Question

What event settled the Shapley-Curtis debate about spiral nebulae?

Answer 1

Hubble measured the distance to the Andromeda Spiral Nebula and showed that it lies outside the Milkyway galaxy. This settled the debate about the nature of spiral nebulae.

Explanation:

Once large telescopes were built it was found that the sky is filled with many spiral shaped nebulae. It was not know what these objects are made of. There were two hypotheses regarding the nature of the spiral nebulae and the astronomers were divided into two camps.

One side thought that the spiral nebulae are island universes (galaxies likes our own milkyway galaxy but are far away from our galaxy). The other side thought that these were sub-galactic objects located inside our own galaxy like glowing gas clouds. This side also maintained the view that the Universe has one one galaxy which is our milkyway.

This confusion resulted in the Shapley-Curtis Debate in 1920. Harlow Shapley, who was known for his estimation of the extent of milkway galaxy represented the side that thought that the spiral nebulae were sub-galactic structures inside our milkyway galaxy. Heber Curtis represented the side that thought they are island universes - galaxies themselves that lie outside the milkyway galaxy.

The question was settled in 1923 when Edwin Hubble, using the `100`" at Mount Wilson, discovered Cepheid Variable stars in the andromeda spiral nebula and used the perio-luminosity relation of Henrietta Leavitt to find the distance of these objects. It clearly showed that these distances are far above the extent of milkyway galaxy estimated by Shapley.