M28, Globular Cluster in Sagittarius

Image by Jim Dixon
At about 18,000 to 19,000 light years distance, M28 with its linear diameter of 60 light years appears considerably smaller and more compressed than its more impressive neighbor, M22. It is slightly elliptical shaped according to H. Shapley. To resolve it into stars, larger instruments are required; it was William Herschel who first described it as a "star cloud".

M28 contains, in addition its only 18 known RR Lyrae variables, a W Virginis variable (Type II, or population II Cepheid) with a period of 17 days, and a second long period variable (Variable # 17, possibly of RV Tauri type, according to Burnham).

M28 was the second globular clusters where a millisecond pulsar was discovered, in 1987 (the first was M4). This one, 1620-26, is spinning around its axis once every 11 milliseconds.

Globular cluster M28 is one of the authentic discoveries of Charles Messier, who cataloged it on July 27, 1764.