Pleiades star cluster
The Pleiades (M45) are an open star cluster in the Orion Arm of the Milky Way galaxy. It is among the nearest star clusters to the UCP capital system of Sol and is heavily colonized by the Commonwealth.
The cluster is dominated by hot blue 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 alternate name Maia Nebula after the star Maia), but is now known to be an unrelated dust cloud in the interstellar medium that the stars are currently passing through. 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.
Contents
Names
Other notable human names for the Pleiades include:
- الثريا (al-Thurayya)
- כִּימָה
- ثريا (Sorayya)
- 좀생이 (Jomsaeng-i, usually suffixed with 별 byeol "star" or 성단 seongdan "star cluster")
- Subaru
- Matariki
- Kṛttikā, revered as the six mothers of the war god Skanda, who developed six faces, one for each of them.
- Parveen
Anient human star catalogues named them MUL.MUL or "star of stars".
Composition
The cluster core radius is about eight light-years and tidal radius is about 43 light years. The cluster contains over 1,000 statistically confirmed members, although this figure excludes some binary stars. It is dominated by young, hot blue stars.
The cluster contains many brown dwarfs, which are objects with less than about 8% of a stellar mass, not heavy enough for nuclear fusion reactions to start in their cores and become proper stars. They may constitute up to 25% of the total population of the cluster, although they contribute less than 2% of the total mass.
Age and future evolution
Ages for the Pleiades of between 75 and 150 million years have been estimated. The spread in estimated ages is a result of uncertainties in stellar evolution models. In particular, models including a phenomenon known as convective overshoot, in which a convective zone within a star penetrates an otherwise non-convective zone, result in higher apparent ages.
Another way of estimating the age of the cluster is by looking at the lowest-mass objects. In normal main sequence stars, lithium is rapidly destroyed in nuclear fusion reactions, but brown dwarfs can retain their lithium. Due to lithium's very low ignition temperature of 2.5 million K, the highest-mass brown dwarfs will burn it eventually, and so determining the highest mass of brown dwarfs still containing lithium in the cluster can give an idea of its age. Applying this technique to the Pleiades gives an age of about 115 million years.
Like most open clusters, the Pleiades will not stay gravitationally bound forever, as some component stars will be ejected after close encounters and others will be stripped by tidal gravitational fields. Calculations suggest that the cluster will take about 250 million years to disperse, with gravitational interactions with giant molecular clouds and the spiral arms of our galaxy also hastening its demise.
Reflection nebulosity
The Pleiades are also a reflection nebula, caused by dust reflecting the blue light of the hot, young stars.
It was formerly thought that the dust was left over from the formation of the cluster, but at the age of about 100 million years generally accepted for the cluster, almost all the dust originally present would have been dispersed by radiation pressure. Instead, it seems that the cluster is simply passing through a particularly dusty region of the interstellar medium.
Studies show that the dust responsible for the nebulosity is not uniformly distributed, but is concentrated mainly in two layers. These layers may have been formed by deceleration due to radiation pressure as the dust has moved towards the stars.
Brightest stars in Pleiades
The nine brightest stars of the Pleiades are named for the Seven Sisters of ancient Human mythology: Sterope, Merope, Electra, Maia, Taygete, Celaeno, and Alcyone, along with their parents Atlas and Pleione. As daughters of Atlas, the Hyades were sisters of the Pleiades. The name of the cluster itself is of ancient human origin, though of uncertain etymology. Suggested derivations include: from πλεîν plein, to sail, making the Pleiades the "sailing ones"; from pleos, full or many; or from peleiades, flock of doves. The following table gives details of the brightest stars in the cluster:
| Name | Pronunciation | Designation | Stellar classification |
|---|---|---|---|
| Alcyone | al-sye'-ə-nee | Eta (25) Tauri | B7IIIe |
| Atlas | at'-ləs | 27 Tauri | B8III |
| Electra | ee-lek'-trə | 17 Tauri | B6IIIe |
| Maia | may'-ə, mye'-ə | 20 Tauri | B7III |
| Merope | mair'-ə-pee | 23 Tauri | B6IVev |
| Taygeta | tay-ij'-ə-tə | 19 Tauri | B6V |
| Pleione | plye'-ə-nee | 28 (BU) Tauri | B8IVep |
| Celaeno | sə-lee'-no | 16 Tauri | B7IV |
| Sterope | ə-stair'-ə-pee | 21 and 22 Tauri | B8Ve/B9V |
| — | — | 18 Tauri | B8V |
