Pleiades
Pleiades | |
---|---|
Apparent magnitude (V) | 1.6[6] |
Apparent dimensions (V) | 2° [6] |
Physical characteristics | |
Mass | 800 M☉ |
Radius | 8 light-years (core radius) |
Estimated age | 75 to 150 million years |
Other designations | Seven Sisters,[1] M45,[1] Cr 42,[1] Mel 22[1] |
Associations | |
Constellation | Taurus |
The Pleiades (
The cluster is dominated by hot blue luminous stars that have formed within the last 100 million years. Reflection nebulae around the brightest stars were once thought to be left over material from their formation, but are now considered likely to be an unrelated dust cloud in the interstellar medium through which the stars are currently passing.[10] This dust cloud is estimated to be moving at a speed of approximately 18 km/s relative to the stars in the cluster.[11]
Computer simulations have shown that the Pleiades were probably formed from a compact configuration that resembled the Orion Nebula.[12] 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.[13]
Together with the open star cluster of the Hyades, the Pleiades form the Golden Gate of the Ecliptic.
Origin of name
The name of the Pleiades comes from
Astronomical role of M45 in antiquity
The M45 group played an important role in ancient times for the establishment of calendars thanks to the combination of two remarkable elements. The first, which is still valid, is its unique and perfectly identifiable aspect on the celestial vault near the ecliptic. The second, essential for the Ancients, is that in the middle of the third millennium BC., this asterism (a prominent pattern or group of stars that is smaller than a constellation) marked the vernal point.[18] The importance of this asterism is also evident in northern Europe, on the Nebra sky disc, dating around 1600 BC. and where it is represented beside the Sun and the Moon.
It is also this asterism that indicates the beginning of the ancient calendars. Several examples can be given:
- In ancient India, it constitutes, in the Atharvaveda, compiled around 1200-1000 BC, the first nakṣatra (Sanskrit name for lunar stations), which is called क्रृत्तिका Kṛittika, a revealing name since it literally means "the Cuttings"[19], i.e. "Those that mark the break of the year".[20] This is so before the classic list lowers this nakṣatra to third place, henceforth giving the first to the couple βγ Arietis which, notably in Hipparchus, at that time, marks the equinox.
- In Mesopotamia, the MUL.APIN compendium, the first known Mesopotamian astronomy treatise, discovered at Nineveh in the library of Assurbanipal and dating from no later than 627 BC., presents a list of gods [holders of stars] who stand on "the path of the Moon", a list which begins with mul.MUL.[21]
- In Greece, the Πλειάδες, are a group whose name is probably functional before having a mythological meaning, as André Lebœuffle points out, who has his preference for the explanation by the Indo-European root *pe/ol-/pl- which expresses the idea of multiplicity, crowd, assembly.[22]
- We find a similar thing among the Ancient Arabs who begin their old parapegma type calendar, that of the anwā, with M45 under the name of الثريّا al-Ṯurayyā.[23] And this before their classic calendar, that of the manāzil al-qamar or "lunar stations", also begins with the couple βγ Arietis whose name, الشرطان al- Šaraṭān, is literally "the Two Marks [of entering the equinox]"[24]
So when M45 leaves the vernal point, the asterism still remains important, both functionally and symbolically. In addition to the changes we have just seen in the calendars based on the lunar stations among the Indians and the Arabs, consider the case of an ancient Yemeni calendar in which the months are designated according to an astronomical criterion which caused it to be named Calendar of the Pleiades: the month of ḫams, literally "five", is that during which the Sun and al-Ṯurayyā, ie the Pleiades, deviate away from each other by five movements of the Moon, i.e. five times the path that the "Moon" travels on average in one day and one night, to use the terminology of ᶜAbd al-Raḥmān al-Sūfī al-Ṣūfī.[25]
Nomenclature and mythology
The Pleiades are a prominent sight in winter in the
The earliest-known depiction of the Pleiades is likely a Northern German
On numerous cylinder seals from the beginning of the 1st millennium BC., M 45 is represented by seven points, while the Seven Gods appear, on low-reliefs of Neo-Assyrian royal palaces, wearing long open robes and large cylindrical headdresses surmounted by short feathers and adorned of three frontal rows of horns and a crown of feathers, while carrying both an ax and a knife, as well as a bow and a quiver[44]
Subaru
In Japan, the cluster is mentioned under the name Mutsuraboshi ("six stars") in the 8th-century Kojiki.[45] The cluster is now known in Japan as Subaru.[46]
It was chosen as the name of the
It was chosen as the brand name of Subaru automobiles to reflect the origins of the firm as the joining of five companies, and is depicted in the firm's six-star logo.[48]
Observational history
Galileo Galilei was the first astronomer to view the Pleiades through a telescope.[49] 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. 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 surmised that the Pleiades and many other clusters of stars must be physically related.[50] 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.[51]
Edme-Sébastien Jeaurat then drew in 1782 a map of 64 stars of the Pleiades from his observations in 1779, which he published in 1786.[52][53][54]
Distance
The distance to the Pleiades can be used as a key first step to calibrate the cosmic distance ladder. As the cluster is relatively close to the Earth, the distance should be 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 the Hipparcos satellite and independent means (e.g., the Hyades, Coma Berenices cluster, etc.).[3]
Measurements of the distance have elicited much controversy. Results prior to the launch of the Hipparcos satellite generally found that the Pleiades were about 135 parsecs (pc) away from Earth. Data from Hipparcos yielded a surprising result, namely a distance of only 118 pc by measuring the parallax of stars in the cluster—a technique that should yield the most direct and accurate results. Later work consistently argued that the Hipparcos distance measurement for the Pleiades was erroneous.[3][4][5][55][56][57] In particular, distances derived to the cluster via the Hubble Space Telescope and infrared color-magnitude diagram fitting (so-called "spectroscopic parallax") favor a distance between 135 and 140 pc;[3][55] a dynamical distance from optical interferometric observations of the Pleiad double Atlas favors a distance of 133 to 137 pc.[57] However, the author of the 2007–2009 catalog of revised Hipparcos parallaxes reasserted that the distance to the Pleiades is ~120 pc and challenged the dissenting evidence.[2] In 2012, Francis and Anderson[58] proposed that a systematic effect on Hipparcos parallax errors for stars in clusters biases calculation using the weighted mean and gave a Hipparcos parallax distance of 126 pc and photometric distance 132 pc based on stars in the AB Doradus, Tucana-Horologium, and Beta Pictoris moving groups, which are all similar in age and composition to the Pleiades. Those authors note that the difference between these results can be attributed to random error. More recent results using very-long-baseline interferometry (VLBI) (August 2014) and preliminary solutions using Gaia Data Release 1 (September 2016) and Gaia Data Release 2 (August 2018), determine distances of 136.2 ± 1.2 pc,[59] 134 ± 6 pc[60] and 136.2 ± 5.0 pc,[61] respectively. The Gaia Data Release 1 team were cautious about their result and the VLBI authors assert "that the Hipparcos-measured distance to the Pleiades cluster is in error".
The most recent distance estimate of the distance to Pleiades based on the
Year | Distance (pc) | Notes |
---|---|---|
1999 | 125 | Hipparcos[63] |
2004 | 134.6 ± 3.1 | Hubble Fine Guidance Sensor[55] |
2009 | 120.2 ± 1.9 | Revised Hipparcos[2] |
2014 | 136.2 ± 1.2 | Very-long-baseline interferometry[59] |
2016 | 134 ± 6 | Gaia Data Release 1[60]
|
2018 | 136.2 ± 5.0 | Gaia Data Release 2[61]
|
2023 | 135.74±0.10 pc | Gaia Data Release 3[62]
|
Composition
The cluster core radius is about 8
The cluster contains many brown dwarfs, which are objects with less than about 8% of the Sun's mass, making them not heavy enough for nuclear fusion reactions to start in their cores and thus 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.[66] Astronomers have made great efforts to find and analyse brown dwarfs in the Pleiades and other young clusters, because they are still relatively bright and observable, while brown dwarfs in older clusters have faded and are much more difficult to study.
Brightest stars
The nine brightest stars of the cluster are named the
were sisters of the Pleiades. The following table gives details of the brightest stars in the cluster:Name | Pronunciation (IPA) | Designation
|
Apparent magnitude | Stellar classification | Distance (ly)[67] |
---|---|---|---|---|---|
Alcyone | /ælˈsaɪ.əniː/ | Eta (25) Tauri | 2.86 | B7IIIe | 409±50 |
Atlas | /ˈætləs/ | 27 Tauri | 3.62 | B8III | 387±26 |
Electra | /əˈlɛktrə/ | 17 Tauri | 3.70 | B6IIIe | 375±23 |
Maia | /ˈmeɪ.ə/ | 20 Tauri | 3.86 | B7III | 344±25 |
Merope | /ˈmɛrəpiː/ | 23 Tauri | 4.17 | B6IVev | 344±16 |
Taygeta
|
/teɪˈɪdʒətə/ | 19 Tauri | 4.29 | B6IV | 364±16 |
Pleione | /ˈpliːəniː, ˈplaɪ-/ | 28 (BU) Tauri | 5.09 (var.) | B8IVpe | 422±11 |
Celaeno | /səˈliːnoʊ/ | 16 Tauri | 5.44 | B7IV | 434±10 |
Asterope or Sterope I | /əˈstɛrəpiː/ | 21 Tauri | 5.64 | B8Ve | 431.1±7.5 |
— | — | 18 Tauri | 5.66 | B8V | 444±7 |
Sterope II | /ˈstɛrəpiː/ | 22 Tauri | 6.41 | B9V | 431.1±7.5 |
— | — | HD 23753 | 5.44 | B9Vn | 420±10 |
— | — | HD 23923 | 6.16 | B8V | 374.04 |
— | — | HD 23853 | 6.59 | B9.5V | 398.73 |
— | — | HD 23410 | 6.88 | A0V | 395.82 |
Age and future evolution
Ages for star clusters can be estimated by comparing the Hertzsprung–Russell diagram for the cluster with theoretical models of stellar evolution. Using this technique, ages for the Pleiades of between 75 and 150 million years have been estimated. The wide spread in estimated ages is a result of uncertainties in stellar evolution models, which include factors such as convective overshoot, in which a convective zone within a star penetrates an otherwise non-convective zone, resulting in higher apparent ages.[citation needed]
Another way of estimating the age of the cluster is by looking at the lowest-mass objects. In normal
The cluster is
Reflection nebulosity
With larger amateur telescopes, the nebulosity around some of the stars can be easily seen; especially when long-exposure photographs are taken. Under ideal observing conditions, some hint of nebulosity around the cluster may even be seen with small telescopes or average binoculars. It is 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.[10]
Studies show that the dust responsible for the nebulosity is not uniformly distributed, but is concentrated mainly in two layers along the line of sight to the cluster. These layers may have been formed by deceleration due to radiation pressure as the dust has moved towards the stars.[71]
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.[72]
Videos
Gallery
-
A star chart of the Pleiades and their nebulae
-
A widefield view of The Pleiades showing the surrounding dust. Image taken with 7 hours of total exposure time.
See also
- Australian Aboriginal astronomy § Seven Sisters
- Stozhary
- Matrikas
- The Seven Sages
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External links
- The Pleiades on
- Information on the Pleiades from SEDS