A star that looks bright in the night time sky might really be a very dim star that happens to be nearby; conversely, a faint star might really be a very luminous star that is extremely far away. Astronomers need to know the true brightness of stars; the true brightness tells us how much power is emitted from the surface in the form of starlight, and it can be calculated once the distance to the star is known. What process causes stars to emit light energy?
A. nuclear fusion
B. oxygen fusion
C. oxygen fusion
D. hydrogen fusion