M9 is a Class VIII globular cluster located in the southern constellation Ophiuchus at right ascension 17h 19m 11.78s and declination -18°30’58.5”. The cluster has the designation NGC 6333 in the new General Catalogue. It is also known as ESO 586-SC005, h 1979, or h 3677. It lies at a distance of 25,800 light-years from Earth. With an apparent magnitude of 8.42, M9 is too faint to be seen with the naked eye. M9 has a total luminosity about 120,000 times that of the Sun and an absolute magnitude of -8.04. Messier 9 is one of the nearest globular clusters to the centre of the Milky Way. It lies at an approximate distance of 5,500 light-years from the galactic centre. As a result, it appears a bit flattened due to the gravitational influence of the Milky Way core pulling it slightly out of shape. The cluster is moving away from us at the very high velocity of 224 km/s. Messier 9 has an angular diameter of 12 arc minutes, corresponding to a linear diameter of 90 light-years. The best time of year to observe the cluster is in the months of May, June and July.
The cluster was discovered by Charles Messier, who added it to his catalogue on May 28, 1764. William Herschel was the first to resolve individual stars in M9 almost two decades after Messier had discovered it. English astronomer Admiral William Henry Smyth observed M9 in July 1834.
There have been 24 variable stars found in M9: 21 RR Lyrae variables, plus a long-period variable, Type II Cepheid, and an eclipsing binary. Based upon the periods of the RR Lyr variables, this cluster is classified as an Oosterhoff type II globular, which precludes an extra-galactic origin. The brightest stars in M9 have a visual magnitude of 13.5. The cluster has an overall spectral type of F2. The stars in it are about twice as old as the Sun and have a significantly different composition. They generally lack heavier elements like oxygen, iron and carbon, which are scarce in globular clusters because the stars in them formed when the universe was much younger and heavier elements were present in far smaller quantities. The heavier elements were formed gradually in the cores of stars and released into space in supernova explosions to become part of a new generation of stars. They only existed in tiny amounts some 12 billion years ago and each new generation of stars has had more heavy elements than the previous one. 13 variable stars have been discovered in the cluster. 10 of them were found by the German astronomer Walter Baade.
Messier 9 lies at the edge of Barnard 64, a dark nebula, and the cluster’s light is obscured by interstellar dust. The core region of Barnard 64 is located about 25′ west of M9, but the nebula itself stretches almost all the way to the cluster. Another dark nebula, Barnard 259, lies to the southeast of M9. Messier 9 can be located about 3 degrees southeast of the star Eta Ophiuchi. The star, also known by its traditional name Sabik, belongs to the spectral class A2 V and has an apparent magnitude of 2.43. It is the second brightest star in Ophiuchus. There are two other globular clusters lying in the vicinity of M9; NGC 6356 lies roughly 80′ to the northeast, while NGC 6342 is located 80′ to the southeast. NGC 6356 has a visual magnitude of 8.25 and is slightly smaller than M9, with an apparent diameter of 8 arc minutes. NGC 6342 is even fainter, with an apparent magnitude of 9.7, and spans 3 arc minutes. Credits: Messier Objects, NASA, Universe Today, Wikipedia.