Messier 28 or M28, also known as NGC 6626, is a
globular cluster of stars in the center-west of
Sagittarius. It was discovered by French astronomer
Charles Messier in 1764.[11][a] He briefly described it as a "nebula containing no star... round, seen with difficulty in 31⁄2-foot telescope;
Diam 2
′."[12]
In the sky it is less than a degree to the northwest of the 3rd
magnitude star
Kaus Borealis (Lambda
Sgr). This cluster is faintly visible as a hazy patch with a pair of binoculars[11] and can be readily found in a small telescope with an 8 cm (3.1 in)
aperture, showing as a nebulous feature spanning 11.2
arcminutes. Using an aperture of 15 cm (5.9 in), the core becomes visible and a few distinct stars can be resolved, along the periphery. Larger telescopes will provide greater resolution,[5] one of 25 cm (9.8 in) revealing a dense 2′ core, with more density within.[11]
It is about 18,300
light-years away from
Earth.[3] It is about 551000M☉[6] and its metallicity (averaging −1.32 which means more than 10 times less than our own star), coherency and preponderence of older
stellar evolution objects, support its dating to very roughly 12 billion years old.[9] 18
RR Lyrae type
variable stars have been found within.
^Shapley, Harlow; Sawyer, Helen B. (August 1927), "A Classification of Globular Clusters", Harvard College Observatory Bulletin, 849 (849): 11–14,
Bibcode:
1927BHarO.849...11S.
^
abOliveira, R. A. P.; Ortolani, S.; Barbuy, B.; Kerber, L. O.; Maia, F. F. S.; Bica, E.; Cassisi, S.; Souza, S. O.; Pérez-Villegas, A. (2022). "Precise distances from OGLE-IV member RR Lyrae stars in six bulge globular clusters". Astronomy & Astrophysics. 657: A123.
arXiv:2110.13943.
Bibcode:
2022A&A...657A.123O.
doi:
10.1051/0004-6361/202141596.
S2CID239998638.
^"Messier 28". SEDS Messier Catalog. Retrieved 28 April 2022.
^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,
Bibcode:
2001AJ....121..916T,
doi:10.1086/318752