The Great Debate on the Nature of Nebulae


By Robert Hazen, Ph.D.George Mason University

Let’s review the history of the great debate on the nature of the nebula. This was a controversy that culminated with the discoveries of Edwin Hubble in the 1920s. Hubble’s research, as much as any scientific discoveries of the 20th century, have changed our view of ourselves and our place in the cosmos.

A 3D illustration of a nebula.
In 1920, there was a great debate on the nature and origin of the nebula at the National Academy of Sciences. (Image: NASA images/Shutterstock)

Different Types of Stars in the Cosmos

The most obvious objects in the heavens are stars, countless millions and billions of stars that are systematized on the Hertzsprung-Russell diagram.

On the Hertzsprung-Russell diagram, there are main-sequence stars. These are stars that burn hydrogen; from the small, cool, brown dwarfs up to the super-giant stars, the most energy-emitting stars in the universe, that are super hot as well. 

There are other kinds of stars as well on the Hertzsprung-Russell diagram. For example, the red giant stars which are large stars that are emitting large amounts of energy but are relatively cool on their surface (just a few thousand degrees). Then there are the white dwarfs which are intensely hot stars, tens of thousands of degrees at their surface, and yet not emitting very much energy at all, because they’re so small.

The stars have various fates. Many stars larger than the Sun explode, and when they explode, they provide the raw material for the next generation of stars. If you think about that, it leads you to an inevitable conclusion: all stars depend on the presence of prior groups of stars to seed them, to provide the raw materials for those stars; so stars must occur in groups. As we look into the sky with even modest-sized telescopes, we can see the hazy band of the Milky Way, and we see that it’s actually composed of countless stars.

This is a transcript from the video series The Joy of ScienceWatch it now, on Wondrium.

Great Debate on Nebulae

A illustration of the universe with stars, nebula, and galaxies.
Nebulae appear as fuzzy masses in the cosmos. (Image: pixelparticle/Shutterstock)

There are also other objects that appear as fuzzy masses; these are called nebula, and the nebulae were enigmatic objects. With telescopes, in 1900, no one could be sure, and there was a great debate going on. 

Some astronomers thought nebulae were merely dust clouds—that is, clouds of debris that were too small to be resolved by any telescope; they also were fairly close by, within our own galaxy. Other astronomers thought that nebulae were actually distant clusters of stars, so far away that no telescope could resolve those individual stars. 

This controversy came into sharp focus on April 26, 1920, when rival astronomers—the American Harlow Shapley and his fellow countryman Heber D. Curtis—engaged in a public debate at the National Academy of  Sciences, in Washington, D.C. They debated the origin of the nebula.

Learn more about the nebular hypothesis.

Shapley and Curtis: What They Thought about Nebula

Shapley had come into great prominence by discovering that the Milky Way galaxy was vastly larger than what anyone had previously thought. In fact, he showed it was more than 100,000 light-years across. It’s sort of ironic that Shapley, who greatly increased the size of the known universe, could not really accept how much larger it really was. 

Curtis, on the other hand, argued that nebulae are much more distant galaxies like our own. He looked in his telescopes and saw spiral shapes that were reminiscent of what people thought the Milky Way might actually look like from a distance. 

Both Shapley’s and Curtis’s hypotheses made specific predictions, just like any good scientific hypothesis would. To Shapley, nebulae were clouds of dust and gas, so even the most powerful telescope wouldn’t be able to resolve any specific structures in those nebulae. But to Curtis, nebulae were clusters of stars, so if you had a powerful enough telescope, you would be able to see those stars. A new telescope, a larger telescope, quite literally could resolve this issue.

Portrait photograph of Edwin Hubble.
Edwin Hubble discovered the true nature of galaxies. (Image: Johan Hagemeyer/Public domain)

Learn more about the past and future of the universe.

Edwin Hubble

The discovery of the true nature of galaxies was made by the American astronomer Edwin Hubble, in 1924. Hubble was a remarkable scientist who excelled in many different fields. He went to college at the University of Chicago, where he was a star athlete and an honors student as well. 

For a time, he thought of becoming a professional heavyweight boxer, but he decided instead to go to grad school. He won a Rhodes scholarship to Oxford. He studied law at Oxford because he was fascinated by the British legal system.

When Hubble came back to the United States, he opened a law practice in Louisville, Kentucky, but in less than a year, he decided that law wasn’t really for him; he wanted to be an astronomer, so he went back to the University of Chicago for a Ph.D. in astronomy. World War I intervened, and Hubble enlisted as a private in 1917. At the end of the war, in 1919, he joined the Carnegie Institution of Washington and went to their new telescope facility at Mount Wilson, near Los Angeles, California.

Common Questions about the Great Debate on the Nature of Nebulae

Q: What did Shapley think about nebulae?

To Shapley, nebulae were clouds of dust and gas, so even the most powerful telescope could not resolve any specific structures in those nebulae.

Q: What did Curtis think about nebulae?

Curtis considered the nebulae to be clusters of stars. He believed that with a very powerful telescope, those stars could be seen in nebulae.

Q: Who was Edwin Hubble?

Edwin Hubble was a very prominent American astronomer who discovered the true nature of galaxies. He excelled not only in astronomy but also in many disciplines such as boxing and law.

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The Golden Age of Astronomy
Astronomy: Understanding the Characteristics of Stars
Hubble Images—Exploring the Birthplace of Stars