M30 (mag. +7.4) is a dense globular cluster in the southern constellation of Capricornus. Located 26,100 light-years from Earth, it's visible in binoculars appearing as a slightly elongated smudge of light. M30 is unusual in that it orbits the galaxy in the opposite direction (retrograde) to most other globulars, suggesting that it was acquired from a satellite galaxy rather than forming within the Milky Way. It's best seen during July, August and September from tropical and Southern Hemisphere locations.
The globular was discovered by Charles Messier on August 3, 1764, who noted it as "a round nebula without stars". It was later described in John L. E. Dreyer's New General Catalogue (NGC) as a "remarkable globular, bright, large, slightly oval." Since it's located in the dim constellation of Capricornus, M30 is one of the more challenging Messier objects to locate. This is especially true for Northern Hemisphere based observers, where the cluster never climbs very high above the southern horizon.
Capricornus is the second faintest zodiac constellation after Cancer. It's positioned to the east of Sagittarius, south of Aquarius and southeast of Aquila. A good starting point to find M30 is the brightest star in the constellation, Deneb Algiedi (δ Cap - mag. +2.9). Next move about 7 degrees southwest to arrive at stars 36 Cap (mag. +4.5) and zeta Cap (ζ Cap - mag. +3.8). Located just over 3 degrees southeast of this pair is M30. Right next to M30 on the eastern side is mag. +5.2 star 41 Cap, which acts as a good marker.
M30 can be easily viewed with a pair of 10x50 binoculars where it appears as a non-stellar, slightly elongated hazy patch of light with a brighter center. Telescopes with apertures starting at 100mm (4-inch) will begin to resolve the outer parts of the cluster. Through a medium size 200mm (8-inch) scope, M30 appears bright with a small core and a large halo. Many stars are resolved in the halo, which extends up to 12 arc minutes in diameter. The compressed core covers only about 1 arc minute. It's a rewarding globular especially with medium to large size telescopes.
M30 is located at a distance of about 29,400 light-years from Earth and is estimated to be nearly 13 billion years old. This cluster is similar to at least 20 of the 150 or so globulars in the Milky Way Galaxy. It exhibits an extremely dense stellar population and has undergone a core collapse (like M15, M70 and probably M62). Sadly, M30 is not loved by Messier Marathoners. These astronomers aim to observe all of the Messier objects in a single evening and usually attempt this during a moonless night in March. M30 is probably the most difficult target for the Marathoners and is often missed.
M30 Data Table
|Object Type||Globular cluster|
|RA (J2000)||21h 40m 22s|
|DEC (J2000)||-23d 10m 45s|
|Apparent Size (arc mins)||12 x 12|
|Age (years)||12.93 Billion|
|Number of Stars||150,000|