Answer:
Sun is bright because it is closer to the earth and the radiation that reaches the earth's surface gives out the heat.
Explanation:
The thermonuclear fusion reactions are responsible for the heat and light emitted by any star.
As a result of the fusion reactions that occur in its core, sun's core is very very hot. This heat is radiated outward and after passing through several zones of the sun itself, the surface temperature of the Sun cools to about about 6000 K.
Sun radiates electromagnetic radiation that can travel through space and then penetrate the earth's atmosphere. Ozone layer which is present in the ozonosphere of the earth's atmosphere filters the UV rays coming from the sun. The radiation in the form of heat and light is absorbed by the particles of air in the earth's atmosphere, there by heating the surface.