| Messier 28 | |
| Epoch: | J2000 |
| Class: | IV |
| Constellation: | Sagittarius |
| Ra: | [1] |
| Appmag V: | 6.8[2] |
| Size V: | 11′.2 |
| Radius Ly: | 30 ly[3] |
| Metal Fe: | –1.32 |
| Age: | 12.0 Gyr |
| Notes: | Contains first pulsar discovered in a globular |
| Names: | GCl 94, M 28, NGC 6626 |
Messier 28 or M28, also known as NGC 6626, is a globular cluster of stars in the center-west of Sagittarius. It was discovered by French astronomer Charles Messier in 1764. He briefly described it as a "nebula containing no star... round, seen with difficulty in 3-foot telescope; Diam 2′."
In the sky it is less than a degree to the northwest of the 3rd magnitude star Kaus Borealis (Lambda). This cluster is faintly visible as a hazy patch with a pair of binoculars and can be readily found in a small telescope with an 8cm (03inches) aperture, showing as a nebulous feature spanning 11.2 arcminutes. Using an aperture of 15cm (06inches), the core becomes visible and a few distinct stars can be resolved, along the periphery. Larger telescopes will provide greater resolution, one of 25cm (10inches) revealing a dense 2′ core, with more density within.
It is about 18,300 light-years away from Earth. It is about and its metallicity (averaging -1.32 which means more than 10 times less than our own star), coherency and preponderence of older stellar evolution objects, support its dating to very roughly 12 billion years old. 18 RR Lyrae type variable stars have been found within.
It bore the first discovery of a millisecond pulsar in a globular cluster - PSR B1821–24. This was using the Lovell Telescope at Jodrell Bank Observatory, England.[4] A total of 11 further of these have since been detected in it with the telescope at Green Bank Observatory, West Virginia. As of 2011, these number the third-most in a cluster tied to the Milky Way, following Terzan 5 and 47 Tucanae.