M4, also known as NGC 6121, is a globular cluster located approximately 5,500 light-years away from Earth, making it one of the closest globular clusters to our Solar System. Its relatively close proximity allows astronomers to study its stars in greater detail compared to more distant clusters.
Physical Properties
Spanning about 75 light-years in diameter, M4 contains tens of thousands of stars gravitationally bound into a dense, spherical formation. The cluster is notable for its low metallicity, which indicates it formed during the early stages of the Milky Way’s evolution, about 12.2 billion years ago. One of M4’s remarkable features is the presence of a central bar-like structure formed by stars, which is uncommon in globular clusters. Additionally, M4 is known to harbour exotic stellar remnants such as white dwarfs, pulsars, and binary star systems, including one of the oldest known millisecond pulsars, PSR B1620-26.
Magnitude
M4 has an apparent magnitude of +5.6, making it one of the few globular clusters visible to the naked eye under excellent viewing conditions. With its brightness concentrated in a relatively small angular area of about 36 arcminutes, it appears as a compact, faint smudge to the unaided eye or through binoculars.
The cluster has an absolute magnitude of -7.2, which translates to a luminosity approximately 40,000 times that of the Sun. This brightness is a result of the numerous old, red giant stars within the cluster, along with a population of horizontal branch stars contributing to its distinct glow.
Observation
M4 is best observed during the summer months (June to August) in the Northern Hemisphere, as it lies in the constellation Scorpius, near the bright star Antares. Its position at southern declination -26° makes it particularly favourable for observers in the Southern Hemisphere, where it appears higher in the night sky.
Even a small telescope reveals M4’s core and some individual stars, which appear resolved due to the cluster’s relatively low stellar density compared to other globular clusters. Large amateur telescopes or astrophotographic equipment can capture its central bar structure and reveal intricate details of its star population. A clear, dark sky significantly enhances its visibility, making it a popular target for both novice and experienced stargazers.