There are good reasons why the Pleiades, or M45, as it is also known has played such a prominent part in the astronomical traditions of virtually all the peoples of Earth since before the dawn of recorded history. Not only is it visible from every part of the planet from the North Pole to below the southernmost tip of South America, but it is also one of the most beautiful objects in the entire night sky. But what is this cluster of blue stars really, and how did it come to be?
Coordinates: RA 03h:47m:24s, Dec +24°07’
Distance: 442 light years
Star Type: Blue (B8IVpe)
Mass: 800 solar masses
Designations: Pleiades, Messier 45 (M45), Melotte 22, The Seven Sisters.
The Pleiades can easily located by imagining a line running through Orion’s belt, past the red star Aldebaran in the constellation Taurus, where one can then find this beautiful open cluster of blue stars. To the naked eye, up to 6 stars are visible in the Pleiades, and possibly as many as 9 on a clear, dark night. In the northern hemisphere, the Pleiades are associated with the winter months, but can be seen from autumn all the way through to April. In the southern hemisphere, it can be seen during late spring and summer.
Long before the advent of telescopes, German astronomer Michael Maestlin (1550-1631) had accurately recorded 11 naked eye star observation in the Pleiades, while Kepler, not to be outdone, recorded 14 stars in his observations. In 1610, the cluster was first observed through a telescope by Galileo who could see more than forty stars using his refractor.
Astronomers today estimate the cluster to contain anything between 400 and 1,000 stars, although the actual number will remain the subject of controversy until the specific star membership of the cluster has been determined.
Structure and Composition
Spread out over an area of 8 light years, the cluster is made up of two distinct regions, one being a central, almost spherical core that contains around 100 stars over an area of just more than 4.5 light years, and the other is an outer, less densely populated region. The oldest stars in the Pleiades are less than 100 million years old, which makes the cluster one of the youngest known, with an estimated total life span of only about 250 million years as a recognizable cluster, before it disperses due to the relatively weak gravitational bonds between the components. The group itself is made visible by 14 or so hot blue stars with luminosities that range between 40 and 1,000 times the Sun’s luminosity. Other stars found in this cluster include a large numbers of brown dwarfs, and several white dwarfs.
The Pleiades contains the following stars in order of brightness; Alcyone (2.86 mag); Atlas (3.62), Electra: (3.70), Maia: (3.86), Merope: (4.17), Taygeta: (4.29), Pleione: (5.09v),and Celaeno: (5.44). The cluster contains a relatively large number of binary star systems, and several triple star systems, with one notable example being Alcyone (eta Tauri), a massive, 10 solar mass giant that is orbited by three faint companions. Another notable feature of some Pleiades stars is that many of them have extremely rapid rates of rotation, which while a common feature of A-B type stars, in the case of the Pleiades includes some giant’s rotating at up to 300 km/sec.
The nebulosity of the Pleiades is not the remains of the material from which the cluster formed. Instead, recent radio and infrared investigations have found that the nebulosity in the cluster is the result of a chance encounter with a passing molecular cloud, but moreover, that the encounter appears to involve two clouds, instead of just one, associated with the Taurus-Auriga cloud complex. The image shows some of the nebulosity in close proximity to the star Merope in the Pleiades. This is a part of Tempel’s Nebula (NGC 1432), and the “wave” like appearance of the cloud is the result of the action of Merope’s solar wind on the different constituents of the cloud.
Only one meteor shower, the Taurids, is associated with the Pleiades, and runs from November 3rd to November 13th. However, the Taurids only produce low numbers of meteors, but the upside is that they are very luminous and slow-moving, which might make them worthwhile to observe.