A small nucleus of a comet is its only hard part, it focuses almost all her weight. Therefore, the core of the first cause of all the rest of the complex comet phenomena. The nucleus of the comet is still not yet available telescopic observations, as they variruyutsya the luminous matter, continuously flowing from the nuclei. Using large increase, you can look into the deeper layers of luminous gas-dust shell, but what's left of its size, is still significantly exceed the real size of the kernel.

Central thickening, visible in the atmosphere of a comet visually and photographs, is called photometric kernel. It is believed that at its center is actually the core of a comet, that is located the center of mass. However, as shown by Soviet astronomer D. O. Mohnach, the center of mass may not coincide with the most vibrant area of the luminosity of the nucleus. This phenomenon is called the effect of Mohnach.

Misty atmosphere surrounding photometric kernel, called the coma. Coma together with the core consists of the head of the comet - gas shell, which is formed by the warming up of the engine when approaching the Sun. Away from the Sun's head looks symmetrical, but with the approach to it, it gradually becomes oval, then extended stronger and in the opposite side of the Sun from it develops the tail is composed of gas and dust forming part of the head.

The kernel is the most important part of a comet. However, there is still no unanimous opinion that it is actually. During the time of the Laplace there was an opinion that the nucleus of the comet - rigid body, consisting of volatile substances such as ice or snow, quickly turning into a gas under the influence of solar heat. This classic ice model of the comet's nucleus was significantly expanded in recent years. The greatest recognition was developed by Iplom model kernel - conglomerate of refractory rocky particles and frozen volatile components (methane, carbon dioxide, water and other). In this ice core layers of frozen gases alternate with dust layers. As the heating gases, steaming, carry dust clouds. This helps to explain the formation of gas and dust tails of comets, as well as the capacity of small nuclei to gassing.

According to Umplu mechanism expiration of substance of a nucleus is explained as follows. In comets, committed small number of passes through the perihelion, the so - called "young" comets - protective surface crust had not yet formed, and the kernel surface covered with ice, so the release is intensive by direct evaporation. In the spectrum of such comets dominated by reflected sunlight, that allows to distinguish spectral "old" comet from "young". Typically "young" called comets that have large axis of the orbits, because it assumes that the first time they penetrate into the inner region of the Solar system. "Old" comet is a comet with a short period of revolution around the Sun, repeatedly held its perihelion. The "old" comets formed on the surface of refractory screen, because when you return to the Sun surface ice, Poltava, "polluted". This screen is well protected underneath the ice from exposure to sunlight.

Model wipe explains many comet phenomena-rich gas emission from small engines, the cause of the non-gravitational forces, deflecting the comet from the calculated route. Streams flowing from the kernel, create the reactive forces that lead to the centuries old accelerations or decelerations in the movement of short-period comets.

There are also other models that deny the existence of a monolithic kernel: one is the kernel as Roy snowflakes, and the other as a mass of stone ice blocks, third suggests that the kernel periodically condensed particle meteor swarm under the influence of gravity of the planets. Yet most plausible is the model wipe.

The masses of the nuclei of comets currently defined very vaguely, so we can talk about the possible mass range, from a few tons (microcamera) to several hundreds, and perhaps thousands of billion tons (10 to 10 to 10 tons).

Coma of comets surrounding the nucleus in the form of a hazy atmosphere. Most comets coma consists of three main parts, significantly differ in their physical parameters:

1) most closely adjacent to the core of the region - domestic, molecular, chemical and photochemical coma,

2) visible coma, or coma radicals,

3) the ultraviolet or nuclear coma.

At a distance of 1 a. that is from the Sun, the average diameter of the inner coma D = 10 km, visible D = 10 - 10 km and ultraviolet D = 10 km.

In the inner coma occur most intensive physico-chemical processes: chemical reactions, dissociation and ionization of neutral molecules. In the visible coma, consisting mainly of radicals (reactive molecules) (CN, OH, NH and others), the process of dissociation and excitation of molecules under the action of solar radiation continues, but less intensively than in the inner coma.

L. M. Shulman on the basis of dynamic properties of the substances proposed to divide the cometary atmosphere on the following areas:

1) the parietal layer (evaporation and condensation of particles on the ice surface),

2) oraloader area (the area of the gas-dynamic movement of substances),

3) transitional region,

4) scope the free-molecular scattering cometary particles in interplanetary space.

But not for any of the comet should be obligatory presence of all these atmospheric regions.

As we approach the comet to the Sun diameter of visible scalp grows from day to day, after passing the perihelion of its orbit head again increases and reaches its maximum size between the orbits of Earth and Mars. In General for the whole population of comets diameters heads enclosed in wide range: from 6000 km to 1 million km

Head of comets at the motion of the comet in its orbit, which take many forms. Away from the Sun, they all, but as it approaches the Sun, under the influence of solar pressure, head takes the form of a parabola or a catenary.

C. Century Orlov proposed the following classification comet goals, considering their shape and internal structure:

1. Type E; - observed in comets with bright comas, framed by the Sun shone parabolic shells, the focus of which lies in the nucleus of the comet.

2. Type C; - observed in comets, the head of which is four times weaker heads of type E and appearance reminds onion.

3. Type N; - observed in comets that are missing and coma and shell.

4. Type Q; - observed in comets, have low ledge towards the Sun, that is abnormal tail.

5. Type h; - observed in comets, in the head of which is generated uniformly expanding ring - Galasy with the center in the kernel.

The most impressive part of a comet is its tail. Tails almost always points away from the Sun side. Tails consist of dust, gas and ionized particles. Therefore, depending on the composition of the particles tails proceed in the opposite from the Sun towards the forces emanating from the Sun.

F. Bessel, exploring the shape of the tail of Halley's comet, for the first time explained its action repelling forces emanating from the Sun. Subsequently, F. A. Bredikhin has developed a more perfect mechanical theory of comet tails and offered to split it into three separate groups, depending on the size of repulsive acceleration.

The mechanism of luminescence comet molecules was deciphered in 1911 by K. Schwarzschild and that is the Crown that had come to the conclusion that this mechanism fluorescence, i.e. the re-emission of sunlight.

Sometimes comets observed quite unusual structures: beams coming from different angles from the kernel and which together form a radiant tail; Galasy system widening concentric rings; shrinking shell - emergence of multiple shells, constantly moving to the kernel; cloud education; preformed bends tails that appears when the inhomogeneities in the solar wind.