Messier 28 orM28, also known asNGC 6626, is aglobular cluster of stars in the center-west ofSagittarius. It was discovered by French astronomerCharles 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 3rdmagnitude starKaus Borealis (LambdaSgr). 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 18,300light-years away fromEarth.[3] It is about551000M☉[6] and its metallicity (averaging −1.32 which means more than 10 times less than our own star), coherency and preponderence of olderstellar evolution objects, support its dating to very roughly 12 billion years old.[9] 18RR Lyrae typevariable 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. Retrieved21 July 2024.
^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
![Messier 28 taken by Hubble.[14]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aNebulous%2c_but_no_nebula_Messier_28.jpg)
