Question

Which two classes of objects were critical to

the Shapley-Curtiss Debate of 1920?
emission nebulae and spiral nebulae
O globular clusters and spiral nebulae
O white dwarfs and red giants
O open clusters and emission nebulae
O planetary nebulae and white dwarfs

Answer 1

globular clusters and spiral nebulae

Step-by-step explanation:

The Shapley-Curtis Debate of 1920, also known as the "Great Debate," was about the scale of the universe and the nature of spiral nebulae – whether they were within our galaxy or were entire galaxies themselves. Harlow Shapley argued that the Milky Way was the entire universe and that spiral nebulae were just part of our galaxy, while Heber Curtis contended that the Milky Way was one of many galaxies and that spiral nebulae were, in fact, other galaxies.

Two classes of objects were central to their arguments:

Shapley used globular clusters to estimate the size of the Milky Way and argued for a much larger galaxy.

Curtis used observations of spiral nebulae (later understood to be galaxies) and their characteristics, such as "novae" that suggested they were much further away than previously thought, indicating that they were separate galaxies.

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