Degree of difficulty 3 (of 5)

Minimum aperture 30mm

Designation NGC 7099

Type Open cluster

Class V

Distance 29,460 ly (R2005) 30,730 ly (CMD, 1999)

Size 100 ly

Constellation Capricornus

R.A. 21h 40.4min

Decl. –23° 11′

Magnitude 7.3

Surface brightness –

Apparent diameter 12′

Discoverer Messier, 1764

History On the 8th of August 1764, Charles Messier found a “nebula” in Capricorn and noted: “It can be seen with difficulty in a simple refractor of 3½ ft; it is round & does not contain any star, 2′ diameter.”

In 1783, with his superior home-made reflectors, William Herschel was able to resolve M 30 into individual stars. He noted: “Towards the north are two rows of bright stars, four or five in a line.” Almost 50 years later, his son John commented in more detail: “Fine cluster; irregularly round, with two projections at its northern side. One is directed from the central brightness and consists of three or four bright stars of 12th magnitude; its position taken with micrometer is 350.4°; the other originates in the preceding side of the center, and is directed in a position 331.7° in a line not passing the center; diameter 6′, stars of 12th magnitude; has a star of 9th magnitude preceding it.” Smyth, observing with a significantly smaller aperture (5.9 inches) than John Herschel, described: “A fine pale white cluster. This object is bright, and from the straggling streams of stars on its northern verge, has an elliptical aspect, with a central blaze; and there are but few other stars, or outliers, in the field.”

Astrophysics The globular cluster M 30 lies at a distance of 30,000 light-years from us, below the galactic plane. Its orbit around the galactic center is inclined at 50° to the galactic disk and it takes 160 million years to complete.