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tail, cometary
tail, cometary The part of a comet containing dust and gas released from the comet's head. Many comets fail to develop a tail but, when present, tails are always directed away from the Sun, so that comets move tail-first after perihelion. Tails do not normally develop until a comet is within about 2 AU of the Sun, and are usually most impressive shortly after perihelion. Comet tails have two main components: the Type I or gas tail (also known as the ion tail or plasma tail), and the Type II or dust tail. Some comets may have a third form of tail, known as Type III, of neutral sodium, which lies between the gas and dust tails. This was first seen in 1997 in Comet Hale–Bopp. Gas tails consist of ionized gas carried away from the coma by the solar wind, and is more or less straight. They can reach 108 km or more in length. Gas tails appear bluish or greenish and are dominated by emission from singly ionized carbon monoxide (CO+) at 420 nm wavelength, resulting from excitation by solar ultraviolet radiation. They are subject to disconnection events. The dust tail, by contrast, appears yellowish because it shines by reflected sunlight, and often appears markedly curved. Dust tails are usually shorter than gas tails, but can still reach 107 km. They consist of micrometre-sized solid particles which are pushed away from the head by radiation pressure on parabolic trajectories. Larger particles of dust (millimetre- to centimetre-sized) shed by comets give rise to meteor streams. See also antitail.
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"tail, cometary." A Dictionary of Astronomy. 1997. Encyclopedia.com. 31 May. 2012 <http://www.encyclopedia.com>. "tail, cometary." A Dictionary of Astronomy. 1997. Encyclopedia.com. (May 31, 2012). http://www.encyclopedia.com/doc/1O80-tailcometary.html "tail, cometary." A Dictionary of Astronomy. 1997. Retrieved May 31, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O80-tailcometary.html |
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nucleus, cometary
nucleus, cometary The small solid body, composed of frozen water and gas plus embedded dusty material, at the centre of a comet's head. It is the source of activity in a comet, and contains essentially all its mass. The nucleus of Halley's Comet, the first to be directly observed (by the Giotto space probe in 1986), is irregularly shaped, 16 × 8 km, with a dark crust; most nuclei are probably smaller. When very distant from the Sun, a comet's nucleus is inert. Within 3–4 AU, however, solar heating leads to gas sublimation, and production first of a coma, then a tail (see coma, cometary; tail, cometary). Near perihelion, surface temperatures may reach 25 °C. Cometary nuclei have low densities (typically 0.2g/cm3), and are prone to fragmentation. Nuclei which have made several perihelion passages are thought to be covered by a dark crust, as Halley's Comet. Gas emerges through fissures in the crust, carrying off dust which pursues its own orbit as a meteor stream. Perhaps only 10 % of the nucleus consists of such active regions at any one time. Emerging jets are the cause of non-gravitational forces which affect the orbits of comets. Pristine nuclei from the Oort Cloud may lack a dark crust and experience less heating than those of well-evolved comets, as happened with Comet Kohoutek. Some Apollo asteroids may be old, outgassed nuclei.
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Cite this article
"nucleus, cometary." A Dictionary of Astronomy. 1997. Encyclopedia.com. 31 May. 2012 <http://www.encyclopedia.com>. "nucleus, cometary." A Dictionary of Astronomy. 1997. Encyclopedia.com. (May 31, 2012). http://www.encyclopedia.com/doc/1O80-nucleuscometary.html "nucleus, cometary." A Dictionary of Astronomy. 1997. Retrieved May 31, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O80-nucleuscometary.html |
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coma, cometary
coma, cometary The envelope of gas and dust that surrounds the solid nucleus of an active comet (see nucleus, cometary). The coma often appears as a teardrop, being largely shaped by the solar wind flowing around the comet. Near perihelion, the coma may be 100 000 km wide. A coma does not usually form until the comet is within 3–4 AU of the Sun. Coma production has, however, been recorded from Chiron at over 11 AU.
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Cite this article
"coma, cometary." A Dictionary of Astronomy. 1997. Encyclopedia.com. 31 May. 2012 <http://www.encyclopedia.com>. "coma, cometary." A Dictionary of Astronomy. 1997. Encyclopedia.com. (May 31, 2012). http://www.encyclopedia.com/doc/1O80-comacometary.html "coma, cometary." A Dictionary of Astronomy. 1997. Retrieved May 31, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O80-comacometary.html |
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head, cometary
head, cometary The main part of a comet, consisting of the solid central nucleus and the surrounding coma of gas and dust (see coma, cometary). While the head may sometimes appear to contain a star-like point (the pseudo-nucleus), Earth-based observations do not reveal the true nucleus itself.
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Cite this article
"head, cometary." A Dictionary of Astronomy. 1997. Encyclopedia.com. 31 May. 2012 <http://www.encyclopedia.com>. "head, cometary." A Dictionary of Astronomy. 1997. Encyclopedia.com. (May 31, 2012). http://www.encyclopedia.com/doc/1O80-headcometary.html "head, cometary." A Dictionary of Astronomy. 1997. Retrieved May 31, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O80-headcometary.html |
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