Question

what are comets made up of what changes takes place in comment when it approaches the sun

Answer 1

Comets are basically dusty snowballswhich orbit the Sun. They are made of ices, such as water, carbon dioxide, ammonia and methane, mixed with dust. These materials came from the time when the Solar System was formed. Comets have an icy center (nucleus) surrounded by a large cloud of gas and dust (called the coma).
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Answer 2

ices ,water,carbon dioxide,ammonia and methane mixed with dust form a comet .as a comet approaches sun it grows warmer and some of the ices heat up and become gas (sublimating)releasing the dust grains that were trapped in the ice.this dusty gas coming from the nucleus is heated by the sun and forms a glowing hot cloud around the comet called the coma