Comets are Solar system bodies having the form of vague objects, usually with pale clot kernel in the center and tail. They are among the most beautiful celestial bodies. hazy Bright shell surrounding a core of hazy, long tail, extending sometimes to half the sky, fast motion among the stars – all this makes comet different from other heavenly bodies. The comet can be observed when a small ice body, called the nucleus of the comet is approaching the Sun at a distance that is less than 4-5.e. warmed by the sun, and it begins to stand out gases and dust that are visible as a result of their illumination by the Sun.
Gases and dust released from the nucleus, creating around him a hazy shell – atmosphere of the comet, components along with the core of the head of the comet. The atmosphere of a comet continuously dissipates in interplanetary space: under the action of radiation pressure and interaction with the solar wind gases and dust are carried away in the direction from the Sun forming the comet’s tail.
Most of the comets in the middle of the head there is a bright star-like “core”, representing the illumination of the Central, most dense zone of true gases around the comet’s nucleus. The head of the comet and its tail do not have sharp outlines. Their apparent sizes depend on the intensity of the emission of gas and dust from the nucleus, the size of the nucleus and its proximity to the Sun, and on the other hand, the circumstances of the observations,primarily from the brightness of the background sky. From time to time or that the comet moves closer to any massive planet, and this leads to a drastic change of its orbit.
The diameter of the head of a comet is typically tens and hundreds of thousands of kilometres, but, for example, the 1680 comet and a bright comet of 1811 he had over a million kilometers, i.e. almost equal to the diameter of the Sun. Along the tail of the comet brightness decreases gradually, and therefore the length of the visible part of the tail where it merges with the background sky, depending on the blackness of the sky, telescope used, and other reasons. Usually the length of the visible part of the tail is millions and tens of millions of kilometers. But the bright comet of 1680 had a giant head, the tail was visible for over 300 million km. i.e. its length was twice the distance from the Earth to the Sun.
Observations of bright comets has allowed astronomers valuable information about comets ‘ tails, which served as the basis for studying their nature.
As demonstrated by telescopic observation, the illumination of the membranes of the head and tail of the comet is created mostly by gas molecules and dust. Head and tail of the comet is completely transparent. When the comet is between the Earth and any star, the star light reaches us without the slightest weakening. So, the gases and dust in the comet is extremely sparse.
According to the classification proposed in the 70-ies of the XIX century by the Russian astronomer F. A. Bredihina, all comet tails are divided into three types: type I tails pointing directly away from the Sun; the type II tails are curved and deviate back towards the orbital motion of the comet; type III tails are almost straight, but significantly deviate ago. When some mutual positions of the Sun, comet and Earth, the tails of the second and third type seem to be the earth observer pointing to the Sun, i.e., they form the so-called anomalous tails. Modern research has allowed to establish that the tails of the first type – plasma, have a striated structure and are composed of ionized molecules that are more accelerated fly away from the nucleus due to electromagnetic interaction with the solar wind. The tails of the second type formed dust particles of different sizes, continuously released from the nucleus.
Tails of the third type appear in the case when the from the core while out of a great cloud of dust. Dust particles of different size get different acceleration under the action of light pressure, and therefore such a cloud in the shape of a stripe is the tail of a comet. Occasionally there is direct sodium tail is directed approximately along the plasma tail (the tail of the first type). Neutral molecules present in the head of the comet, acquire under the action of light pressure about the same acceleration as dust particles and should therefore move in the direction of the tail of the second type. However, the life-time before ionization by solar radiation only a few hours, and they do not have time to move far in the tail of the second type. Sometimes they can be seen in small numbers in the initial segment of the tail.
About 1950 he established that the nucleus of comets are relatively small icy bodies, consisting of frozen gases mixed with a certain amount of non-volatile rocky substances. The widths of the cores is usually from several hundreds meters to several kilometers, and therefore no nuclei visible.
When the comet passes closer to the Sun and evaporation increases, reveals a hazy sheath of the head of the comet, and sometimes a sparse flow of gases, distills away from the nucleus by the repulsive action of the Sun. Along with the gas leaving the core and the rocky dust of non-volatile substances. Such streams of gas and dust to form one or more tails of the comet.
Not only dust particles but also larger particles leave the nucleus, entrained by a stream of evaporating gases. Cometary nuclei are so small that the force of gravity on their surface in tens of thousands of times less than on Earth.
The illumination of gases in the comet is the re-radiation of sunlight, and pereklokayutsia only rays of certain wavelengths characteristic of the molecules.
As the study of spectra, almost all of the comets head is generated by the radiation of neutral molecules consisting of 2 or 3 atoms. A few years ago it was found the presence in comets the atomic oxygen, hydrogen and carbon. In 1974, for the first time managed to detect the radio emission of cometary molecules.
Comets are members of the Solar system. They move around the Sun in elongated elliptical orbits of different sizes, arbitrarily oriented in space. There are about 100 short-period comets, which in a few years or decades, is approaching the Sun, dissipating part of your core. Most comets have an orbit thousands of times the large diameter of the planetary system. As they approach the Sun through intervals of time in millions of years. Therefore, unlike short-period comets, predict their occurrence is impossible. These comets when they are very far from the sun, the orbits change under the action of gravity of the nearest stars. At the same time all comets during their movement in the area occupied by the planets, orbits change under the influence of planetary attractions. The changes are particularly large during close approaches of comets with planets-giants. Occasionally must be the collision of the comet with the planets. Part of the craters on the moon, mercury and Mars were formed as a result of strikes of comet nuclei.