Messier 28 is a globular cluster in the constellation Sagittarius. It was discovered by Charles Messier in July of 1764; it was William Herschel who first described it as a "star cloud".
With an apparent diameter of 11.2', M 28 appears considerably smaller and more compressed than its impressive neighbor, M 22. Messier 28 has a glowing core which fades rapidly outward into a halo; a scattering of stars can be resolved around its rim. It is slightly elliptical in shape. To resolve it into stars, larger instruments are required; this globular cluster has a total magnitude of 6.8.
M 28 is about 18,000 light-years away, and has a linear diameter of about 60 light years. Some 18 RR Lyrae-type variable stars have been observed in this cluster. In 1986, M 28 became the first globular cluster where a millisecond pulsar was discovered.