Why does a comet have a tail? Short and long period comets. Orbit and speed of a comet. The structure of a comet, the nucleus, coma and tail of a comet. Degenerate comets

If the comet is far from the Sun, it is only a nucleus; she has neither a head nor a tail yet. The diameter of this ice ball can be tens of kilometers or just a couple of kilometers. This is very small by astronomical standards, and since the nucleus glows only from reflected light from the Sun, the distant comet is almost invisible and therefore difficult to detect.

Photos of the nucleus of Comet Halley taken by the European Space Agency (ESA) probe showed that this irregularly shaped blob of ice has a dark-colored crust (much like a scoop of vanilla ice cream dipped in chocolate). Alas, comets are not so tasty, but for the eyes they are a real pleasure! But as soon as the Sun slightly heats the surface of the core, jets of gas and dust begin to burst out of it, like geysers, into the surrounding space. (What a bark! No sense!)

As the comet approaches the Sun, the ice at its core begins to evaporate, sending streams of gas and dust into space. Gas and dust form around the core something like a hazy glowing cloud called coma(coma); this term comes from the Latin word for "hair" and has nothing to do with the comatose state of the patient (joke). Almost everyone confuses a coma with the head of a comet, but the head, strictly speaking, consists of a coma and a nucleus.

The glow of a comet's coma is partly the light of the Sun reflected by millions of tiny dust particles, and partly weak radiation emanating from the atoms and molecules of the coma.

The dust and gas contained in the comet's coma are subject to disturbing forces, causing the comet to form tails.

Under the influence of the solar wind, dust particles are thrown in the direction opposite to the Sun (Fig. 4.4), forming a dust tail comets.

Rice. 4.4. The comet's tail is directed in the opposite direction from the Sun.

The dust tail glows with reflected light from the Sun. It is smooth, sometimes with a slight curvature, and pale yellow.

Coma again?

The first rule of comet watching is: get out of town! Although the comet's nucleus may be only 8-16 km in diameter, the coma that forms around it sometimes reaches tens of thousands or even hundreds of thousands of kilometers in diameter. Gases are released from the core in the same way as smoke from a cigarette. As they disperse, they gradually disappear from view. Therefore, the size of a comet's coma depends not only on how much material the comet releases, but also on the sensitivity of the human eye or photographic film (or electronic sensor). The apparent size of the coma also depends on the degree of darkness of the sky. The bright comet in the city center appears much smaller than outside the city, where the sky is much darker.

Some gas in coma ionized, i.e., it acquires an electric charge under the influence of ultraviolet radiation from the Sun. In this state, gases are exposed to solar wind, an invisible stream of electrons and protons emitted by the Sun into outer space (more details in Chapter 10). The solar wind throws the electrified cometary gas in a direction also opposite the Sun, resulting in the formation of an ion or plasma comet tail. The plasma tail is like a windsock at an airport: it shows astronomers observing the comet which way the solar wind is blowing at the point in space where the comet is located.

Unlike a dust tail, a comet's plasma tail is blue and "stringy" in appearance, and sometimes even twisted or torn.

Sometimes some part of the plasma tail breaks off from the comet and flies off in the direction that the tail "points." Then the comet (like a lizard) forms a new plasma tail. The length of comet tails can range from millions to hundreds of millions of kilometers.

When a comet's head faces the Sun, its tail (or tails) flutter behind it. When a comet circles the Sun and heads out of the Solar System, its tail is still pointing away from the Sun, so the comet is now following its tail! Thus, the comet behaves in relation to the Sun as a courtier behaves in relation to the emperor: it never turns its back on its master. As shown in Fig. 4.4, the comet can move clockwise or counterclockwise, but in any case its tail will always be directed in the direction opposite to the Sun.

The coma and tails of a comet are components of the process of its disappearance. The core releases gas and dust, forming a coma, and the tails are already lost by the comet forever - they simply dissipate. By the time the comet moves far beyond the orbit of Jupiter (which is where most comets come from), only one nucleus will remain from it again. But the dust it lost could one day “fall” on Earth as a meteor shower if it crosses its orbit.