87th Anniversary of the “Great Debate”

Although not as notable as some recent events in astronomical history, today is the 87th anniversary of what set the stage to be a major revolution in the astronomical field.

Prior to the early 1900’s, the scale of the universe was thought to be quite small. Astronomical distances at that time were ones that could be dealt with without having to resort to scientific notation. We had our nice little home in our galaxy. There were stars, dust, and some nebulae. But of particular interest were a set of what were then dubbed “spiral nebulae”.

On April 26, 1920 two prominent astronomers, Herber Curtis and Harlow Shapley got together for a debate on this issue. The question was whether or not these odd structures were relatively nearby objects, within our own galaxy, or far more distant than astronomers until that point had dared imagined, lying far beyond our own galaxy as their own, unique “island universes.”

Shapley held the former position, suggesting the extent of our galaxy was effectively the entire universe. He required that it be fairly large to do this (about twice the size we estimate today). Another prominent astronomer of the time, Adriaan vanMaanen, supported him.

On the other side, Curtis claimed that our own galaxy was much smaller, being at most about 30,000 light years in diameter. This left room elsewhere for more distant objects.

Shapley began the talk, first presenting his side. One of the first points he made is that, our own galaxy is so large, if we assume that these spiral nebulae were similar sizes, that would put them at inconceivably large distances. He also pointed out differences in the surface brightness between our galaxy and what was observed for these spiral nebulae. Our own was much fainter, which would suggest that they are different beasts. His main piece of evidence, however, was based on observations from vanMaanen in which he claimed to have shown that the spiral nebula M31 rotated once every 10⁵ years. If these objects were really that far away, there would be no way we could observe them rotating. Additionally, if they really were that large, their outer edges would have to be moving much faster than the speed of light to complete a full rotation that frequently.

Curtis followed, making several points to suggest that these objects were much further. His first point was that there was a large range in the observed sizes of the spiral nebulae. Some were big (like M31 and M100). Others were much smaller. It did not seem plausible that the same processes could make objects with such disparate sizes. As such, they were probably all similar sizes, but at varying distances, which would require that they be much further than Shapley claimed. Curtis also observed novae in the Andromeda nebula (M31) and noticed that they were extremely faint compared to ones in our own galaxy. He assumed that, if they were indeed the same process, then the faintness must be due to extreme distances. Lastly, he noted that the Doppler shifted spectra of these objects did not match with other objects we knew to be in our own galaxies. They traveled at much higher relative velocities, and as such, must be independent systems. He also wondered about the distribution of these nebulae. Most non-spiral nebulae seemed to be confined to near the plane of the Milky Way whereas these objects had no apparent preference.

While this debate didn’t definitively decide the matter, it did set the stage for more research, leading up to Edwin Hubble’s discovery of Cepheid variables in M31 in 1923. What was interesting was the evidence provided by both sides.

Shapley’s first argument against a large universe was simply an argument from incredulity, very similar to the ones we see today from the ID proponents. His second, that the surface brightnesses were dissimilar between our galaxy and those spiral nebulae was a major blow at the time, but ended up being solved upon the discovery of absorption caused by dust in our own galaxy which made ours seem much dimmer. The observations he relied upon from vanMaanen ended up wrong just due to sloppiness on vanMannen’s part.

But Shapley wasn’t the only one that got things wrong.

The novae Curtis cited as subluminous weren’t actually just regular novae. They were supernovae, which put M31 even further away than was realized. Even Hubble didn’t get things quite right. At that time, the distance calibration for Cepheid stars wasn’t terribly accurate because we didn’t have well-established distances for the calibration stars (now we have trigonometric parallaxes which are the most accurate distance measurement). It was also later realized that there’s two distinct populations of Cepheids with slightly different calibrations. His original distance estimate was about half the current.

Since then, we have discovered ever more about our galaxy, and our universe and most certainly, the future will bring even more understanding.