By Joe Luna

A century or so ago, the Milky Way was considered the extent of the universe, though astronomers concerned with the size of the universe faced a puzzle. They had seen clouds (“nebulae”), and observed that these nebulae had unusual properties. For one thing, the nebulae displayed line spectra that matched those of star clusters, not interstellar dust. There were more stellar explosions (“novae”) observed in these clusters as well. Reasonable explanations were given for these and other observations that still fit these nebulae within the Milky Way, but some astronomers doubted the basic premise and proposed something altogether much larger: that these nebulae were entirely OTHER galaxies far beyond our own.

Harlow Shapley (1885-1971)
Harlow Shapley (1885-1971)

Astronomers took sides and argued. They made observations for and against multiple galaxies. It all came to a head in 1920, when the National Academy of Sciences sponsored a debate on the size of the universe between two pre-eminent astronomers: Harlow Shapley (born 2 November 1885) and Heber Curtis (born 27 June 1872). Shapley believed that the universe was solely the Milky Way and that the Sun was in the outer arms of this giant galaxy. Curtis disagreed, and contended that there are many galaxies, of which the Sun is near the center of a smaller Milky Way.

With the size of the universe on the line, the two men met at the Smithsonian on the 26th of April for what is still rightly known as The Great Debate.

There were fourteen issues under debate, fourteen rounds as it were, with each fighter casting blows against his opponents chiseled arguments. Ok, maybe that’s a bit much, but suffice to say that the debate was extraordinary show of mutual intellectual smackdown. Shapley argued that the rate of novae in the Milky Way to be a few per year. Curtis countered that if this were true, then the novae in these weird nebulae (if in the Milky Way) should only occur once every 500 years (it’s a big sky after all). This was not the case: numerous novae had been observed in the Andromeda “nebulae” in the past 20 years. Curtis landed a haymaker!

Curtis Heber (1872-1942)
Curtis Heber (1872-1942)

Shapley asserted that the idea of the Sun near the center of the galaxy was an observational illusion. Curtis offered little to counter and felt it was God’s truth. Turns out that dust does indeed skew our measured placement in the galaxy, we’re actually in its outer arms. Shapley left hook!

This went on for the whole day and revealed notable questions for discoveries to be made decades later. Case in point: it was noticed that the weird nebulae seemed to be moving farther away. Why was this? Shapley suggested that there might be some repulsive “radiation pressure” in the outer reaches of the galaxy. Curtis thought that the large, positive wavelength shifts (indicative of this away motion), were something intrinsic to these nebulae (and to galaxies in general). These weren’t explanations really. We now know these observations are explained by an expanding universe. Both were knocked out by not enough data!

The aftermath of the debate was presented in dual papers in the Bulletin of the Astronomical Society in 1921 and was in many ways a draw (both Shapley and Curtis claimed victory at some point). There were still two universes, Shapley’s singular large Milky Way, and Curtis’s small Milky Way, one of many galaxies.

Edwin Hubble (born 20 November 1889) settled the matter four years later. In 1925, Hubble presented data of pulsating stars in the Andromeda nebula and unequivocally measured their distance to be far beyond even Shapley’s bloated galaxy. There were galaxies beyond our own; the debate was finally over.

Shapley at Harvard, receiving a letter from Hubble, remarked to Cecilia H. Payne-Gaposchkin, Harvard’s first female PhD student in astronomy, “Here is the letter that destroyed my universe.” There are few who can say this, and mean it.

Don’t cry for Shapley just yet: he was correct that the Sun is nowhere near the center of the galaxy. This insight (and a glittering career as director of the Mt. Wilson Observatory) earned him the Bruce Medal in 1939, among the highest honors in Astronomy.

Sources:

1) For a great and highly readable play-by-play of The Great Debate, check out: Trimble, V. The 1920 Shapley-Curtis Discussion: Background, Issues, and Aftermath. Pub. Astro. Soc. Pac. 1995 December;107:1133-1144.
2) Shapley’s and Curtis’ dual 1921 papers. All in one place. It’s mostly in greek to this biochemist reader.
3) Shapley’s Bruce Medal Citation: Jeffers, H.M., Address of the Retiring Presdient of the Society in Awarding the Bruce Gold Medal to Professor Harlow Shapley. Pub. Astro. Soc. Pac. 1939 April; 51(300): 77-84