Messier 30 (also known as NGC 7099) is a globular cluster in Capricornus. Discovered by Charles Messier in August of 1764, this globular was first resolved into stars by William Herschel in 1783.
Messier 30 is a bright (magnitude 7.2), well-resolved 12'-diameter disk of stellar points; it is a fine object for small telescopes. The cluster is distinctly elongated east to west; its brightest stars are about of visual magnitude 12.1. The star density increases dramatically near the center. The outer halo is filled with numerous star chains.
M 30 has a diameter of approximately 100 light years, and is about 26,000 light years away. The cluster contains at least 200,000 stars.
The core of M 30 exhibits an extremely dense stellar population, and has undergone a collapse similar to M 15, M 70, and possibly 20 other globulars in the Milky Way. Consequently, M 30's core is very small, less than a light year across. Its half-mass radius is 8.7 light years, about the distance from us to Sirius. On the other hand, its tidal radius is large: 139 light years. Beyond that distance, member stars would escape because of the Milky Way galaxy's gravitational forces.
Despite its compressed core, close encounters of M 30's member stars seem to have been rare. The cluster appears to contain only a few X-ray binary stars. These stellar systems are thought to form in close encounters that occur in the denser zones of globular clusters. A dwarf nova has occurred in M 30.