Degree of difficulty 3 (of 5)

Minimum aperture 30mm

Designation NGC 6333

Type Globular cluster

Class VIII

Distance 46,090 ly (RR Lyr, 1999)

Size 150 ly

Constellation Ophiuchus

R.A. 17h 19.2min

Decl. –18° 31′

Magnitude 7.6

Surface brightness –

Apparent diameter 11′

Discoverer Messier, 1764

History Charles Messier discovered M 9 on the 28 th of May 1764, as a “nebula without star” and added: “it is round & its light faint, 3' diameter.” Exactly 20 years later, William Herschel reported that this object was actually a very rich star cluster. In the 1830s, Admiral Smyth observed M 9 in more detail and noted: “This fine object is composed of a myriad of minute stars, clustering into a blaze in the center, and wonderfully aggregated, with numerous outliers seen by glimpses.” Lord Rosse remarked: “The outline not round; on south side is an outlying portion separated from the chief portion by a dark passage.” The German observer Heinrich d'Arrest, by contrast, believed he saw an “almost double-core cluster” and noted an elongation in the north-south direction.

In 1918, Curtis remarked that M 9 maintained the relatively small diameter of 3' on photographic plates. But modern studies, based on much deeper exposures, have almost quadrupled this number.

Astrophysics M 9 has a distance of about 14,000 light-years from the galactic center, situated on its far side, just beyond the galactic bulge. The distance to us is 46,000 light-years, and its diameter of 150 light-years is quite average, as is its total mass of about 300,000 solar masses.

Because of its position behind the fields of dusty interstellar clouds in Ophiuchus, M 9 suffers from about one magnitude of interstellar absorption, in particular in its northern and western parts. Also, according to Shapley, M 9 did not appear exactly spherical, he rather regarded it as slightly elliptical (0.9).

The brightest individual star of M 9 reaches magnitude 13.5, the mean magnitude of the 25 brightest cluster members is 15.5. 16 RR Lyrae stars have been cataloged for M 9, and two other variables, one of which is a type II cepheid.