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