The Great Debate on the nature of galaxies

Image on the left: The Andromeda Galaxy (M31) photographed by the Hubble Space Telescope. Image on bottom right: Picture of Galaxy M31, taken by Edwin Hubble in 1923. The Cepheid star identified by Hubble is noted as VAR! Image on top right: The same Cepheid star photographed by the Hubble Space Telescope in 2011.
Credit: NASA, ESA, The Hubble Heritage Team (STScI/AURA)
The Great Debate on the nature of galaxiesLe grand débat sur la nature des galaxies
The Great Debate on the nature of galaxies

A century ago, a debate was organized in an attempt to solve the mystery of one of the enigmas of astronomy in that period: what is the nature of spiral nebulae?

For a long time, astronomers wondered about the nature of those fuzzy spiral nebulae that they glimpsed through large telescopes. Were these objects that belonged to our galaxy, the Milky Way, or were they from distant galaxies?

In those days, measuring the distance of these blurry objects was difficult. Nevertheless, astronomers believed that the objects were probably located at the furthest reaches of the Milky Way.

In 1920, the partly self-taught American astronomer George Ellery Hale persuaded the National Academy of Sciences in Washington, DC to organize a debate as a way of deciding the issue of the nature of spiral nebulae.

For the debate, he recruited two parties: his young colleague from the Mount Wilson Observatory, Harlow Shapley, and the eminent Heber Curtis from the Lick Observatory.

Shapley argued that spiral nebulae were objects in the Milky Way. For him, the size of our galaxy would be 300,000 light years, and the Sun would be situated towards the outer edge.

Curtis, meanwhile, claimed that spiral nebulae are “island universes” located far outside the Milky Way. He believed that the Milky Way was only 30,000 light years in diameter and that the Sun was situated near the center.

The great discussion

After lengthy negotiations, the debate turned into a discussion with the title “The Scale of the Universe.” The two protagonists made a 40-minute presentation before the members of Washington’s National Academy of Sciences, and their arguments were published in the annals of the institution.

The great event took place on April 26, 1920. Shapley was nervous, not being comfortable in this sort of public presentation, and did a monotone reading of a highly generalized and didactic 19-page text (he devoted six pages to defining what a light year is).

Curtis, on the contrary, was an outgoing and experienced speaker. He made a technical presentation backed by the use of slides.

The debate finished in a draw, because neither man really succeeded in making the better case.

Hubble puts an end to the debate

It was finally Edwin Hubble, known as the titan of astronomy in America, who would settle the debate in 1923. Hubble at the time was working at Mount Wilson on these famous spiral nebulae. In Andromeda’s, he discovered Cepheid variable stars.

The luminosity of those stars varies in a regular way. A few years earlier, Henrietta Leavitt had shown that the period of variation of these stars was proportional to their intrinsic luminosity. In measuring the variation period and the visible brightness of a Cepheid, we can calculate its distance.

Using that method, Hubble would show that the distance of Andromeda is 900,000 light years, in other words well beyond the limits of the Milky Way. Spiral nebulae were in fact “island universes.” (It should be noted here that Hubble’s calculations would be refined over the years. Today the estimated distance of the Andromeda galaxy is two million light years.)

In the end, the two protagonists in the debate were each partly right. Spiral nebulae are indeed objects outside the Milky Way, as Curtis maintained. But his model of our galaxy turned out to be incorrect.

These days the dimensions of the Milky Way are estimated at 100,000 light years, and our Sun is situated at its furthest reaches. This is the model advanced by Shapley during the debate. However, he overestimated the Milky Way’s dimensions.

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