M28
Messier 28 or M28, also known as NGC 6626, is a globular cluster of stars in the constellation Sagittarius. It was discovered by French astronomer Charles Messier on July 27, 1764. He briefly described it as a "nebula containing no star... round, seen with difficulty in 3½-foot telescope; Diam 2′." The 2′ at the end indicates an angle of two arcminutes.
In the sky it is less than a degree to the northwest of the 3rd magnitude star Kaus Borealis. This cluster is faintly visible as a hazy patch with a pair of binoculars and can be readily found in a small telescope with a 8 cm (3.1 in) aperture, showing as a nebulous feature spanning 11.2 arcminutes. At 15 cm (5.9 in), the core becomes visible and a few individual stars can be resolved along the periphery. Larger telescopes will provide greater resolution, with a 25 cm (9.8 in) telescope revealing a 2′ core.
M28 is at a distance of about 17,900 light-years away from Earth. It has a combined 551,000 times the mass of the Sun and is 12 billion years old. 18 RR Lyrae type variable stars have been observed in this cluster. In 1986, M28 became the first globular cluster where a millisecond pulsar, PSR B1821–24, was discovered with the Lovell Telescope at Jodrell Bank Observatory. A total of 11 additional millisecond pulsars have since been detected in the cluster with the Green Bank Telescope. As of 2011, this is the third largest known population of pulsars in a cluster following Terzan 5 and 47 Tucanae.
