Open cluster in Cassiopeia
M103
The last entry in the catalogue as Messier published it — everything above 103 was added by later astronomers from his and Méchain's notes. A small fan of stars with one red giant in it, which is the only thing that makes it memorable.
It is around twenty-five million years old and lies about eight and a half thousand light-years away, which puts it among the more distant open clusters anybody bothers with. The red giant is Struve 131, and it is probably a foreground star rather than a member — which leaves the cluster with nothing memorable at all, honestly stated.
- Catalogue
- M103
- Type
- Open cluster
- Constellation
- Cassiopeia
- Distance
- 8,500 light-years
- Apparent magnitude
- 7.4
- Surface brightness
- 19.9 mag/□″
- Apparent size
- 6′
- Right ascension
- 1h 33m
- Declination
- +60.70°
- Found by
- Pierre Méchain, 1781
Magnitude 7.4 for the cluster as a whole, though that is a sum rather than a thing you see: what reaches you is a scatter of separate stars, and whether you catch it depends on how bright the brightest of them are. Why surface brightness is the number that matters.
Could you see it?
Straightforward with the naked eye from Bortle 1 or darker; with 10×50 binoculars from Bortle 9.
| Sky | Eye | 10×50 | 80mm | 130mm | 200mm |
|---|---|---|---|---|---|
| Bortle 1 21.9 mag/□″ | Easy | 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 | Easy | Easy | Easy | Easy |
| Bortle 5 20.3 mag/□″ | No | Easy | Easy | Easy | Easy |
| Bortle 6 19.3 mag/□″ | No | Easy | Easy | Easy | Easy |
| Bortle 7 18.6 mag/□″ | No | Easy | Easy | Easy | Easy |
| Bortle 8 18.1 mag/□″ | No | Easy | Easy | Easy | Easy |
| Bortle 9 17.8 mag/□″ | No | Easy | Easy | Easy | Easy |
At the eyepiece
A small fan of about twenty-five stars with one obviously red one, and a bright double at the apex. Easy in a small telescope and quickly exhausted. Its interest is where it sits in the story, not in the eyepiece.
When, and from where
Highest in the middle of the night in October. Circumpolar from most of Europe, Canada and the northern United States — it never sets there at all. From south of 29°S it never rises.
| Latitude | Highest it gets | |
|---|---|---|
| 60°N Oslo, Anchorage | 89° | Never sets |
| 51°N London, Calgary | 80° | Never sets |
| 40°N New York, Madrid | 69° | Never sets |
| 30°N Cairo, Houston | 59° | Never sets |
| 0°N the equator | 29° | |
| 33°S Sydney, Cape Town | — | Never rises |
Nearby in the sky
The whole catalogue →Nothing else in the catalogue is especially close; the nearest is 9.2° apart, which is a separate trip rather than a nudge of the tube. Everything below is measured from M103 itself, not from the middle of Cassiopeia.
- M76 Little Dumbbell 9.2° apart Perseus · magnitude 10.1 · binoculars from Bortle 4 or darker
- M52 Open cluster 15° apart Cassiopeia · magnitude 6.9 · binoculars from Bortle 9 or darker
- M110 Galaxy 21° apart Andromeda · magnitude 8.5 · binoculars from Bortle 3 or darker
- M34 Open cluster 21° apart Perseus · magnitude 5.5 · binoculars from Bortle 9 or darker
Terms on this page
- Red giant What a star becomes when the hydrogen in its core runs out.
One of 110. Positions J2000; the sky is the final authority.