Messier 45



In astronomy, the Pleiades or Seven Sisters (Messier 45 or M45), is an open star cluster containing middle-aged hot B-type stars located in the constellation of Taurus. It is among the nearest star clusters to Earth and is the cluster most obvious to the naked eye in the night sky. The celestial entity has several meanings in different cultures and traditions.

The cluster is dominated by hot blue and extremely luminous stars that have formed within the last 100 million years. Dust that forms a faint reflection nebulosity around the brightest stars was thought at first to be left over from the formation of the cluster (hence the alternative name Maia Nebula after the star Maia), but is now known to be an unrelated dust cloud in the interstellar medium, through which the stars are currently passing. Computer simulations have shown that the Pleiades was probably formed from a compact configuration that resembled the Orion Nebula. Astronomers estimate that the cluster will survive for about another 250 million years, after which it will disperse due to gravitational interactions with its galactic neighborhood.

The name of the Pleiades comes from Ancient Greek. It probably derives from plein ('to sail') because of the cluster's importance in delimiting the sailing season in the Mediterranean Sea: 'the season of navigation began with their heliacal rising'. However, the name was later mythologised as the name of seven divine sisters, whose name was imagined to derive from that of their mother Pleione, effectively meaning 'daughters of Pleione'. However, in reality the name of the star-cluster almost certainly came first, and Pleione was invented to explain it.

History
The Pleiades are a prominent sight in winter in the Northern Hemisphere, and have been known since antiquity to cultures all around the world, including the Celts, Māori, Aboriginal Australians, the Persians, the Arabs (known as Thurayya), the Chinese, the Japanese, the Maya, the Aztec, and the Sioux and Cherokee. In Hinduism, the Pleiades are known as Krittika and are associated with the war-god Kartikeya (Murugan, Skanda), who derives his name from them. The god is raised by the six Krittika sisters, also known as the Matrikas. He is said to have developed a face for each of them.

The Babylonian star catalogues name the Pleiades MUL.MUL or "star of stars", and they head the list of stars along the ecliptic, reflecting the fact that they were close to the point of vernal equinox around the 23rd century BC. The earliest known depiction of the Pleiades is likely a bronze age artifact known as the Nebra sky disk, dated to approximately 1600 BC. Some Greek astronomers considered them to be a distinct constellation, and they are mentioned by Hesiod, and in Homer's Iliad and Odyssey. They are also mentioned three times in the Bible (Job 9:9 and 38:31, as well as Amos 5:8). Some scholars of Islam suggested that the Pleiades (ath-thurayya) are the star mentioned in the sura (chapter) Najm of the Quran.

In Japan, the constellation is mentioned under the name Mutsuraboshi ("six stars") in the 8th century Kojiki and Manyosyu documents. The constellation is also known in Japan as Subaru (“unite”) and is depicted in the logo and name of the Subaru automobile company. The Persian equivalent is Nahid (pronounced "Naheed").

The rising of the Pleiades is mentioned in the Ancient Greek text Geoponica. The Greeks oriented the Hecatompedon temple of 550 BC and the Parthenon of 438 BC to their rising.The rising of the Pleiades before dawn (usually at the beginning of June) has long been regarded as the start of the new year in Māori culture, with the star group being known as Matariki. The rising of Matariki is celebrated as a midwinter festival in New Zealand. In Hawaiian culture the cluster is known as the Makali'i and their rising shortly after sunset marks the beginning of Makahiki, a 4 month time of peace in honor of the god Lono. Animation of proper motion in 400,000 years (cross-eyed viewing Stereogram guide cross-eyed.png). Due to technical limitations on thumbnails, you must click through to the actual image to see the animation.

Galileo Galilei was the first astronomer to view the Pleiades through a telescope. He thereby discovered that the cluster contains many stars too dim to be seen with the naked eye. He published his observations, including a sketch of the Pleiades showing 36 stars, in his treatise Sidereus Nuncius in March 1610. The Pleiades have long been known to be a physically related group of stars rather than any chance alignment. The Reverend John Michell calculated in 1767 that the probability of a chance alignment of so many bright stars was only 1 in 500,000, and so correctly surmised that the Pleiades and many other clusters of stars must be physically related. When studies were first made of the stars' proper motions, it was found that they are all moving in the same direction across the sky, at the same rate, further demonstrating that they were related.

Charles Messier measured the position of the cluster and included it as M45 in his catalogue of comet-like objects, published in 1771. Along with the Orion Nebula and the Praesepe cluster, Messier's inclusion of the Pleiades has been noted as curious, as most of Messier's objects were much fainter and more easily confused with comets—something that seems scarcely possible for the Pleiades. One possibility is that Messier simply wanted to have a larger catalogue than his scientific rival Lacaille, whose 1755 catalogue contained 42 objects, and so he added some bright, well-known objects to boost his list.

Distance
The distance to the Pleiades can be used as an important first step to calibrate the cosmic distance ladder. As the cluster is so close to the Earth, its distance is relatively easy to measure and has been estimated by many methods. Accurate knowledge of the distance allows astronomers to plot a Hertzsprung-Russell diagram for the cluster, which, when compared to those plotted for clusters whose distance is not known, allows their distances to be estimated. Other methods can then extend the distance scale from open clusters to galaxies and clusters of galaxies, and a cosmic distance ladder can be constructed. Ultimately astronomers' understanding of the age and future evolution of the universe is influenced by their knowledge of the distance to the Pleiades. Yet some authors argue that the controversy over the distance to the Pleiades discussed below is a red herring, since the cosmic distance ladder can (presently) rely on a suite of other nearby clusters where consensus exists regarding the distances as established by Hipparcos and independent means (e.g., the Hyades, Coma Berenices cluster, etc.

Brightest Stars
The nine brightest stars of the Pleiades are named for the Seven Sisters of Greek mythology: Sterope, Merope, Electra, Maia, Taygeta,Celaeno, and Alcyone, along with their parents Atlas and Pleione. As daughters of Atlas, the Hyades were sisters of the Pleiades. The English name of the cluster itself is of Greek origin (Πλειάδες), though of uncertain etymology. Suggested derivations include: from πλεῖν plein, "to sail," making the Pleiades the "sailing ones"; from πλέος pleos, "full, many"; or from πελειάδες peleiades, "flock of doves."

Possible Planets
Analyzing deep-infrared images obtained by the Spitzer Space Telescope and Gemini North telescope, astronomers discovered that one of the cluster's stars – HD 23514, which has a mass and luminosity a bit greater than that of the Sun, is surrounded by an extraordinary number of hot dust particles. This could be evidence for planet formation around HD 23514.