What was the physical principle used by edwin hubble to demonstrate that the "spiral nebulae" are very distant and large?

The physical principle used by Edwin Hubble to demonstrate that the "spiral nebulae" are very distant and large was the inverse-square law of light. The Cepheid variable stars in the spiral nebulae obey a period-luminosity relation; by measuring the period of a Cepheid star, its average luminosity can be determined, and then, by comparing this with its average apparent brightness, its distance can be calculated.