Right Ascension	21 : 40.4 (h:m)
Declination	-23 : 11 (deg:m)
Distance	26.1 (kly)
Visual Brightness	7.2 (mag)
Apparent Dimension	12.0 (arc min)

Discovered 1764 by Charles Messier.

Globular cluster Messier 30 (M30, NGC 7099), at about 26,000 light years distance and about 90 light years across, and appears to us under an angular diameter of about 12.0 arc minutes. It is fairly dense (as its concentration class V indicates), and a fine object in even small telescopes. Its brightest red giant stars are about of apparent visual magnitude 12.1, its horizontal branch giants at magnitude 15.1. Only about 12 variable stars have been found in this globular cluster. A color-magnitude diagram (CMD) of M30 can be found in Richer et.al. (1988). Its overall spectral type has been determined as F3, and its color index was given as B-V=0.60 mag. It is approaching us at 181.9 km/s.

The core of M30 exhibits an extremely dense stellar population, and has undergone a core collapse, similar to at least 20 other of the 150 globulars in the Milky Way Galaxy, including M15, M70, and possibly M62. Consequently, M30's core is very small in extension, only about 0.12 arc minutes (7.2 arc seconds, corresponding to a linear diameter of 0.9 light years), and its half-mass radius is 1.15 arc min (8.7 light years); half of this cluster's mass is concentrated in a spherical volume of a radius equal to the distance of Sirius from us, or 17.4 light years diameter. On the other hand, its tidal radius is large: 18.34 arc minutes, corresponding to a linear radius of 139 light years. Beyond that distance, member stars would escape simply because of the Milky Way Galaxy's tidal gravitational forces.

Despite its compressed core, close encounters of the member stars of globular cluster M30 seem to have occurred comparatively rare, as it appears to contain only few X-ray binary stars, according to investigations with the Chandra X-ray Observatory satellite. These particular stellar systems are thought to form in close encounters as they occur occasionally in the denser zones of globular clusters.

Cecilia Payne-Gaposchkin mentions that a dwarf nova had occurred in M30; another one has been detected in M5 and a third in NGC 6712.

M30 was one of the original discoveries of Charles Messier, who cataloged it on August 3, 1764 and like most of his globulars, described it as round nebula, containing no stars. It was first resolved by William Herschel around 1784.

The brightest stars of M30 can be seen in telescopes starting at or even just below 4-inch aperture.

M30 is less loved by Messier Marathoners, as it is often the last missed object of an almost-complete Messier Marathon, a tour for viewing all Messier objects in one night (which is possible near the end of March in moonless nights). This is, however, only because of its location in the sky; otherwise it is a nice object for amateur astronomers.

NGC Online data for M30

References:

• Harvey B. Richer, Gregory G. Fahlman, and Don A. Vandenberg, 1988. Deep CCD photometry in globular clusters. VII - M30. Astrophysical Journal, Vol. 329, p. 187-207 (June 1, 1988) [ADS: 1988ApJ...329..187R]

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Hartmut Frommert
Christine Kronberg
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Last Modification: August 21, 2007