|Right Ascension||18 : 24.5 (h:m)
|Declination||-24 : 52 (deg:m)
|Visual Brightness||6.8 (mag)
|Apparent Dimension||11.2 (arc min)
Discovered 1764 by Charles Messier.
At about 15,000 or 19,000 light years distance, M28 with its linear diameter of 75 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.
Last Modification: 9 Dec 1999, 22:58 MET