Object Name: Messier 18
Alternative Designations: M18, NGC 6613
Object Type: Open Star Cluster
Right Ascension: 18 : 19.9 (h:m)
Declination: -17 : 08 (deg:m)
Distance: 4.9 (kly)
Visual Brightness: 7.5 (mag)
Apparent Dimension: 9.0 (arc min)
Locating Messier 18: Because Messier 18 is nothing more than a small collection of stars which are slightly brighter than the background Milky Way stars, it isn’t easy to distinguish in binoculars or a finderscope if you’ve never seen it before. One of the most sure ways of locating it is to become familiar with Messier 17 and simply aim a couple of degrees (about a field of view) south. While it won’t strike you as a grand object, you will notice that the stars are compressed in this area and that there are several dozen of them which appear brighter than the rest. In a telescope, use your lowest magnification. Since this is a very well spread cluster, it is easily resolved in even modest instruments.
What You Are Looking At: Located about 4,900 light years and spread over a 17 light year expanse of space, this group of around 20 stars is only about 32 million years old. Its hottest members are spectral type B3, yet you will also see many yellow and orange stars as well.
But, is M18 alone in space? Not necessarily. According to research done by R. and C. R. de la Fuente Marcos, M18 may very well be a binary cluster, paired with the open cluster – NGC 6618 – which is harbored inside M17. “We have shown that binary open clusters appear to constitute a statistically significant sample and that the fraction of possible binary clusters in the Galactic disk is comparable to that in the Magellanic Clouds. The spatial proximity of two almost coeval open clusters, compared to the large distances which typically separate these objects, suggests that both objects were formed together. In starforming complexes, one star cluster might capture another to form a bound state in the presence of a third body or of energy dissipation. This mechanism may also be at work within orbital resonances for non-coeval clusters.”
History: M18 was an original discovery of Charles Messier who states in his notes: “In the same night [June 3 to 4, 1764], I have discovered a bit below the nebula reported here above, a cluster of small stars, environed in a thin nebulosity; its extension may be 5 minutes of arc: its appearances are less sensible in an ordinary refractor of 3 feet and a half [FL] than that of the two preceding [M16 and M17]: with a modest refractor, this star cluster appears in the form of a nebula; but when employing a good instrument, as I have done, one sees well many of the small stars: after my observations I have determined its position: its right ascension is 271d 34′ 3″, and its declination 17d 13′ 14″ south.”
In this circumstance, we must give Messier great credit considering his observations were long before we understood stellar movements and the nature of open clusters. While Messier seems to have spotted some nebulosity around the cluster which may have belonged to M17, he takes a later historic cut from Smyth: “A neat double star, in a long and straggling assemblage of stars,below the Polish shield. A 9 and B 11 [mag], both blueish. This cluster was discovered by Messier in 1764, and registered as a mass of small stars appearing like a nebula in a 3 1/2-foot telescope; which affords another instance that the means of that very zealous observer did not quadrate with his diligence.” What a shame Smyth wasn’t around to later know that M18 could be paired with its nebulous neighbor!
B&W image thanks to Palomar Observatory, courtesy of Caltech, and color image thanks to Hillary Mathis, REU program/NOAO/AURA/NSF.