Messier 45 (M45), commonly known as the Pleiades or the Seven Sisters, is a prominent open star cluster located in the constellation Taurus. Visible to the naked eye, the Pleiades has been known since antiquity and is one of the most recognizable star clusters in the night sky. Its name, “Seven Sisters,” originates from Greek mythology, where the cluster represented the seven daughters of the Titan Atlas and the sea-nymph Pleione.
Composition
The Pleiades is a relatively young open star cluster composed of several hundred stars, loosely bound by gravity and spread across a region approximately 14 light-years in diameter. The stars are mostly blue-white, indicating that they are hot, young, and massive. This youth, combined with their intense luminosity, gives the Pleiades a strikingly bright appearance in the night sky. The cluster’s age is estimated to be around 100 million years, quite young in stellar terms, which explains the high proportion of blue-white stars. The Pleiades is located about 444 light-years from Earth, making it one of the closest star clusters to our solar system.
In addition to the main stars, the Pleiades contains a faint, blue nebulosity around some of its members, visible in long-exposure photographs. This nebulosity is thought to be a reflection nebula—interstellar dust that reflects the light from the cluster’s young, luminous stars.
Shape and Appearance
To the naked eye, the Pleiades appear as a compact, bright cluster resembling a small dipper or a cluster of grapes. Though commonly associated with seven main stars, most observers typically see six to seven stars, with the exact number depending on viewing conditions and the observer’s eyesight. The brightest stars in the cluster—Alcyone, Electra, Maia, Taygeta, Asterope, Celaeno, and Merope—form a distinctive, easily recognized pattern.
With binoculars or a telescope, many more stars become visible, unveiling the cluster’s rich stellar population and further enhancing the sense of a dense, cohesive grouping. The Pleiades are often compared to a miniature version of the Big Dipper due to their dipper-like shape.
History
The Pleiades have been noted and celebrated in many ancient cultures worldwide. The Greek astronomer Ptolemy included the Pleiades in his astronomical catalogue in the 2nd century AD. Charles Messier later added it to his catalogue of non-cometary celestial objects in 1769, designating it as M45. Beyond the Western tradition, the Pleiades have significant cultural and navigational importance in Indigenous Australian, Japanese, and Native American cultures, among others, often symbolizing concepts of family or unity.
Observation
The Pleiades cluster is ideally placed for observation in the Northern Hemisphere, where it is visible from late autumn through winter. The best months to observe M45 are November to February, when Taurus is high in the night sky. The cluster becomes visible in the early evening around October, rising low on the eastern horizon and remains observable through early March, though it sets earlier each night as spring approaches.
With an apparent magnitude of 1.6, the Pleiades are easily visible to the naked eye under clear skies, appearing as a bright, tight grouping. Observers with binoculars or a small telescope can enjoy a more detailed view, as the individual stars and subtle structures within the cluster become more apparent. This experience is often a breathtaking sight for amateur astronomers, revealing the cluster’s intricate arrangement of stars and occasional glimpses of the surrounding nebulosity.
