Physics
The different colours of different stars are due to the variation of

Density
Radiation from them
Pressure
Temperature

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Physics
Radiocarbon is produced in the atmosphere as a result of

Collision between fast neutrons and nitrogen nuclei present in the atmosphere.
Action of ultraviolet light from the sun on atmospheric oxygen.
Action of solar radiations particularly cosmic rays on carbon dioxide present in the atmosphere.
Lightning discharge in atmosphere.

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