Messier 28 (M28) is a globular cluster located in the constellation Sagittarius, a region renowned for its rich star fields and an abundance of deep-sky objects. Globular clusters, like M28, are dense assemblies of stars orbiting the cores of galaxies. These clusters typically contain thousands to millions of stars bound together by their mutual gravitational attraction, forming spherical or elliptical structures.
M28 was discovered in 1764 by the French astronomer Charles Messier, who included it as one of the 110 entries in his catalogue of non-cometary celestial objects. This catalogue was designed to help astronomers differentiate between comets and other fixed celestial features.
Composition
M28, like most globular clusters, is composed primarily of ancient stars, with ages exceeding 10 billion years. These stars are typically metal-poor, meaning they contain fewer elements heavier than helium. This low metallicity is characteristic of stars formed during the early stages of the universe, before heavy elements were synthesized in later stellar generations.
The stars in M28 orbit around the cluster’s centre of mass, creating a gravitationally bound system. The densely packed core of the cluster results in complex dynamical interactions, offering astronomers valuable insights into the processes governing globular clusters. By studying these interactions, researchers learn more about the formation and evolution of not only globular clusters but also the structure of galaxies, including the Milky Way.
Appearance
Through a telescope, M28 presents itself as a dense, glowing sphere of light, characteristic of globular clusters. Its stars are so closely packed that most telescopes cannot resolve individual stars, leaving the cluster appearing as a faint, hazy patch. Higher-powered telescopes may reveal the brighter, outer stars, while the core remains unresolved due to its dense stellar population.
M28 spans about 60 light-years in diameter and is located roughly 18,000 light-years from Earth. With an apparent magnitude of 7.66, it is too dim to be seen with the naked eye but becomes visible as a faint fuzzy spot under dark skies with binoculars or a small telescope. Its angular diameter of approximately 11.2 arcminutes means it covers a modest area of the sky, slightly smaller than half the apparent diameter of the full Moon. Observing M28 from light-polluted areas can be challenging, making a dark sky location crucial for appreciating this celestial cluster.
Observation
M28 is best observed from the Southern Hemisphere, particularly during the winter months of June to August. During this time, the constellation Sagittarius, home to M28, is prominently positioned in the night sky, reaching its highest altitude around midnight. This positioning provides ideal viewing conditions for stargazers. The cluster can also be seen starting in May and remains visible until September, though its peak observability occurs in the heart of winter.
M28 is situated near the “Teapot” asterism in Sagittarius, making it relatively easy to locate. Using the spout of the “Teapot” as a guide, amateur astronomers can find the cluster with binoculars or a telescope. Observing under dark skies enhances the visibility of its faint glow and offers a better view of its context within the star-rich region of the Milky Way.