The Pleiades, M45 or Seven Sisters is one of the best known star clusters in the entire sky. It has been known since antiquity to cultures worldwide and is easily visible to the naked eye on Winter evenings from the Northern Hemisphere. There are references to the Pleiades in the Odyssey, the Bible, and the Quran, and it is also revered in Hindu mythology. Located in the constellation of Taurus, the Pleiades forms a tiny dipper-shaped asterism in the shoulder of the Bull.
This open cluster is composed of extremely hot and luminous blue stars concentrated within a sphere of about 15 light years in diameter. Over 1,000 stars have been identified as members of the cluster, of which about 24 are visible to the naked eye. The faint reflection nebula visible around the brightest stars is an unrelated dust cloud that the cluster is currently passing through.
The age of the Pleiades is estimated to be between 75 and 150 million years making it quite young compared to the Sun’s age of 4.5 billion years. It is traveling slowly in the direction of Orion, but gravitational interactions with other stars in the galactic neighborhood will eventually disperse the cluster about 250 million years in the future.
Early measurements of the cluster’s distance placed it about 135 parsecs (440 light years) from Earth. However, more precise measurements from the
Hipparcos satellite yield a distance of 118 parsecs (385 light years) by using the parallax of stars in the cluster. Although this 14% decrease in distance may not seem significant, it is important because the distances of nearby clusters like the Pleiades are a first step in calibrating the cosmic distance scale. More recent observations from Hubble support the original larger distance so the subject remains unsettled.
The 18th-century French comet hunter Charles Messier added the Pleiades to his famous catalog as object M45. The original purpose of the catalog was to identify “fuzzy” comet-like objects (star clusters, nebulae and galaxies) to exclude from his true quest for new comets. Today, the Messier Catalog is recognized as the finest collection of bright deep sky objects by amateur astronomers everywhere.
Now it’s time to go outside on some clear, crisp January evening and view the Pleiades with the naked eye, binoculars or even a small telescope.