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Stars are very far from they appear as point sources of light.....and not an entire sphere...
This is bcoz stars are very very far away from earth and we can see it as a point source of light energy the temperature varies as we come downwards from stars and this makes air lighter or heavier respectively.This stars emit its own light,so when the light travels through this changing temperature and air it gets refracted and we see a blurred image of the star and think it is twinkling but actually it is the light which are continuously coming from one medium to other and continuously refracts and we cant see the stars as spheres but as asterisk(*).