The letter that changed the universe

On February 19, 1924, American astronomer Edwin Hubble wrote a letter to fellow American scientist and astronomer Harlow Shapley, telling him about his discovery. On reading the letter, Shapley is said to have told a colleague, “Here is the letter that destroyed my universe.” A.S.Ganesh talks about the Great Debate and how Hubble finally settled it…

You know that the Andromeda galaxy is the nearest large galaxy to our own Milky Way, don’t you? In case you didn’t, don’t be too hard on yourself. Just a century ago, it was a hotly debated topic, even among astronomers. In fact, it was proven beyond doubt only about 100 years ago, in 1923-24.

Before that, astronomers used to refer to the Andromeda galaxy, also known as Messier 31, M31, or NGC 224, as the Andromeda nebula. In fact, all the many galaxies scattered through the sky were referred to as spiral nebulae. Even though a lot of them, of varying sizes, shapes, and orientations, were visible in the night sky, their nature remained a mystery and their distances were unknown.

The nature of spiral nebulae and the extent of the universe was a keenly contested topic. There were two diverging theories that dominated the scene a little over 100 years ago. While the old theory defined the Milky Way as the universe that had all the spiral nebulae within its boundaries, the new one saw these other spirals as external individual galaxies, referred to as “island universes.”

American scientist and astronomer Harlow Shapley of Mount Wilson Observatory was the main proponent of the former opinion based on his conclusions from observations during the years 1914-17. American astronomer Hebert Doust Curtis of Lick Observatory, who had been studying spiral nebulae for nearly 10 years, was the central figure of the external galaxies view and believed that the Milky Way was one of many galaxies in a huge universe. 

The two scientists faced off in the Great Debate, also called the Shapley-Curtis Debate, that was held at the Smithsonian Museum of Natural History on April 26, 1920. It was less of a debate, however, as Shapley gave a safe lecture, and Curtis made his point with a thorough research presentation.

Even though the debate had no clear winner, the years that followed provided one as one theory was proved to be right. American astronomer Edwin Hubble was the one who made the discovery, and it put him onto greater things.

In 1919, Hubble began working at Mount Wilson and used the 100-inch Hooker telescope, then the largest reflector in the world, and photographic plates to focus on spiral nebulae. He discovered a variable star –  a star whose brightness as seen from Earth changes with time – within what was then referred to as the Andromeda nebula in October 1923.