Up until 1577, comets were believed to be an atmospheric occurrence. They were seen as an omen of evil. People believed that they heralded the coming of plagues, wars or the death of a well-known person.
Today we know that comets are objects circling the Sun. They are distinguished by their long, glowing tail, which can be seen when the comet reaches the point in its orbit nearest the Sun.
Today it is thought that comets originate from a hemispherical cloud, which is named the Oort Cloud after the Dutch astronomer J. Oort. Several hundred million comet cores are to be found there
Every comet is made up of three parts, the head with the core, the coma and the tail. As the US astronomer Whipple predicted in 1950, the core consists of a mixture of water ice, methane or carbon dioxide ice and dust particles of organic materials. For this reason, comets are also called "dirty snow balls".
When a comet draws closer to the Sun, the ice in its core begins to melt. The evaporation of the outer layers releases a gas mixture and forms an immense cloud that also takes dust particles with it. The cloud can have a diameter of between 100,000 and several million kilometers. This envelope of gas is called the coma. Sunlight reflects off the dust particles.
The solar wind, a current of hot gases originating in the Sun's corona, blows the gas envelope away from the comet's coma creating the tail. Whether the comet is moving towards or away from the Sun, the tail always points away from it. When increased solar activity causes the solar wind to become stronger, the tail can even angle off to one side or have other irregularities.
Simultaneously, the dust particles are blown further out by the solar wind to form a second tail called the dust tail. It is shorter than the gas tail and usually has a greater curvature. As the comet moves further away from the Sun, less and less dust and gas are released from it until finally the tails disappear completely. Some tails are visible for an extended period, while others are not visible at all.
