Question

what is the reason behind this comets in the sky ??

Answer 1

Comets are cosmic snowballs of frozen gases, rock and dust that orbit the sun. When frozen, they are the size of a small town. When a comet's orbit bring it close to the sun, it heats up and spews dust and gases into a gaint glowing head larger than most planets and sometimes also a tail which appears behind the head in the sky.
Usually the orbits of comets are very large they are seen in many hundred years.
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Answer 2

A comet is an icy small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of solar radiation and the solar wind acting upon the nucleus of the comet. Comet nuclei range from a few hundred metres to tens of kilometres across and are composed of loose collections of ice, dust, and small rocky particles. The coma may be up to 15 times the Earth's diameter, while the tail may stretch one astronomical unit. If sufficiently bright, a comet may be seen from the Earth without the aid of a telescope and may subtend an arc of 30° (60 Moons) across the sky. Comets have been observed and recorded since ancient times by many cultures.