Why is there a mass-luminosity relation for main sequence stars? The more massive a star is, the lower the rate of fusion in the core needs to be in order for the star not to overheat. When the energy from the fusion reaction reaches the surface it radiates into space with a characteristic luminosity. The gravitational energy of a certain mass releases characteristic radiation that is only detectable on the enormous scale of the masses of stars. The more massive a star is, the higher the rate of fusion in the core is due to the larger weight pressing down on the inner layers. When the energy from the fusion reaction reaches the surface it radiates into space with a characteristic luminosity. The more massive a star is, the lower the rate of fusion in the core is due to the larger weight pressing down on the inner layers. Most of the energy from the fusion reaction is consumed to balance against the weight of the star, and the leftover energy radiates into space with a characteristic luminosity.