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

User Hrishikesh
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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.
User Eunjoo
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