The Spiral Nebulae and the Great Debate

Additional reading from www.astronomynotes.com

• Other Galaxies

We have defined the Milky Way as the conglomeration of objects that are mutually bound to each other by the force of gravity. The Milky Way thus consists of stars, gas, and dust. The gas in the Milky Way takes many forms, but the most visible forms are the different types of bright nebulae.

Many different nebulae were well known to astronomers in the early part of the 20^th century, but their nature was not yet entirely understood. Recall that the Messier Catalogue from the 18^th century included a number of nebulae, and the NGC catalogue of the 19^th century included thousands more. By the early part of the 20^th century, one specific type of object in these catalogues, called “spiral nebulae,” was generating a lot of debate because the nature of these nebulae was not understood. Below is an example image of M51 (unfortunately, it is not very well reproduced in this image), published in 1910 by George Ritchey. Note that the label he uses is "Spiral Nebula Messier 51."

Figure 9.1: Photographic image of M51 spiral nebula by G. W. Ritchey

Credit: G. W. Ritchey, 1910, ApJ 32, Plate IV

The two sides of the argument over the spiral nebulae had to do with the size of the Milky Way and its relationship to the universe as a whole. On the one hand, some astronomers argued that the Milky Way was a large part of the entire universe, and that the spiral nebulae were just one other type of gas cloud inside of our Galaxy. On the other hand, some astronomers argued that these spiral nebulae were “island universes” like the Milky Way, and they were simply so far away that their stars were not resolved into point sources of light but were instead blurred together so they looked like a nebula. This argument culminated in a debate between two astronomers in 1920 that is now referred to as the “Great Debate.”

The topic of the debate was the “Scale of the Universe,” and one of the debaters was Harlow Shapley. Shapley is the astronomer who used globular clusters to determine the size of the Milky Way, and this research was also his contribution to the debate. Heber Curtis was the other participant in the debate. His main assertion was that the spiral nebulae are objects like the Milky Way, not objects contained in the Milky Way. In the late 90s, additional debates were held on modern topics to celebrate the 75^th anniversary of the Great Debate, and a lot of information on the original program was collected and published on The Shapley-Curtis Debate website.

The data used by both Shapley and Curtis in their debate were not of high enough quality to conclusively solve the debate over the nature of the spiral nebulae. However, even though they drew other conclusions that have since been proven incorrect, both astronomers made points that fundamentally altered the understanding of our place in the Universe. Shapley did show that the Milky Way is larger than it was believed at the time and that the Sun is offset from the center. His incorrect conclusion was that he believed that the Milky Way was so large that it could encompass the spiral nebulae. Curtis' main contribution was to argue that the data available were not of sufficient quality to conclude that the spiral nebulae were inside of the Milky Way, and he believed that we would eventually find them to be external objects similar to the Milky Way. His argument relied on his belief that the Milky Way was much smaller than it truly is, however. So, although Curtis was proven correct about the nature of the spiral nebulae, he came to that conclusion based on a faulty assumption!

One observation was made shortly after the Great Debate that conclusively settled the debate on the nature of the spiral nebulae. Using the 100-inch telescope on Mount Wilson, Edwin Hubble took images of M31, the Andromeda Nebula. He discovered that M31 was composed of stars, and he even identified several Cepheid variable stars useful for measuring the distance to M31. If you remember our discussion of Cepheid stars, they have a very specific relationship between their variability period and their luminosity. Thus, if you measure the period of a Cepheid variable, you can estimate its luminosity. Then, if you measure the apparent brightness of the Cepheid (which you can do using the same observations you took to get its period), you can measure its distance. When Hubble calculated the distance to Andromeda, he found that it was much larger than the size of the Milky Way, confirming that this was another galaxy like the Milky Way, and not a “spiral nebula” inside of the Milky Way.

The discovery that the spiral nebulae are other galaxies similar to the Milky Way again caused a large shift in our understanding of the Universe and our place in it. For example, prior to Hubble's discovery, Shapley believed that the Universe was filled almost entirely by the Milky Way. Hubble showed that, in reality, the Milky Way is just one object in a universe filled with many billions of other similar objects, which forces us to realize that the universe is much larger than the Milky Way. One popular astronomy textbook has a nice expression that summarizes our current understanding: “Galaxies are the fundamental units of the Universe, just as stars are the basic units of galaxies." (The Cosmos: Astronomy in the New Millenium. Pasachoff & Filippenko , 3rd edition, p. 367.)