Glossary term: La chevelure (ou coma) de comète

Also known as Coma

Description: Lorsqu'une comète s'approche du Soleil, une partie des substances solides liées au noyau glacé de la comète se transforme en gaz. Ce gaz, qui se compose principalement d'eau mais aussi d'autres substances chimiques telles que le monoxyde de carbone, le dioxyde de carbone, l'ammoniac, le méthane et le méthanol, ainsi que des particules de poussière, entoure le noyau comme un nuage sphérique et flou, appelé atmosphère cométaire, chevelure de comète, ou encore coma. À mesure que la comète se rapproche du Soleil, de plus en plus de molécules sont scindées par les photons ultraviolets du Soleil, ce qui chauffe la coma et fait croître ses régions extérieures. Les régions extérieures finissent par être ionisées, formant la queue ionique de la comète.

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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 have been approved by a research astronomer and a teacher

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".

Related Media

A comet with two tails, one is yellowish and gradually spreading away from the nucleus, the other is blue and compact

Comet C/2020F3 (Neowise) with separate dust and ion gas tails and a green glowing coma, by Dietmar Gutermuth, Germany

Caption: Second place in the 2021 IAU OAE Astrophotography Contest, category Comets. Comets have a very interesting structure comprising of four main parts: the nucleus, composed of rock, dust and frozen gases, typically spanning a few kilometres, although bigger ones have been observed; a small atmosphere of gas surrounding the nucleus (only present when the comet approaches its closest point to the Sun), called coma; and the two distinctive cometary tails (there is at times third tail). The green colour of the coma is due to carbon and nitrogen present in the coma reacting with the Sun’s ultraviolet radiation. The tail that we are mostly used to observing – dust tail and is composed of micron sized dust particles, the second tail composed of charged particles – ion or gas tail. The tails are released only when the comet approaches the Sun at a distance where the heat and radiation emanating from our star is intense enough to vaporize the frozen gases. The dust tail is curved, while the gas tail is straight and always points away from the Sun as this is carried by the solar wind - flow of charged particles emitted by the Sun. As comets are formed by leftover material, they carry with them important information about the early stages of the Solar System’s formation. This beautiful image shows the comet C/2020 F3 (Neowise), as seen from Germany in July 2020, with three of the four structures clearly visible – coma, gas, and dust tail.
Credit: Dietmar Gutermuth/IAU OAE

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