Astrophysics is the science that applies principles of physics to many fields of astronomy.
It tries to determine the physical nature, origin and development of the solar system, stars, galaxies and the universe. Much can be learned about the physical nature of cosmic objects by studying the wavelengths of the electromagnetic waves that they emit. Astrophysicists estemate the motion of a star or galaxy by measuring theshift in the wavelengths of its light. The shift in the blue light of a cosmic object toward the spectrum's red region or longer wavelengths is called red shift. The enormous red shifts of distant galaxies and quasars indicate that they are rapidly moving away from the earth.