Question

Discuss the “Great Debate” that took place between Harlow Shapley and Heber Curtis in 1920. (a) What were the major arguments on each side of the question of whether the spiral nebulae were Galactic (in the Milky Way) or extragalactic (outside the Milky Way)? (b) Briefly outline how Shapley estimated the size of the Milky Way.

Answer 1

IN 1920, there was a great debate between Shapley and Curtis. This was about the milky way galaxy. Shapley believed that distant nebulae were relatively small and lay within the outskirts of Earth's home galaxy, while Curtis held that they were, in fact, independent galaxies, implying that they were exceedingly large and distant.

a) Shapley was arguing in favour of the Milky Way as the entirety of the universe. He believed that "spiral nebulae" such as Andromeda are simply part of the Milky Way. He could back up this claim by citing relative size if Andromeda were not part of the Milky Way, then its distance must have been on the order of 10⁸ light-years.

But According to Curtis, Andromeda is a separate galaxy or Island like the milky way. He showed that there were more novae in Andromeda than in the Milky Way. If Andromeda is also a part of the Galaxy then why there are a large number of the nova in a small section of the galaxy then other. This lead to conclude that Andromeda is a separate galaxy.

b) Shapley believed that everything in the universe was located within our galaxy. He thought that the Galaxy was quite large,300,000 light-years in diameter and that the sun was not at the centre. Shapley also argued that spiral nebulae (spiral-shaped patches of light observed through telescopes) were nearby clouds of gas located well within the Milky Way.

Shapley used his ideas on the size of the Milky Way on observations of globular star clusters. The distance to one prominent globular cluster, M13(Andromeda), had been determined. Shapley assumed that all globular clusters were roughly the same size, and so used their apparent size on the sky to determine their actual distances. He found that the globular clusters formed a loose halo around the flat, disk-shaped body of the rest of the galaxy. In the same way, he determined that the galaxy was 300,000 light-years in diameter and that the sun was located 50,000 light-years away from the centre.

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