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Glossary term: Cometary Tail

Description: When a comet is close to the Sun, the Sun's radiation heats up the comet's surface. Ice on the surface turns to gas (it "sublimates"), taking rocky, dusty material with it. The resulting mix forms a cloud around the nucleus of the comet, which is called the coma. In general, a comet will have two tails: The ejected dust particles form the comet's dust tail, which has a characteristically curved shape. It is made up of dust particles released from the surface, which follow the comet along its orbit around the Sun. Dust tails can be millions of kilometers or more in length. They reflect sunlight, and if the conditions are right, their whitish, diffuse shape accounts for most of what can be observed when a comet is visible to the naked eye.

A substantial fraction of the gas is blown away and ionized by the solar wind – the electrically charged particles emitted by the Sun. Those ions form the comet's ion tail, which typically has a blueish color. The ion tail always points straight away from the Sun. If the comet itself is moving away from the Sun, the ion tail precedes the comet.

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Term and definition status: 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

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


A woman in silhouette appears to greet a comet that appears behind bands of light cloud

Hello Comet, shall we dance?, by Robert Barsa, Slovakia

Caption: Third place in the 2021 IAU OAE Astrophotography Contest, category Comets. This beautiful and poetic image taken from Slovakia in July 2020 captures the comet C/2020 F3 (Neowise). The direction of the tails of the comet provides a clue as to the position of the Sun. In the past, the appearance of a comet in the skies could be accompanied by apprehension and even fear from those who did not know what these objects really are. Through careful observations and the applications of knowledge from physics, chemistry and geology, we now understand that comets are objects left over from the earliest days when the Solar System formed. The most distinctive features of a comet are the bluish ion (gas) tail, and whitish dust tail, which can extend for tens of millions of kilometres. These distinctive features, easily observable with the unaided eye together with an understanding of the science, are no longer cause for fear, rather they help us understand the history of our Solar System, and bring awe, joy and contemplation, as portrayed in this image.
Credit: Robert Barsa/IAU OAE

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


Image of a typical comet with a wide white tail and a second blue tail tilted 30 degrees counter-clockwise to the white tail.

Comet Hale-Bopp

Caption: Image of comet C/1995 O1 (Hale-Bopp), taken on 4 April 1997, with an exposure time of 10 minutes. The field shown is about 6.5°x6.5°. Two tails extend from the bright coma: one white-yellowish dust tail and a bluish gas tail, always pointing away from the Sun.
Credit: E. Kolmhofer, H. Raab; Johannes-Kepler-Observatory, Linz, Austria credit link

License: CC-BY-SA-3.0 Creative Commons Attribution-ShareAlike 3.0 Unported icons

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