object. In 1969 an amateur astronomer, William C. Williams, worked out that Messier had made an error in measuring the position from a nearby reference object, and that the galaxy he actually saw was NGC 4548. That identification is now generally accepted.