Galaxy in Coma Berenices
M91
Lost for two hundred years: Messier measured its position from M89 and then applied the figures to M58 by mistake, so nothing was where the catalogue said it was. An amateur, William Williams, spotted the slip in 1969 and matched the entry to NGC 4548.
Williams was working from the published positions and simply did the arithmetic that nobody had bothered to redo in two centuries. It is a barred spiral with an unusually strong bar, and the bar has funnelled so much gas inward that the outer disc has almost nothing left to form stars from.
- Catalogue
- M91
- Type
- Galaxy
- Constellation
- Coma Berenices
- Distance
- 63 million light-years
- Apparent magnitude
- 10.2
- Surface brightness
- 22.1 mag/□″
- Apparent size
- 5′ × 4′
- Right ascension
- 12h 35m
- Declination
- +14.50°
- Found by
- Charles Messier, 1781
Small enough that its 10.2 magnitudes arrive concentrated rather than smeared: 22.1 per square arcsecond, which is why it stands out from the sky better than its brightness alone suggests. Why surface brightness is the number that matters.
Could you see it?
Straightforward with 10×50 binoculars from Bortle 3; with a 200mm from Bortle 4.
| Sky | Eye | 10×50 | 80mm | 130mm | 200mm |
|---|---|---|---|---|---|
| Bortle 1 21.9 mag/□″ | No | Easy | Easy | Easy | Easy |
| Bortle 2 21.7 mag/□″ | No | Easy | Easy | Easy | Easy |
| Bortle 3 21.4 mag/□″ | No | Easy | Easy | Easy | Easy |
| Bortle 4 20.9 mag/□″ | No | No | Easy | Easy | Easy |
| Bortle 5 20.3 mag/□″ | No | No | Challenge | Challenge | Challenge |
| Bortle 6 19.3 mag/□″ | No | No | No | Challenge | Challenge |
| Bortle 7 18.6 mag/□″ | No | No | No | No | No |
| Bortle 8 18.1 mag/□″ | No | No | No | No | No |
| Bortle 9 17.8 mag/□″ | No | No | No | No | No |
At the eyepiece
About as faint as galaxies here get, and correspondingly awkward: a dim oval that needs a dark night and averted vision, with the bar strictly a large-aperture proposition. Given that it went missing for two centuries, being hard to see is at least consistent.
When, and from where
Highest in the middle of the night in March. Passes overhead at 15°N and rises from everywhere except the far south — below 75°S it never clears the horizon.
| Latitude | Highest it gets | |
|---|---|---|
| 60°N Oslo, Anchorage | 45° | |
| 51°N London, Calgary | 54° | |
| 40°N New York, Madrid | 65° | |
| 30°N Cairo, Houston | 75° | |
| 0°N the equator | 76° | |
| 33°S Sydney, Cape Town | 43° |
Nearby in the sky
The whole catalogue →This is a busy patch. 4 more of the catalogue sit within five degrees, which is a binocular field or two — worth planning a session around rather than visiting once. Everything below is measured from M91 itself, not from the middle of Coma Berenices.
- M88 Galaxy 50′ apart Coma Berenices · magnitude 9.6 · binoculars from Bortle 5 or darker
- M90 Galaxy 1.4° apart Virgo · magnitude 9.5 · binoculars from Bortle 4 or darker
- M89 Galaxy 2.0° apart Virgo · magnitude 9.8 · binoculars from Bortle 4 or darker
- M87 Virgo A 2.4° apart Virgo · magnitude 8.6 · binoculars from Bortle 5 or darker
One of 110. Positions J2000; the sky is the final authority.