Messier 21 (M21) is an open star cluster located in the constellation Sagittarius. Visible in the southern sky from most locations, M21 is part of the rich tapestry of celestial objects in this region of the Milky Way. It lies near the prominent Sagittarius Star Cloud (Messier 24), enhancing its observational appeal. The cluster was first discovered by Giovanni Battista Hodierna before 1654, predating its inclusion in Charles Messier’s famous catalogue of deep-sky objects.
An informal name, “Webb’s Cross,” is occasionally used to describe M21 due to its perceived cross-like shape when viewed under specific conditions. However, this term has no official astronomical recognition.
Composition
M21 is situated approximately 4,250 light-years from Earth and spans about 13 light-years in diameter. Like most open clusters, it originated from a single molecular cloud composed of gas and dust, which collapsed to form its stars. Despite being loosely bound by gravity, the cluster’s stars remain in close proximity, traveling together as they orbit the galactic centre.
Astronomers estimate the cluster’s age to be relatively young at around 4.6 million years. This makes M21 significantly younger than many other open clusters, such as the Pleiades, which is over 100 million years old. Its youth is evident in its stellar composition, which predominantly consists of hot, blue stars of spectral type B. These stars shine brightly against the dense backdrop of the Milky Way, making M21 a visually striking target for observation. The cluster is confirmed to host at least 57 member stars, though the total count may be higher when accounting for fainter, less detectable stars.
Observation
M21 has an apparent magnitude of about 6.5, placing it just beyond the threshold of naked-eye visibility. However, it can be easily spotted using binoculars or a small telescope. Through these instruments, the cluster reveals its individual stars, presenting a dazzling array of bright points against the densely packed star fields of Sagittarius.
The cluster is best observed from the Southern Hemisphere during the winter months, from May to September, when Sagittarius dominates the evening sky. During this time, the constellation rises in the southeast after sunset and reaches its highest point near midnight, offering optimal viewing conditions.
While M21 can also be seen from the Northern Hemisphere, its lower altitude above the horizon makes it more challenging to observe, especially at higher northern latitudes. Observers closer to the equator will have a better vantage point, as Sagittarius appears higher in the sky there.
For the best experience, it is essential to observe M21 under dark skies with minimal light pollution. A clear, moonless night will enhance its visibility, allowing stargazers to fully appreciate the cluster’s subtle details and the surrounding richness of the Milky Way.