Messier 28

Lore & Background

Messier 28 was discovered by Charles Messier in 1764, who briefly described it as a 'nebula containing no star... round, seen with difficulty in 3½-foot telescope; Diam 2′.' In the sky, it lies less than a degree to the northwest of the 3rd magnitude star Kaus Borealis (Lambda Sgr). The cluster is faintly visible as a hazy patch with binoculars and can be readily found in a small telescope with an 8 cm (3.1 in) aperture, showing as a nebulous feature spanning 11.2 arcminutes. With a 15 cm (5.9 in) aperture, the core becomes visible and a few distinct stars can be resolved along the periphery. Larger telescopes provide greater resolution; a 25 cm (9.8 in) telescope reveals a dense 2′ core with more density within.

The cluster is about 18,300 light-years from Earth, with a mass of about 551,000 M☉. Its metallicity averages −1.32 (more than 10 times less than our own star), and its coherency and preponderance of older stellar evolution objects support its dating to very roughly 12 billion years old. Eighteen RR Lyrae-type variable stars have been found within the cluster.

Reader's Guide

Messier 28 holds a significant place in astronomical history as the first globular cluster in which a millisecond pulsar was discovered. This pulsar, designated PSR B1821–24, was detected using the Lovell Telescope at Jodrell Bank Observatory in England. Since then, a total of 11 further millisecond pulsars have been detected in the cluster using the telescope at Green Bank Observatory in West Virginia. As of 2011, these 12 pulsars number the third-most in a cluster tied to the Milky Way, following Terzan 5 and 47 Tucanae. The cluster's age of roughly 12 billion years and its low metallicity (more than 10 times less than the Sun) indicate it is an ancient stellar population, typical of globular clusters. Its visibility in binoculars and small telescopes makes it a popular target for amateur astronomers, while its pulsar population continues to be of interest for studies of stellar evolution and compact objects.

Did You Know?

• It is about 18,300 light-years away from Earth.
• It contains 18 RR Lyrae-type variable stars.
• The first millisecond pulsar discovered in a globular cluster, PSR B1821–24, was found in M28.