Which is the brightest star in sky?

Sun

Answer:

The brightest star can be identified either on an apparent brightness (how bright it seems to an observer on Earth) or absolute luminosity. Obviously the Sun is the brightest star in the sky at any time. If only the night sky is considered, it is Sirius. The brightest star discovered, based on its energy output, is Cygnus OB2-12 (a very bright blue hypergiant). It is more than 6 million times brighter than the Sun.