Glossary term: Milieu interstellaire

Redirected from 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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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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Caption: 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.
Credit: Dario Giannobile/IAU OAE

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

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Credit: Mohamed Usama/IAU OAE

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

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