M28 is a mag. +7.2 globular cluster located among the rich Milky Way star fields of Sagittarius. The cluster is visible with binoculars, although unspectacular in appearance. However, telescopes fair better and on nights of good seeing and transparency, it's possible to resolve a few stars with just a medium size scope.
The globular was discovered by Charles Messier on July 27, 1764. It's located 18,000 light-years from Earth and spans 11 arc minutes of apparent sky, which corresponds to a relatively small spatial diameter of 60 light-years. In total, M28 contains about 50,000 stars and is best seen from southern and equatorial regions during the months of June, July and August. From northern temperate locations, it never rises particularly high above the southern horizon.
M28 is one of the easier globulars to locate as it's positioned less than a degree northwest of mag. +2.8 star Kaus Borealis (λ Sag). Visible in the same binocular field of view as M28 is M22, a much larger and brighter globular and one of the finest objects of its type in the night sky.
Through a small 80mm (3.1-inch) refractor, M28 appears non-stellar and without a well-defined core. A 150mm (6-inch) telescope at high powers will begin to resolve individual stars, although scopes of 250mm (10-inch) aperture or more are essential to fully appreciate this object. On closer examination, M28 appears slightly elliptical in shape. It's noticeable how considerably smaller and compressed it is when compared to M22.
M28 contains 18 RR Lyrae type variable stars and in 1986 a millisecond pulsar was discovered. Since then many more pulsars have been detected.
M28 Data Table
|Object Type||Globular cluster|
|RA (J2000)||18h 24m 33s|
|DEC (J2000)||-24d 52m 12s|
|Apparent Size (arc mins)||11.2 x 11.2|
|Age (years)||12 Billion|
|Number of Stars||50,000|
|Notable Feature||Contains millisecond pulsars|