What was the key breakthrough for measuring distances to spiral nebulae?

Answer

Recognizing certain stars within nebulae as Cepheid variables.

Before Hubble could quantify recession velocity in relation to location, accurate distance measurement was essential. The key breakthrough involved identifying Cepheid variables within the distant spiral nebulae. Henrietta Swan Leavitt had previously established that these stars pulsate with an intrinsic luminosity directly related to their cycle period. By measuring this period of variation, astronomers could determine the star's true brightness (luminosity). Comparing this true luminosity to how faint the star appeared from Earth allowed for a precise distance calculation, which was necessary to confirm that these nebulae were indeed separate stellar systems existing far outside the Milky Way.

What was the key breakthrough for measuring distances to spiral nebulae?

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