M45, commonly known as the Pleiades or Seven Sisters, is the finest open cluster in the sky. It's a breathtaking site, known since antiquity and easily visible to the naked eye. Located in the large and prominent zodiac constellation of Taurus, this showpiece object is best seen during the Northern Hemisphere Winter and the Southern Hemisphere Summer months.
Finding M45 is easy. The cluster is positioned about 14 degrees northwest of orange giant star Aldebaran (mag. +0.9), the brightest star in Taurus. At first glance with the naked eye, it's obvious that something is special about this small section of sky. On closer inspection, M45 reveals itself to be a beautiful cluster of about half a dozen white stars, covering an area much greater than that of the full Moon. At the heart of the cluster is a set of stars that form a small dipper shape, similar to the brightest stars of Ursa Major. Even under light polluted skies the dipper shape is readily visible. From dark sites, the Pleiades is an outstanding naked eye object. The main stars appear bright and striking, with up to 10 or more visible under ideal conditions.
In total, the Pleiades cover nearly two degrees of sky. Due to its large apparent size, it's best seen with either binoculars, a small or wide field telescope or even a finderscope. The view through a pair of 10x50 binoculars is fantastic. The main stars are bright and distinct with dozens of more fainter stars visible. A trail of 7th magnitude stars extends to the southeast from one corner of the dipper. It's a breathtaking sight when viewed through a small telescope of around 80mm (3.1-inch) or 100mm (4-inch) aperture. The cluster neatly fills the field of view at low magnifications, with many bright bluish-white stars standing out brilliantly against the dark background. Many fainter background stars are also visible, especially when using averted vision. Due to its large apparent size, some of the awe of the Pleiades is lost in larger scopes. A 200mm (8-inch) telescope reveals dozens of additional stars. However, the observer is limited to eyeing only part of the cluster and then scanning around to see the remainder.
M45 is a young cluster of hot blue and extremely luminous stars that formed about 115 million years ago. The brightest stars are all of class B:
|Pleione||28 Tauri||+5.09 (var)|
There is a faint reflection nebulosity surrounding the brightest stars that's easy to image or photograph, although more difficult to observe visually. Under excellent seeing conditions some faint nebulosity may be seen in small/medium sized telescopes, especially around the star Merope (23 Tauri). Originally, it was thought at first that the nebulosity was left over from the formation of the cluster. It's now believed to be an unrelated dust cloud in the interstellar medium that the stars are passing through.
The Pleiades are located 425 light-years from Earth. In total, it's estimated to contain around 500 members.
M45 Data Table
|Object Type||Open cluster|
|RA (J2000)||03h 47m 06s|
|DEC (J2000)||+24d 07m 00s|
|Apparent Size (arc mins)||110 x 110|
|Age (years)||115 Million|
|Number of Stars||500|
|Other Names||Collinder 42, Melotte 22|