I have had a passion for galaxies since I was a child. I was born at a time when it was understood that the Earth revolved around a star (called the “sun”) which was part of a massive collection of stars (“billions upon billions” in the words of astronomer Carl Sagan) called the Milky Way. It was also accepted that the Milky Way was an unexceptional example of something called a “galaxy”, of which there were billions and billions in the universe. The concept of a vast universe filled with gigantic galaxies, of which one was the Milky Way, has always been very powerful to me.
One hundred years ago, in 1920, very few people accepted this idea of a galaxy. The word galaxy comes from the Greek, galaxias kyklos, meaning “milky circle”. The name “milky way” and the word “galaxy” were the same thing. There was one galaxy, and our solar system was inside it. The milky way was effectively the universe.
For years there had speculation that perhaps some of the visible nebulae were more distant, including comments by Immanuel Kant in 1755 and the speculations of William Herschel a few years later. Yet it was one hundred years ago that our perception of our place in the universe began changing forever. Before 1917 there was little evidence of other galaxies, in 1920 the scientific debate was at its peak, and by 1925 every astronomer knew that there were a vast number of galaxies.
The “New General Catalogue of Nebulae and Clusters of Stars” was completed in 1908. Many of the objects cataloged were galaxies (each object is named “NGC” followed by a catalogue number), but in 1920 they were nebulae of uncertain nature.
The situation heated up in 1917 when a Nova (suddenly brightening star) was discovered in what we now know as galaxy NGC6946. Soon a list of eleven Novae had been compiled that were located in seven different nebulae, four in the M31 Andromeda nebula (…galaxy) alone. These Novae were compared with Novae that had appeared nearby and their characteristics strongly supported the “island universe” theory, namely that these nebulae were galaxies outside the Milky Way.
Here is my own image of NGC6946, from September 2021. Note the red star-formation regions in this distant "island universe".
The scientific debate was on: Harlow Shapley of Harvard University argued powerfully that the island universe theory was false, and that these nebulae were just odd objects within the Milky Way. Clearly resolved stars within the Andromeda Nebula were referred to as “nebulous stars”, which were considered to be unlike regular “stars”. Real problems in the island universe theory couldn’t be explained.
H.D. Curtis, director of the Allegheny Observatory (U. Pittsburgh), was convinced of the existence of “island universes” from his work studying nebulae. Kurt Lundmark of Sweden entered the debate with his 1920 Doctoral Thesis. His thesis assumed that the Novae in the odd nebulae were of the same physical type as those previously discovered and proceeded to estimate the distances to the nebulae, clearly identifying them as distant galaxies.
The Shapley-Curtis debate of April 26, 1920 before the National Academy of Sciences allowed both sides a chance to state their case, but an impasse had been reached.
All debate ended in 1925 when Edwin Hubble (for whom the telescope is named) published research on variable stars observed in nearby galaxies. Such stars had well-known brightness/variation characteristics. By observing their variation and apparent brightness their distance could be determined. These stars were far outside the milky way, in island universes far away, in “galaxies”.
Suddenly humanity understood the Milky Way that we see in our night sky to be an “island universe” among many islands. Without realizing it, astronomers had for decades been observing and photographing other island universes at unimaginable distances, and these other island universes were incredibly large and luminous.
Our world changed dramatically one hundred years ago.
The image below is a photograph of an entry in the Hubble Atlas of Galaxies (1961). The galaxy is NGC3623 (M65), photographed at Palomar Observatory in November 1924. When this image was taken it was not generally accepted to be a galaxy. The second image is also of M65, observed at Lac Teeples in March 2021.
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