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Terme du glossaire : Milieu interstellaire

Redirigé depuis Nuage de poussière

Description : Le milieu interstellaire (ISM en anglais) est un terme qui désigne l'ensemble du gaz et de la poussière qui se trouvent entre les systèmes stellaires d'une galaxie. Notre système solaire se trouve dans le disque de la Voie lactée, où la majeure partie du MIS est constituée d'hydrogène atomique mélangé à de l'hélium atomique et à de la poussière.

Le milieu interstellaire a une densité très faible par rapport aux atmosphères planétaires, avec une densité typique inférieure à une particule par centimètre cube, soit environ 50 milliards de milliards de fois moins dense que l'atmosphère terrestre. Cette densité varie considérablement, de même que la température, à travers la galaxie, le milieu interstellaire étant divisé en plusieurs composantes différentes.

Les composants les plus importants en volume dans le disque galactique sont le gaz atomique chaud et le gaz ionisé chaud, dont les températures avoisinent les 8 000 kelvins (K) et dont la densité est d'environ un demi-atome ou ion par centimètre cube. Un volume plus petit se présente sous la forme de gaz atomique plus froid et plus dense, dont la température est d'environ 40 K. Un volume encore plus petit du milieu interstellaire se présente sous la forme de nuages d'hydrogène moléculaire plus denses (jusqu'à un million de molécules par centimètre cube) et plus froids (<20 K). Certains de ces nuages moléculaires s'effondreront sous l'effet de leur propre gravité, entraînant la formation de nouvelles étoiles. Les nuages moléculaires de la Voie lactée se trouvent principalement dans ses bras spiraux. Le gaz entourant le disque de la Voie lactée est très chaud (des millions de kelvins) et de très faible densité.

Les étoiles rejettent du gaz et de la poussière dans le milieu interstellaire par l'intermédiaire des vents stellaires et des supernovae. Le gaz et la poussière renvoyés dans le milieu interstellaire contiennent une plus grande proportion d'éléments lourds (métaux), ce qui enrichit la galaxie au fil du temps. Le gaz et la poussière présents dans le milieu interstellaire sont la principale cause de l'extinction interstellaire.

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Statut du terme et de sa définition : La définition initiale de ce terme en anglais a été aprouvée par un·e spécialiste de la recherche en astronomie et un·e spécialiste de l’éducation
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Media associé


Above a volcano, a bow-tie-shaped Orion is peppered with bright sweeps of nebular gas

Orion Rises Over Mount Etna

Légende : Honourable mention in the 2022 IAU OAE Astrophotography Contest, category Still images of celestial patterns.   Taken in February 2021, this image is a composite of an astronomy picture in the background and Mount Etna, the famous volcano in Sicily, Italy, in the foreground. Prominently, we see the red hydrogen clouds in space in the area of Orion. Barnard’s Loop is the gigantic bow with the Great Orion Nebula and the Horsehead Nebula in its centre. The deeper-coloured Horse Head is below the southernmost stars in Orion’s Belt, which is the line of white stars above the red nebula. Clearly visible is also the division between the Small and the Great Orion Nebula, the circular and the trapezium-shaped structure in light pink within which one of the nearest star-forming regions is located. The nebula is only a bit more than a thousand light-years away. In the middle-left, close to the edge of the image, the small red structure is the Monkey Head Nebula still in the constellation Orion. It hosts a young star cluster and the deep red colour of this hydrogen cloud indicates its potential to build new stars in the future if the material is compressed again. All these reddish objects are strongly processed in this image, as they are not visible to the unaided eye. Still, this image provides an interesting feature; the red supergiant star Betelgeuse lies in the middle of the image and it seems to be directly above the active volcano Mount Etna. At the foot of this volcano is an ancient settlement, the city of Catania. We consider both Betelgeuse and Mount Etna somehow dangerous — but which of them will erupt first? Ok, we know that Etna occasionally erupts. Normally it exhibits only small eruptions, but the bigger ones happen every few centuries. We also know that Betelgeuse as a giant star will become a supernova in the future. Astronomers call the timescale for the potential supernova short, implying that it will be only 10 000 or maybe 100 000 years until this star explodes. This is “soon” for astronomers, meaning that on Earth, two to four precession cycles will pass by (with the consequence that the Sahara will turn green and dry again two to four times), continental drift will take Africa further north and cause the Alps to grow in height, the Niagara falls in America will wash the rock completely away and only after all this (and much more) happening on Earth will Betelgeuse explode as a supernova. Mount Etna is much more dangerous for the people in Sicily, and Catania in particular, because it will erupt sooner.
Crédit : Dario Giannobile/IAU OAE

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


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

The Pleiades M45 with Majestic Dust

Légende : 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.
Crédit : Mohamed Usama/IAU OAE

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


A red patch mass of gas with a few dark bubbles with lighter edges and several lighter colored clusters and filaments

Herschel’s view of new stars and molecular clouds

Légende : This image shows the Westerhout 3, 4 and 5 star formation regions. This area has huge amounts of gas and dust. This gas and dust hides the physical processes going on in this region from studies using visible light. This image was taken in infrared light by the Herschel Space Observatory. This infrared light allowed Herschel to see deep into these star forming regions. In Westerhout 3, 4 and 5, huge, cold clouds of molecular hydrogen have collapsed into dense knots and filaments. Within these new structures the gas is dense and cold enough for it to collapse and form stars. These new stars give off powerful winds of charged particles, like stronger versions of the solar wind our sun gives off. These winds have combined to blow massive bubbles in the surrounding gas and dust. These are visible as the large darker voids in the image.
Crédit : ESA/Herschel/NASA/JPL-Caltech; acknowledgement: R. Hurt (JPL-Caltech) Lien vers les crédits

License: CC-BY-3.0-IGO Creative Commons (CC) Attribution 3.0 Organisations Internationales Icônes


A field of stars with a large blue wispy feature is interrupted by a huge black void covering almost a quarter of the image

Dust Clouds and Nebulae near R Coronae Australis

Légende : The image shows a dark and dusty cloud and some bright reflection nebulae near the binary star R Coronae Australis. The dark cloud spans several light-years and is located in the constellation Corona Australis near the constellation Sagittarius, in the direction of the Milky Way's center. The cloud appears to swallow the light of distant stars behind it as the dust particles in it scatter light passing through in all directions. This gives it the appearance of a void in the sky. This dark nebula is part of the broader Corona Australis Molecular Clouds. R Coronae Australis forms part of the Coronet Cluster, a collection of young stars which formed at some point in the last two million years. Around R Coronae Australis in the center of the image is the small reflection nebula NGC 6729 with two blueish reflection nebulae NGC 6726 and NGC 6727 lying to the upper right of it. In these nebulae the dust scatters light from bright stars near the nebula towards an observer on Earth, making it glow in this image.
Crédit : ESO Lien vers les crédits

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

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