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Glossary term: Cluster ouvert

Description: Les amas ouverts sont des amas d'étoiles que l'on trouve dans notre Voie lactée et dans d'autres galaxies. On pense qu'ils sont le résultat d'un seul événement de formation d'étoiles et qu'ils peuvent être constitués de plusieurs centaines, voire de milliers d'étoiles. Les amas ouverts sont beaucoup moins étroitement liés à la gravitation que les amas globulaires et, dans la Voie lactée, ils se trouvent généralement dans le disque galactique. Cette faible liaison signifie qu'au cours de centaines de millions d'années, les amas ouverts se dissolvent progressivement dans la population stellaire générale de la Voie lactée. En tant que groupes d'étoiles ayant un âge commun, les amas ouverts constituent des laboratoires idéaux pour étudier l'évolution stellaire. Les Pléiades sont peut-être l'amas ouvert le plus célèbre du ciel.

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Term and definition status: The original definition of this term in English have been approved by a research astronomer and a teacher
The translation of this term and its definition is still awaiting approval

The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".

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A cluster of brilliant blue stars illuminate the surrounding nebular gas.

The Pleiades M45 with Majestic Dust

Caption: Honourable mention in the 2022 IAU OAE Astrophotography Contest, category Still images of celestial patterns.   Taken in Dar Eid in Saint Catherine/Sinai, Egypt, in October 2021, this image shows the Pleiades, an open cluster also known as The Seven Sisters. The Pleiades are located in the north-western part of the constellation Taurus, the Bull. This constellation originates from ancient Babylonian or even Sumerian belief, where it was designated the Bull of Heaven, a mighty creature owned by the sky god. In Late Babylonian times, the Pleiades were called The Bristle at the hunchback of The Bull. In China, the asterism is also called The Hair, but this does not necessarily imply any relationship between the East Asian and West Asian names of this asterism, although exchange is hypothesised with the establishment of the Silk Road. In ancient Babylonian texts the term The Hair does not appear. Instead, the Pleiades are only called The Star Cluster in Sumerian, and the Sumerian term was used in later languages as a loanword. The Sumerian and early Babylonian religion associated all constellations with specific deities, including gods, demons, messengers of gods. The Star Cluster was associated with a deity of the Netherworld that was called The Seven and was considered an ensemble of seven speaking weapons or strongly armed gods. The later Greek name of the Seven Sisters might possibly have sprung from an intercultural misunderstanding of this older religious association, since, in fact, seven stars are not seen in this cluster. The star cluster of the Pleiades is really prominent in the sky, and thus was used for several cultural purposes, such as determining the calendar and the spring equinox. However, its significance is frequently overstated in cultural astronomy. As the tradition of representing it with seven dots originates from an ancient Sumerian belief, we should be careful about interpreting any group of seven dots on cave walls and archaeological sites across Europe, Asia and America from the Stone Age onwards as a representation of the Pleiades. Modern astrophysics has found that the star cluster of the Pleiades is extraordinarily young, so there was certainly not an additional star in ancient times. Furthermore, we know that the bright stars are only the core region of an open star cluster that consists of hundreds of stars scattered over an area of the sky which exceeds the bright core by one or two of its diameters in any direction. The photograph does not even show the whole cluster. The group is thought to be about 400 light-years away from Earth, which is relatively close in astronomical terms.
Credit: Mohamed Usama/IAU OAE

License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons

Related Diagrams


Les étoiles brillantes d'Andromède forment un Y. Pégase en bas à droite. Au centre se trouve M31, marquée d'une ellipse rouge.

Carte de la constellation d'Andromède

Caption: La constellation d'Andromède montrant ses étoiles brillantes et les constellations environnantes. Andromède est entourée (dans le sens des aiguilles d'une montre à partir du haut) de Cassiopée, du Lézard, de Pégase, des Poissons, du Bélier, du Triangle et de Persée. L'étoile la plus brillante d'Andromède (Alpheratz) se trouve dans la partie inférieure de la constellation. Avec trois étoiles de Pégase, elle forme l'astérisme connu sous le nom de "Grand carré de Pégase". Les deux autres étoiles brillantes de la constellation (Mirach et Almach) forment une ligne qui s'étend au nord-est d'Alpheratz. Andromède est une constellation septentrionale et est surtout visible le soir, à l'automne dans l'hémisphère nord. Elle est visible depuis tout l'hémisphère nord et la plupart des régions tempérées de l'hémisphère sud, mais n'est pas visible depuis les régions antarctiques et subantarctiques. L'objet le plus célèbre d'Andromède, la galaxie d'Andromède, est marqué ici d'une ellipse rouge et de son numéro de catalogue Messier M31. Le cercle jaune à gauche indique la position de l'amas ouvert NGC 752 et le cercle vert à droite indique NGC 7662 (la nébuleuse de la boule de neige bleue), une nébuleuse planétaire. L'axe des y de ce diagramme est en degrés de déclinaison avec le nord en haut et l'axe des x est en heures d'ascension droite avec l'est à gauche. La taille des étoiles marquées ici correspond à la magnitude apparente de l'étoile, une mesure de sa luminosité apparente. Les points les plus gros représentent les étoiles les plus brillantes. Les lettres grecques indiquent les étoiles les plus brillantes de la constellation. Elles sont classées par ordre de luminosité, l'étoile la plus brillante étant étiquetée alpha, la deuxième plus brillante bêta, etc., bien que cet ordre ne soit pas toujours respecté à la lettre. Les lignes pointillées marquent les délimitations des constellations selon l'UAI et les lignes vertes pleines marquent l'une des formes couramment utilisées pour représenter les figures des constellations. Ni les limites des constellations, ni les lignes reliant les étoiles n'apparaissent sur le ciel.
Credit: Adapté par le Bureau de l'astronomie pour l'éducation de l'AIU à partir de l'original de l'AIU/Sky & Telescope

License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons


Scorpius appears as a letter T joined to a letter J. The ecliptic runs ESE to WNW and clips one arm of the T

Scorpius Constellation Map

Caption: The constellation Scorpius (often commonly called Scorpio) along with its bright stars and surrounding constellations. Scorpius is surrounded by (going clockwise from the top) Ophiuchus, Serpens Caput, Libra, Lupus, Norma, Ara, Corona Australis and Sagittarius. Scorpius’s brightest star Antares appears in the heart of the constellation with the famous tail of Scoprius in the south-east (lower left). Scorpius lies on the ecliptic (shown here as a blue line), this is the path the Sun appears to take across the sky over the course of a year. The Sun only spends a short amount of time in late November in Scorpius. The other planets of the Solar System can often be found in Scorpius. Scorpius lies south of the celestial equator. The whole constellation is not visible from the most arctic regions of the world with parts of Scorpius obscured for observers in northern parts of Asia, Europe and North America. Scorpius is most visible in the evenings in the northern hemisphere summer and southern hemisphere winter. The yellow circles mark the positions of the open clusters M6, M7 & NGC 6231 while the yellow circles with plus signs superimposed on them mark the globular clusters M4 and M80. The y-axis of this diagram is in degrees of declination with north as up and the x-axis is in hours of right ascension with east to the left. The sizes of the stars marked here relate to the star's apparent magnitude, a measure of its apparent brightness. The larger dots represent brighter stars. The Greek letters mark the brightest stars in the constellation. These are ranked by brightness with the brightest star being labeled alpha, the second brightest beta, etc., although this ordering is not always followed exactly. The circle around Antares indicates that it is a variable star. The dotted boundary lines mark the IAU's boundaries of the constellations and the solid green lines mark one of the common forms used to represent the figures of the constellations. The blue line marks the ecliptic, the path the Sun appears to travel across the sky over the course of one year. Neither the constellation boundaries, nor the line marking the ecliptic, nor the lines joining the stars appear on the sky.
Credit: Adapted by the IAU Office of Astronomy for Education from the original by IAU/Sky & Telescope

License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons


Sagittarius is shaped like a teapot pouring tea south west. The ecliptic runs WSW to ENE at the top of the constellation

Sagittarius Constellation Map

Caption: The constellation Sagittarius along with its bright stars and surrounding constellations. Sagittarius is surrounded by (going clockwise from the top) Aquila, Scutum, Serpens Cauda, Ophiuchus, Scorpius, Corona Australis, Telescopium, Microscopium and Capricornus. The brighter stars in Sagittarius form a distinctive teapot shape. Sagittarius lies on the ecliptic (shown here as a blue line), this is the path the Sun appears to take across the sky over the course of a year. The Sun is in Sagittarius from mid December to mid January. The other planets of the Solar System can often be found in Sagittarius. Sagittarius lies south of the celestial equator. The famous teapot asterism is visible for all but the most arctic regions of the world but the most southerly parts of the constellation are not visible in northern parts of Asia, Europe and North America. Sagittarius is most visible in the evenings in the northern hemisphere summer and southern hemisphere winter. The supermassive black hole Sagittarius A* which lies at the center of our Milky Way Galaxy is sits on the western (here right-hand) edge of Sagittarius. Due to it covering an area at the center of our Galaxy, Sagittarius is home to many star clusters including open clusters (marked here with yellow circles) and globular clusters (marked here with yellow circles with + signs superimposed on them). Three nebulae are also marked here with green squares. The y-axis of this diagram is in degrees of declination with north as up and the x-axis is in hours of right ascension with east to the left. The sizes of the stars marked here relate to the star's apparent magnitude, a measure of its apparent brightness. The larger dots represent brighter stars. The Greek letters mark the brightest stars in the constellation. These are ranked by brightness with the brightest star being labeled alpha, the second brightest beta, etc., although this ordering is not always followed exactly. The dotted boundary lines mark the IAU's boundaries of the constellations and the solid green lines mark one of the common forms used to represent the figures of the constellations. Neither the constellation boundaries, nor the line marking the ecliptic, nor the lines joining the stars appear on the sky.
Credit: Adapted by the IAU Office of Astronomy for Education from the original by IAU/Sky & Telescope

License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons


Aquarius appears as a sprawling series of connected lines. The ecliptic runs through its center from WSW to ENE.

Aquarius Constellation Map

Caption: The constellation Aquarius along with its bright stars and surrounding constellations. Aquarius is surrounded by (going clockwise from the top) Pegasus, Equuleus, Aquila, Capricornus, Piscis Austrinus, Sculptor, Cetus and Pisces. Aquarius lies on the ecliptic (shown here as a blue line), this is the path the Sun appears to take across the sky over the course of a year. The Sun is in Aquarius from mid February to mid March. The other planets of the Solar System can often be found in Aquarius. Aquarius spans the celestial equator and is thus visible at some time in the year from all of planet Earth. In the most arctic or antarctic regions of the world, some parts of the constellation may not be visible. Aquarius is most visible in the evenings in the northern hemisphere autumn and southern hemisphere spring. The yellow circles with plus symbols superimposed on them mark the globular clusters M2 and M72. The green circles superimposed on plus symbols mark the planetary nebulae NGC 7293 (the Helix Nebula) and NGC 7002 (the Saturn Nebula). M73 (marked with an x symbol) is a coincident grouping of stars previously erroneously classified as an open cluster. The y-axis of this diagram is in degrees of declination with north as up and the x-axis is in hours of right ascension with east to the left. The sizes of the stars marked here relate to the star's apparent magnitude, a measure of its apparent brightness. The larger dots represent brighter stars. The Greek letters mark the brightest stars in the constellation. These are ranked by brightness with the brightest star being labeled alpha, the second brightest beta, etc., although this ordering is not always followed exactly. The dotted boundary lines mark the IAU's boundaries of the constellations and the solid green lines mark one of the common forms used to represent the figures of the constellations. Neither the constellation boundaries, nor the line marking the ecliptic, nor the lines joining the stars appear on the sky.
Credit: Adapted by the IAU Office of Astronomy for Education from the original by IAU/Sky & Telescope

License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons


The four bright stars of Crux form a kite shape with the long axis pointing vertically

Crux Constellation Map

Caption: The constellation Crux (commonly known as the Southern Cross or Crux Australis) showing its bright stars and surrounding constellations. The Southern Cross is surrounding by (going clockwise from the top) Centaurus, Carina and Musca. The brightest star is alpha Crucis which appears at the bottom of the constellation's famous kite shape. The Southern Cross is visible from southern and equatorial regions of the world. In more southerly parts of the world it is circumpolar so is always above the horizon. In other parts of the southern hemisphere and in equatorial regions it is most visible in the evenings in the southern hemisphere autumn. The yellow circles show the locations of two open clusters, NGC 4755 (known as the Jewel Box) and NGC 4609. The line joining gamma and alpha Crucis (the third and first brightest stars in the Southern Cross) points in the approximate direction of the South Celestial Pole. This has led to the Southern Cross playing an important role in celestial navigation, allowing navigators from different astronomical traditions to find their bearings. The y-axis of this diagram is in degrees of declination with north as up and the x-axis is in hours of right ascension with east to the left. The sizes of the stars marked here relate to the star's apparent magnitude, a measure of its apparent brightness. The larger dots represent brighter stars. The Greek letters mark the brightest stars in the constellation. These are ranked by brightness with the brightest star being labeled alpha, the second brightest beta, etc., although this ordering is not always followed exactly. The dotted boundary lines mark the IAU's boundaries of the constellations and the solid green lines mark one of the common forms used to represent the figures of the constellations. Neither the constellation boundaries, nor the lines joining the stars appear on the sky.
Credit: Adapted by the IAU Office of Astronomy for Education from the original by IAU/Sky & Telescope.

License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons