How Edwin Hubble won the great debate

By the early 20th century, little had changed in the 400 years since Galileo discovered the four moons of Jupiter and confirmed Copernicus’ theory of a Sun-centered solar system.

In the early 1900s, astronomers disagreed about whether the Universe was home to many galaxies, so-called “island universes,” or whether it was entirely contained within the Milky Way Galaxy. At that time, no one had yet presented evidence confirming one side or the other.

Leavitt’s Special Stars

In 1912, the director of the Harvard College Observatory, Edward Pickering, published in the observatory’s circular the report of a little-known assistant named Henrietta Swan Leavitt. The paper, entitled “Periods of 25 Variable Stars in the Small Magellanic Cloud,” was intended to change the course of our understanding of the Universe.

Leavitt discovered a unique characteristic of Cepheid variable stars, a type of pulsating star with a reliable relationship between brightness and the length of its period (from dimmest to brightest). The brighter the star, the slower it blinks.

This was an important discovery. This meant that it was possible to create a formula relating the period and luminosity relationship of these stars to their distance. In short, astronomers could start measuring the cosmos.

Connected: Meet Henrietta Leavitt, the woman who gave us the universal ruler.

A year later, astronomers Einar Hertzsprung and Henry Norris Russell independently demonstrated how Cepheids, named after the prototype Delta(δ) Cephei, could be used to determine relative distances in space, using the Sun’s orbit as a baseline.

Harlow Shapley updates Copernicus

Meanwhile, in 1914, astronomer Harlow Shapley began his career at Mount Wilson Observatory near Los Angeles. Shapley’s interest lay in studying the Milky Way, and he began using Cepheid variables in the 69 then known globular clusters (huge, gravitationally bound clusters of stars) to calculate cluster sizes and shapes using a 60-inch reflecting telescope.

In a historic series of papers that culminated in 1918, Shapley, using the work of Leavitt and Hertzsprung, concluded that the Milky Way had a diameter of 300,000 light years. By his calculations, it was disk-shaped and the solar system was about 50,000 light-years from its center.

This was an ingenious method of attack (and it was what Hubble used to make his revolutionary discovery a few years later). In one of the greatest exploratory expeditions of modern cosmology, Shapley updated the data of Copernicus and Galileo. But while Shapley was convinced that his new universe was all there was, others remained skeptical.

“Great Debate” of 1920.

In 1920, a debate took place between Harlow Shapley and Heber Curtis, a leading proponent of the island universe theory and newly appointed director of the Allegheny Observatory at the University of Pittsburgh.

The debate took place on April 26 at the Smithsonian Institution in Washington, DC. Curtis, 48, spoke on behalf of the island universes, and Shapley, 34, essentially spoke on his own behalf.

The Curtis Galaxy was only 10,000 light-years in diameter, a tenth of its actual size. It was also shaped like a bun with the Sun in the center. On the other hand, Shapley’s galaxy was three times larger in size, and although his idea was correct that the solar system was located far from its center, he placed it at twice the actual distance. Shapley’s statement about the disk-shaped galaxy was more accurate than Curtis’s statement.

Despite what in retrospect appears to be a victory for Shapley, the result of the debate for those present was essentially a draw. The answer to the question about the nature of nebulae was still in doubt.

Enter Edwin Hubble.

Hubble and the 100-inch Hooker Telescope

Hubble arrived at Mount Wilson in the fall of 1919, just as the 100-inch Hooker Telescope began operating. His timing couldn’t have been better. He had been studying so-called spiral nebulae for years, and now he had access to the largest and most technologically advanced telescope in the world, which he could use to study his targets in more detail.

Fast forward four years later to October 5/6, 1923. Hubble was viewing images he took of Andromeda (M31) with the 100-inch reflector during a routine search for novae (exploding stars) when he noticed a distinctive signature of something that looked like a Cepheid variable. By comparing the brightness of the star on previously made plates, he soon confirmed that it was in fact a Cepheid. Unable to contain his excitement, Hubble wrote on the back of the record with a red wax pen: “Var!”

Over the next month, Hubble continued to study the star and, using the Shapley measurement system, calculated the distance to M31 to be 930,000 light-years from Earth. Although this was less than half the actual distance to Andromeda, it was more than three times larger than the entire Shapley Universe.

Connected: The Star That Changed Space: M31-V1

Edwin Hubble collected the first evidence that galaxies are distant, huge clouds of stars, gas and dust. Over the following months, he discovered more Cepheids in M31 and its nearby companion M33 and measured their distances to confirm that both galaxies were far from the Milky Way.

Hubble allowed his discovery to become available New York Times November 22, 1924, and his official report was read aloud by Henry Norris Russell at a joint conference of the American Association for the Advancement of Science (AAAS) and the American Astronomical Society (AAS) on January 1, 1925.

This act of cosmic archeology would prove to be the purest and most objective discovery of Hubble’s career. As Carnegie Observatory Director John Mulchay recently told me, “It is very difficult to emphasize how important Hubble’s discovery of external galaxies has been for astronomy. He instantly changed our view of the Universe. In my opinion, this discovery is the most significant in all of astronomy over the past 400 years, starting with the work of Copernicus and Galileo. Simply put, Hubble’s work shaped our modern understanding of the Universe. Everything we study today is based on his work.”

After 400 years of uncertainty, Edwin Hubble won the Great Debate.