|Observation data (J2000 epoch)|
|Right ascension||18h 24m 32.89s|
|Declination||–24° 52′ 11.4″|
|Distance||17.9 kly (5.5 kpc)|
|Apparent magnitude (V)||+7.66|
|Apparent dimensions (V)||11′.2|
|VHB||15.55 ± 0.10|
|Estimated age||12.0 Gyr|
|Notable features||Contains first pulsar discovered in a globular|
|Other designations||M 28, NGC 6626, GCl 94|
Messier 28 (also known as M28 or NGC 6626) is a globular cluster in the constellation Sagittarius. It was discovered by French astronomer Charles Messier on July 27, 1764. He briefly described it as a "nebula containing no star... round, seen with difficulty in 3½-foot telescope; Diam 2′." The 2′ at the end indicates an angle of two arcminutes.
In the sky it is less than a degree to the northwest of the 3rd magnitude star Kaus Borealis. This cluster is faintly visible as a hazy patch with a pair of binoculars and can be readily found in a small telescope with a 8 cm (3.1 in) aperture, showing as a nebulous feature spanning 11.2 arcminutes. At 15 cm (5.9 in), the core becomes visible and a few individual stars can be resolved along the periphery. Larger telescopes will provide greater resolution, with a 25 cm (9.8 in) telescope revealing a 2′ core.
M28 is at a distance of about 17,900 light-years away from Earth. It has a combined 551,000 times the mass of the Sun and is 12 billion years old. 18 RR Lyrae type variable stars have been observed in this cluster. In 1986, M28 became the first globular cluster where a millisecond pulsar, PSR B1821–24, was discovered with the Lovell Telescope at Jodrell Bank Observatory. A total of 11 additional millisecond pulsars have since been detected in the cluster with the Green Bank Telescope. As of 2011, this is the third largest known population of pulsars in a cluster following Terzan 5 and 47 Tucanae.
- Shapley, Harlow; Sawyer, Helen B. (August 1927), "A Classification of Globular Clusters", Harvard College Observatory Bulletin (849): 11–14,
- "SIMBAD Astronomical Database". Results for NGC 6626. Retrieved 2006-11-16.
- Boyles, J.; et al. (November 2011), "Young Radio Pulsars in Galactic Globular Clusters", The Astrophysical Journal 742 (1): 51,
- Inglis, Mike (2004), Astronomy of the Milky Way: Observer's guide to the northern sky, Patrick Moore's Practical Astronomy Series 1, Springer, p. 21,
- distance × sin( diameter_angle / 2 ) = 30 ly radius
- Testa, Vincenzo; et al. (February 2001), "Horizontal-Branch Morphology and Dense Environments: Hubble Space Telescope Observations of Globular Clusters NGC 2298, 5897, 6535, and 6626", The Astronomical Journal 121 (2): 916–934,
- Koleva, M.; et al. (April 2008), "Spectroscopic ages and metallicities of stellar populations: validation of full spectrum fitting",
- "JBO - Stars".
- Thompson, Robert Bruce; Thompson, Barbara Fritchman (2007), Illustrated Guide to Astronomical Wonders, Diy Science, O'Reilly Media, Inc., p. 402,
- Burnham, Robert (1979), Burnham's celestial handbook: an observer's guide to the universe beyond the solar system, Dover books explaining science 3 (2nd ed.), Dover Publications, p. 1609,
- Bogdanov, Slavko; et al. (April 2011), "Chandra X-ray Observations of 12 Millisecond Pulsars in the Globular Cluster M28", The Astrophysical Journal 730 (2): 81,
- Globular Cluster M28 @ SEDS Messier pages
- Messier 28, Galactic Globular Clusters Database page
- Messier 28 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Astrophoto, Sky Map, Articles and images