M 91 Deep sky · Galaxy in Coma Berenices · M 91 · NGC 4548
M 91
The missing Messier object: for nearly two centuries nobody could find anything where Messier said M 91 was.
Save the chart (SVG) Tactile chart (A4 SVG)
- Type
- Galaxy
- Designations
- M 91 · NGC 4548
- Constellation
- Coma Berenices
- Apparent magnitude
- 11.0
- Apparent size
- 5′ × 4′
- Position (J2000)
- RA 12h 35m · Dec +14° 30′
The full Moon, about 31′ across, is 5.8 times as wide. The Sun comes no nearer to it than 17°, around late September, so it is never lost in the Sun’s glare, which reaches about 15° out.
When to see it
On the night of Sun 28 Mar 2027, when it is highest around midnight, on each city’s own clock:
| From | Rises | Highest | Sets |
|---|---|---|---|
| London GMT+1 | 17:58 | 01:12 · 53° | 08:26 |
| New York EDT | 18:17 | 01:07 · 64° | 07:57 |
| Singapore GMT+8 | 19:17 | 01:17 · 77° | 07:18 |
| Sydney GMT+11 | 19:49 | 01:08 · 42° | 06:28 |
Common questions
- Can I see it from 50° north?
- Yes. It rises and sets, and at its highest it stands 54° up in the south.
- Can I see it from 35° south?
- Yes. It rises and sets, and at its highest it stands 41° up in the north.
- When is the best time of year to see it?
- Around late March, when it stands opposite the Sun and is highest at midnight. It comes round about two hours earlier each month, so by late June it is highest in the early evening, and in late December not until the hours before dawn.
- How bright is it?
- Magnitude 11.0 in total, but that light is spread over 5′ × 4′ of sky: on average 23.0 magnitudes per square arcsecond. Against a dark country sky that average stands out; a city’s glow drowns all but its brightest parts.
- Naked eye, binoculars or telescope?
- Binoculars. At magnitude 11.0 it is past the eye, but a 50 mm pair under a dark sky reaches about magnitude 11.0.
History
The position Messier recorded holds nothing. In 1969 an amateur, William C. Williams, traced the slip in his measurement and showed he must have meant this barred spiral.
Look for
It is among the faintest on the list: a small, round glow in Coma Berenices with a hint of a bar across the middle in a larger telescope.
Look for
From 50° north it climbs to 54°; from 35° north it climbs to 69°; from the equator it climbs to 76°; from 35° south it climbs to 41°.
Look for
Magnitude 11.0 against the faintest star each can show: the eye under a city sky, 4.5 (6.5 short); the eye under a dark country sky, 7.0 (4.0 short); 50 mm binoculars under that dark sky, 11.0 (0.1 to spare); a 100 mm telescope there, 12.5 (1.6 to spare). Those limits are for a point of light; its light is spread over its whole size, so it needs some of that margin in hand. Through 7×50 binoculars, whose field is about 7.1° across, it spans 1% of the field’s width; through a 100 mm telescope at 50×, a field about 1° across, it spans 9% of the field’s width.