Pleiades

Pleiades (star cluster) or M45

The Pleiades cluster or M45, is an open cluster of stars seen in the northern hemisphere in the constellation Taurus. This cluster is the most famous group of stars from the sky, as can be seen without binoculars, even under the lights, our polluted cities. The origin of the name "Pleiades" comes from Greek mythology, the Pleiades were seven sisters, daughters of Atlas and Pleione: Asterope, Merope (or Dryops or Aero), Electra, Maia, Taygeta, Celaeno (or Selene) and Alcyone. The Pleiades cluster is a cluster closest to the Earth. Immersed in a cloud of cosmic dust, the Seven Sisters are famous for reflection nebulae, they feed their radiation. The cluster extends about 2 °, equivalent to 4 times the apparent diameter of the Moon. Its density is relatively low compared to other open clusters. The age of the cluster M45 is estimated at 100 million years, but it will not live long, since it should be separated in 250 million years, partly because of its low density. The group of Pleiades contains over 3 000 stars located 400 light years, over a diameter of only 13 light years. The data from the Spitzer Space Telescope revealed the presence of numerous cold low-mass stars and brown dwarfs that have never reached the critical mass (13 times the mass of Jupiter, or 0.08 times the mass of the Sun) to ignite itself and maintain a sustainable state.

The famous Pleiades cluster is well known for the blue color of its stars. This cluster is visible to the naked eye in the sky to fall in the northern hemisphere. We must look towards the constellation Taurus, to see at least 6 stars among 500 stars, which are the Pleiades.

NB: The reflection nebulae are huge dust clouds that reflect light of several nearby stars and bright enough to make the reflecting dust. When the stars are very hot, they cause ionization of gas clouds, creating an emission nebula.
Reflection nebulae are usually blue. Reflection nebulae and emission nebulae are often combined into a single type, diffuse nebulae.

Image: The nebula of Pleiades or seven sisters. This mosaic of 5 images cover a field of nearly 70 light years. In the vicinity of the Pleiades, there are beautiful reflection nebulae. Credit & Copyright: John Davis

History of the Pleiades 

March 4, 1769, Charles Messier added the cluster M45 in his catalog astronomical. In 1846 the German astronomer Johann Heinrich von Madler comment cohesion in the star cluster M45, where stars have no measurable movement in relation to one another. He concludes that it is a star system, with Alcyone is closest to the center. Astronomers hold that they move in groups and clouds of dust around stars reflect the light of these stars, they are located near or inside the clouds. In 1859, Ernst Wilhelm Tempel discovers around Merope, the nebula NGC 1435. In 1875, NGC 1432 is found around Maia. In 1880, some clouds are seen around Alcyone, Electra, and Celaeno Taygetus. The complex structure of the cloud is finally revealed by the brothers Henry and Isaac Roberts between 1885 and 1888. The nebula of Pleiades or seven sisters, is not a remnant of the dust cloud that gave birth to the Pleiades. Indeed the nebula and the cluster have the same apparent speed, which suggests that the star clusters, this cloud would cross his path.

NB: the Japanese name for the Pleiades cluster is Subaru, which means "unity".
In 1953, five Japanese companies merged to form "Fuji Heavy Industries Ltd". The new group adopted the cluster as Subaru name and logo of the car brand well known.

Image: The nebula Pleiades or Seven Sisters and 500 main stars. In this false-color image taken by the Spitzer Space Telescope, dust shines as the stars because the picture is taken in the infrared. The image covers about one degree on the sky, or 7 light-years at the distance of the Pleiades. Credit: NASA, JPL-Caltech, J. Stauffer (SSC, Caltech)

Seven sisters, daughters of Atlas and Pleione

Alcyone (η Tauri, Eta Tauri) is a multiple star located about 440 light years from Earth in the constellation Taurus. It is the brightest star in the Pleiades cluster. The main star Alcyone A, with a surface temperature of 13,000 Kelvin, has a brightness equal to 1400 times that of the Sun. His speed is very fast (about 215 km/s at the equator). Around the star there is the presence of a disk of gas generated by ejection of material.
Electra (17 Tauri) is a giant star, the third brightest star of the Pleiades.
Maia (20 Tauri) is blue giant star, which emits 660 times more energy than the Sun with a surface temperature estimated at about 12,600 kelvins. It is situated at a distance of 385 light years, is the fourth brightest star in the Pleiades cluster, after Alcyone, Atlas, and Electra.
Merope (23 Tauri) is located about 440 light years from the Sun. Merope with a mean apparent magnitude of 4.17 has a brightness equal to 630 times that of the Sun. Its surface temperature is 14,000 degrees Kelvin. Around Merope Merope Nebula lies, IC 349.

Taygeta (19 Tauri) is a triple star system the Pleiades star cluster. It is located about 440 light years from Earth.
Celaeno is a star in the Pleiades open cluster, apparent visual magnitude 5.44. It is distant about 562 light years.
Asterope or Sterope is the name of two stars, 21 Tauri, 22 Tauri, and are located about 440 light years from the Sun.
Seven sisters, are located between 385 and 562 light years from our solar system.

Image: The 7 Sisters of the Pleiades, in Greek mythology are daughters of Atlas and Pleione. The stars of the Pleiades are known since prehistoric times. A pictorial representation of this cluster is also found on the bronze Nebra disc, found in Germany and dated from the early Bronze Age (1600 BC. JC).