Open cluster in Sagittarius
M25
Somewhere in this loose scattering sits U Sagittarii, a Cepheid that brightens and fades on a seven-day cycle. Watch it over a fortnight and you can see a Messier object change, which is not something the rest of the list offers. That is a rarer thing than it sounds.
Cepheids are the rung of the distance ladder everything above it stands on, and one sitting inside a cluster whose distance can be measured independently is a chance to check the calibration rather than assume it. There are only a handful anywhere.
- Catalogue
- M25
- Type
- Open cluster
- Constellation
- Sagittarius
- Distance
- 2,000 light-years
- Apparent magnitude
- 4.6
- Surface brightness
- 20.8 mag/□″
- Apparent size
- 32′
- Right ascension
- 18h 32m
- Declination
- −19.25°
- Found by
- Philippe Loys de Chéseaux, 1745
Magnitude 4.6 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 7 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/□″ | Easy | Easy | Easy | Easy | Easy |
| Bortle 3 21.4 mag/□″ | Easy | Easy | Easy | Easy | Easy |
| Bortle 4 20.9 mag/□″ | Easy | Easy | Easy | Easy | Easy |
| Bortle 5 20.3 mag/□″ | Easy | Easy | Easy | Easy | Easy |
| Bortle 6 19.3 mag/□″ | Easy | Easy | Easy | Easy | Easy |
| Bortle 7 18.6 mag/□″ | Easy | 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 loose, bright scattering that binoculars handle well. If you note the brightnesses of its stars against each other over two weeks, U Sagittarii visibly shifts — which makes this one of the very few Messier objects where coming back a second time shows you something different.
When, and from where
Highest in the middle of the night in June. Overhead at 19°S and southerly enough to sit low from northern Europe, though never out of reach there — only north of 71°N does it stop rising at all.
| Latitude | Highest it gets | |
|---|---|---|
| 60°N Oslo, Anchorage | 11° | Low — in the murk |
| 51°N London, Calgary | 20° | Low — in the murk |
| 40°N New York, Madrid | 31° | |
| 30°N Cairo, Houston | 41° | |
| 0°N the equator | 71° | |
| 33°S Sydney, Cape Town | 76° |
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 M25 itself, not from the middle of Sagittarius.
- M18 Open cluster 3.5° apart Sagittarius · magnitude 7.5 · binoculars from Bortle 9 or darker
- M24 Sagittarius Star Cloud 3.6° apart Sagittarius · magnitude 2.5 · binoculars from Bortle 9 or darker
- M17 Omega Nebula 4.0° apart Sagittarius · magnitude 6.0 · needs more than binoculars
- M22 Globular cluster 4.8° apart Sagittarius · magnitude 5.1 · binoculars from Bortle 5 or darker
Terms on this page
- Standard candle An object whose true brightness is known, so how faint it looks gives you the distance.
One of 110. Positions J2000; the sky is the final authority.