This image shows the beautiful Andromeda Galaxy with its two smaller companion galaxies. Credit: Terry Hancock
At the dawn of the 20th century, not much had changed in the 400 years since Galileo’s discovery of the four Jovian moons and his confirmation of Copernicus’s Sun-centered solar system.
In the early 1900s, astronomers disagreed about whether the universe was home to a multitude of galaxies, so-called “island universes,” or was contained entirely within the confines of the Milky Way. Up to that point no one had yet provided evidence that proved one side or the other.
Leavitt’s Special Stars
In 1912, Harvard College Observatory director Edward Pickering published a report by a little-known assistant named Henrietta Swan Leavitt in the observatory’s circular. The paper, titled “Periods of 25 variable stars in the Small Magellanic Cloud,” was destined to change the course of our understanding of the universe.
Leavitt had discovered a unique feature 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 a formula could be created that related the period-luminosity relationship of these stars to their distance. In short, astronomers could begin measuring the cosmos.
Related: Meet Henrietta Leavitt, the woman who gave us a universal ruler
A year later, astronomers Ejnar 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. basic.
Harlow Shapley updates Copernicus
Meanwhile, in 1914, astronomer Harlow Shapley began his career at the Mount Wilson Observatory near Los Angeles. Shapley’s interest was in the study of the Milky Way, and he began using Cepheid variables in the then 69 known globular clusters (vast concentrations of gravitationally bound stars) to calculate the sizes and shapes of the clusters using the 60-inch reflecting telescope.
In a landmark series of papers culminating in 1918, Shapley took advantage of the work of Leavitt and Hertzsprung to conclude that the Milky Way was 300,000 light-years in diameter. Its shape, he calculated, was disk-like, and the solar system was about 50,000 light-years from its center.
This was an ingenious mode of attack (and one that Hubble would use to make his breakthrough discovery a few years later). In one of the greatest research expeditions in modern cosmology, Shapley had updated both Copernicus and Galileo. But, although Shapley was convinced that his new universe was all there was, others remained skeptical.
The “Great Debate” of 1920
In 1920 a debate took place between Harlow Shapley and Heber Curtis, a leading proponent of the insular universe theory and the new director of the Allegheny Observatory at the University of Pittsburgh.
The debate took place April 26 at the Smithsonian Institution in Washington, D.C. Curtis, 48, spoke on behalf of the island universes, and Shapley, 34, essentially on his own behalf.
The Curtis Galaxy was only 10,000 light-years in diameter, a tenth of its actual size. It too was shaped like a sandwich, with the Sun in the center. On the other hand, Shapley’s galaxy was three times larger in scale, and although his idea was correct that the solar system is far from its center, he placed it at twice its actual distance. Shapley’s statement about the disk-like shape of the galaxy was more accurate than Curtis’s.
Despite what in hindsight appears to be a victory for Shapley, the outcome of the debate for those present was essentially a draw. The answer to the question of the nature of nebulae was still uncertain.
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 was becoming operational. His timing couldn’t have been better. He had been studying so-called spiral nebulae for years and now had access to the largest and most technologically advanced telescope in the world, with which he could study his targets in more detail.
Fast forward to four years later, October 5/6, 1923. Hubble was reviewing plates he had taken of Andromeda (M31) with the 100-inch reflector during a routine search for novae (exploding stars) when he noticed the telltale signature of what appeared to be a Cepheid variable. Comparing the star’s brightness to previously made plates, he soon confirmed that it was indeed a Cepheid. Unable to control his excitement, Hubble wrote on the back of the plate with red wax pen: “Var!”
Over the next month, Hubble continued to study this star and, using the Shapley measuring system, calculated that the distance of M31 was 930,000 light-years from Earth. Although this is less than half the actual distance to Andromeda, it was more than three times larger than the entire Shapley universe.
Related: The star that changed the cosmos: M31-V1
Edwin Hubble had collected the first evidence that galaxies are distant, enormous clouds of stars, gas and dust. Over the next few months he discovered more Cepheids in M31 and its nearby companion M33 and measured their distances to confirm that they were both galaxies far from the Milky Way.
Hubble leaked the discovery THE New York Times on November 22, 1924, and his formal 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 the director of the Carnegie Observatory, John Mulchaey, told me recently: “It is really difficult to underline how significant Hubble’s discovery of external galaxies was for astronomy. It instantly changed our view of the universe. In my opinion, this discovery is the most significant in all astronomy of the last 400 years, dating back to the work of Copernicus and Galileo. Simply put, Hubble’s work is responsible for defining our modern view of the universe. Everything we study today is based on his work.”
After 400 years of uncertainty, Edwin Hubble had won the Great Debate.
