Comets
Every now and then from Earth we can observe a shiny spot in the sky, something that looks like a star with a long tail. In reality these appear not to be peculiar 'stars', but only pieces of ice and rocky debris, that orbit the Sun: comets.

Oortcloud
The Oortcloud, is in the middle our solar system.
image source: planetarium Amsterdam.
 The Oortcloud.
In the outer reaches of our solar system a huge cloud stretches out, with large quantities of frozen pieces of dust covered with layers of ice, which is known as the Oortcloud. This cloud contains billions of these clumps of ice, as if they lay waiting in a refrigerator to be sent, by the gravity fields of a passing star, in the direction of our Sun. Eventually they will cross our solar system as comets. When such a comet nears our Sun, the ice will melt and the comet will get a huge 'head' and two clearly observable tails.
Many of the comets are too small or are too far away from Earth, to observe them. About once every ten years however, one passes by at a shorter distance and then we can observe it from Earth with the naked eye. One example of this is the Halley comet that visits our solar system regularly; West in 1976 and Hale-Bopp in 1997 are a few other examples.

The ice becomes a comet.
The nucleus of a comet is made out of large quantities of rocky debris, frozen gases and ice. In the neighborhood of the Sun, heat vaporizes the icy material, which produces a large cloud of gases around the nucleus, known as the coma. Normally the nucleus of a comet is usually about one to ten kilometers in diameter (the nucleus of Halley is about 5.5 km), and the coma may reach a diameter of several millions of kilometers, especially when it nears the Sun.

When the ice melts, dust particles are set free from the comet, and these form a long trail behind the comet, which is called the dust tail. The expelled particles are blown away by the solar wind, and the heat and light radiation of the Sun. When a comets is heading toward our solar system, and moves in the direction of the Sun, then the dust tail is more visable behind the comet. As soon as the comet has completed its orbit around the Sun and heads away from it, the solar winds speed the dust particles up, so the dust tail is more visable in front of the comet. The length of such tails may reached between 100 to 200 million kilometers. Dust particles seem to glow because they reflect sunlight, that is why we can see them so well from Earth.

 Hale-Bopp
Comet Hale-Bopp on 1997, March 30th
source: Pic du Midi Observatory,

The gases and water particles are also expelled through this heating up process, these form a blue colored trail,which is known as an ion tail. These gas particles are very volatile and are concentrated more around the comet's head, therefore these tails always point away from the Sun.

The orbit of a comet.
Just like the planets, comets follow an elliptical orbit around the Sun. Their orbits range from a few years to several hundred thousand years. According to the laws of Kepler, the speed of a comet is highest near the Sun and decreases as its drifts away. Comets are therefore are only inside our solar system for a short period of time and are very far away for a large portion of their lifetime.

The comet heats up when it comes near closer to the to the Sun, and this is when its coma and tails are the largest. When it moves away, its size decreases again, until it is so cold, that only the icy nucleus remains. The comets regularly circulate around the Sun, and with every passing by it looses matter, and so gets smaller and smaller. Halley's comet, for instance, may pass only about two to three thousand times, before it is completely evaporated. When the comet has left, the dust particles and small rocks are left behind. Because of their small size they cannot be seen. However when Earth crosses their path they become visible as meteors or even as meteorites.

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