Asterism in Aquarius
M73
Four faint stars in a Y, and nothing else. Messier suspected there was nebulosity between them and there is not; the stars are not even a group, merely a chance alignment. Catalogued honestly as a curiosity and kept out of stubbornness.
Later measurements settled it: the four stars are at different distances and moving in different directions, and there is nothing between them. It survives in the catalogue because Messier had written it down and later editors declined to tidy it away, and the sky is better documented for having an entry that says plainly there is nothing here.
- Catalogue
- M73
- Type
- Asterism
- Constellation
- Aquarius
- Distance
- unrelated stars, 2,000 to 3,000 light-years
- Apparent magnitude
- 9.0
- Surface brightness
- 20.0 mag/□″
- Apparent size
- 3′
- Right ascension
- 20h 59m
- Declination
- −12.63°
- Found by
- Charles Messier, 1780
Magnitude 9.0 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 10×50 binoculars from Bortle 6; with a 200mm from Bortle 9.
| 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 | 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 | No | Easy | Easy | Easy |
| Bortle 8 18.1 mag/□″ | No | No | Easy | Easy | Easy |
| Bortle 9 17.8 mag/□″ | No | No | Easy | Easy | Easy |
At the eyepiece
Four stars. That is the whole object, and they split easily in anything. The reason to visit is that Messier looked at these four stars, suspected something was there, wrote down honestly that he was not sure, and left it in. That is a better lesson than most objects here provide.
When, and from where
Highest in the middle of the night in August. Overhead at 13°S and southerly enough to sit low from northern Europe, though never out of reach there — only north of 77°N does it stop rising at all.
| Latitude | Highest it gets | |
|---|---|---|
| 60°N Oslo, Anchorage | 17° | Low — in the murk |
| 51°N London, Calgary | 26° | |
| 40°N New York, Madrid | 37° | |
| 30°N Cairo, Houston | 47° | |
| 0°N the equator | 77° | |
| 33°S Sydney, Cape Town | 70° |
Nearby in the sky
The whole catalogue →One more of the catalogue sits within five degrees, close enough to take in on the same night without moving far. Everything below is measured from M73 itself, not from the middle of Aquarius.
- M72 Globular cluster 1.3° apart Aquarius · magnitude 9.4 · binoculars from Bortle 4 or darker
- M30 Globular cluster 14° apart Capricornus · magnitude 7.2 · binoculars from Bortle 5 or darker
- M2 Globular cluster 15° apart Aquarius · magnitude 6.5 · binoculars from Bortle 5 or darker
- M75 Globular cluster 16° apart Sagittarius · magnitude 8.5 · binoculars from Bortle 5 or darker
One of 110. Positions J2000; the sky is the final authority.