# Messier 9

Messier 9
Messier 9 by HST
Credit: NASA/ESA
Observation data (J2000 epoch)
Class VIII[1]
Constellation Ophiuchus
Right ascension 17h 19m 11.78s[2]
Declination –18° 30′ 58.5″[2]
Distance 25.8 kly (7.9 kpc)[3]
Apparent magnitude (V) +7.9[4]
Apparent dimensions (V) 9.3′[4]
Physical characteristics
Mass 4.22×105[3] M
Metallicity ${\displaystyle {\begin{smallmatrix}\left[{\ce {Fe}}/{\ce {H}}\right]\end{smallmatrix}}}$ = –1.77[3] dex
Estimated age 12.0 Gyr[5]
Other designations HD 156587, NGC 6333[6]

Messier 9 or M9 (also designated NGC 6333) is a globular cluster in the constellation of Ophiuchus. It is positioned in the southern part of the constellation to the southwest of Eta Ophiuchi, and lies atop a dark cloud of dust designated Barnard 64.[4][7] The cluster was discovered by French astronomer Charles Messier on June 3, 1764, who described it as a "nebula without stars".[8] In 1783, English astronomer William Herschel was able to use his reflector to resolve individual stars within the cluster. He found the cluster to be 7−8′ in diameter with stars densely packed near the center.[9]

M9 has an apparent magnitude of 7.9, an angular size of 9.3′, and can be viewed with a small telescope.[4] It is one of the nearer globular clusters to the center of the Milky Way Galaxy with a separation of around 5,500 light-years from the Galactic Core. Its distance from Earth is 25,800 light-years.

The total luminosity of this cluster is around 120,000 times that of the Sun, the absolute magnitude being -8.04. The brightest individual stars in M9 are of apparent magnitude 13.5, making them visible in moderately sized telescopes. 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. No blue stragglers or SX Phoenicis variables have been discovered. 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.[10]

Nearby, at about 80' to the northeast of M9 is the dimmer globular cluster NGC 6356, while at about 80' to the southeast is the globular NGC 6342.

## References

1. ^ Shapley, Harlow; Sawyer, Helen B. (August 1927), "A Classification of Globular Clusters", Harvard College Observatory Bulletin (849): 11–14, Bibcode:1927BHarO.849...11S.
2. ^ a b Formiggini, Liliana; et al. (May 2002), "Hidden subluminous stars among the FAUST UV sources towards Ophiuchus", Monthly Notices of the Royal Astronomical Society 332 (2): 441−455, arXiv:astro-ph/0210325, Bibcode:2002MNRAS.332..441F, doi:10.1046/j.1365-8711.2002.05327.x.
3. ^ a b c d Boyles, J.; et al. (November 2011), "Young Radio Pulsars in Galactic Globular Clusters", The Astrophysical Journal 742 (1): 51, arXiv:1108.4402, Bibcode:2011ApJ...742...51B, doi:10.1088/0004-637X/742/1/51.
4. ^ a b c d Gilmour, Jess K. (2012), The Practical Astronomer’s Deep-sky Companion, The Patrick Moore Practical Astronomy Series, Springer Science & Business Media, p. 75, ISBN 1447100719.
5. ^ Koleva, M.; et al. (April 2008), "Spectroscopic ages and metallicities of stellar populations: validation of full spectrum fitting", Monthly Notices of the Royal Astronomical Society 385 (4): 1998–2010, arXiv:0801.0871, Bibcode:2008MNRAS.385.1998K, doi:10.1111/j.1365-2966.2008.12908.x
6. ^ "SIMBAD Astronomical Database". Results for NGC 6333. Retrieved 2006-11-15.
7. ^ O'Meara, Stephen James (2014), Deep-Sky Companions: The Messier Objects, Cambridge University Press, p. 71, ISBN 1107018374.
8. ^ Machholz, Don (2002), The Observing Guide to the Messier Marathon: A Handbook and Atlas, Cambridge University Press, p. 23, ISBN 0521803861.
9. ^ Klein, Hermann Joseph (1901), Star Atlas, Society for promoting Christian knowledge, p. 55.
10. ^ Arellano Ferro, A.; et al. (September 2013), "A detailed census of variable stars in the globular cluster NGC 6333 (M9) from CCD differential photometry", Monthly Notices of the Royal Astronomical Society 434 (2): 1220−1238, arXiv:1306.3206, Bibcode:2013MNRAS.434.1220A, doi:10.1093/mnras/stt1080.