The Great Debate

Young Astronomers Blog, Volume 28, Number 10.

We live in a galaxy called the Milky Way. Our galaxy is around 100,000 light years across, and our solar system is located ½ of the way out from the galactic center. The universe is significantly larger containing billions and possibly a trillion other galaxies.

However, for a long time we didn’t know the true size of the universe. We didn’t know if there was anything beyond our galaxy, and we didn’t know the true position of our solar system within the Milky Way.

In the late 18^th century, Charles Messier cataloged several objects that were observable in the night sky. Messier wasn’t that interested in the objects themselves. They got in the way of his is true objective, which was to discover new comets. If these objects were known and cataloged, they wouldn’t be mistaken for a comet.

Messier published his catalog initially in 1774 with a final version in 1784. His version contained 103 objects.  The catalog is still used today and now has 109 distinct objects, the last added in 1967.

Today, for a real challenge, amateur astronomers search for Messier’s objects in something called a Messier Marathon.  The idea is to find all 109 objects in a single night of viewing. The prime time for this is in late March. So, you have another year to plan for it in 2021.

Forty of the objects in the current Messier catalog, including Andromeda, were initially classified as nebulae. In the years after his publication, thousands of these nebulae were observed. However, until a hundred years ago, there was a big debate over the nature of these objects.  On the one side were those who thought our galaxy was the extent of the universe and these objects were nebulae within the bounds of the Milky Way. On the other side were those who thought these various nebulae (including Andromeda) were “island universes”, other galaxies, beyond the Milky Way.

In 1919, it was suggested that a debate be organized to discuss this issue. The debate was held one hundred years ago on April 26, 1920 at the U.S. National History Museum (Smithsonian) in Washington D.C. Harlow Shapley defended the view that the Milky Way was the extent of the universe, while Heber Curtis argued that island universes did exist beyond the bounds of the Milky Way.

Curtis was the older and more established of the two debaters. He worked at the Lick Observatory in Southern California from 1902 to 1920, when he became the director of the Allegheny Observatory in Pittsburgh. A few years earlier, astronomer Jacobus Kapteyn had estimated the Milky Way’s size to be some 30,000 light years across. Curtis, following Kapteyn’s ideas, believed the Sun was at the center of this relatively small Milky Way. Along with many astronomers of the time, he also supported the belief that the various spiral nebulae were galaxies themselves many times farther than the extent of the Milky Way.

Harlow Shapely was the younger of the two. He joined the Wilson Observatory in 1914 and later became the director of the Harvard College Observatory.  He was the first to suggest that the Sun was not at the center of the Milky Way, but that the center was some 30 to 50 thousand light years away. He also thought the Milky Way was immense in size, spanning some 300,000 light years across.

The debate itself consisted of a short presentation by each participant followed by a question-and-answer session. Shapley went first and gave more of a popular account. He was followed by Curtis who made more of a technical presentation. Each participant drafted a summary of their position and they were published the next year by the National Academy of Sciences.

It isn’t clear who actually “won” the debate. Curtis correctly identified that the nebulae were external galaxies.  Shapley, however, recognized that the solar system was not at the center of the galaxy. Shapley also was correct in that the Milky Way was larger than what Kapteyn and Curtis thought, although not as large as his estimate.

Edwin Hubble finally settled the argument in 1925 when he published his measurement of the distance to the Andromeda nebula, which he estimated to be 900,000 light years. This put Andromeda well beyond the bounds of the Milky Way galaxy. Over the next few years, Hubble continued his observations and measured the distance to other nebulae proving they were, in fact, other galaxies and that the universe was significantly larger than just the Milky Way.