Tycho’s Nova, a well-known supernova remnant caused by a Type Ia supernova. Now two groups of astronomers are at odds over the mysterious nature of Supernova 2005E: Was it a Type Ia, Type II or something entirely different? (NASA)

News comes to us across the pond from the BBC that there’s a bit of a scientific kerfuffle at the moment concerning possible explanations for what could turn out to be a new kind of supernova… or not. Two teams take different sides in the journal Nature.

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The team from Harvard Smithsonian Center for Astrophysics maintains that features of the so-called Supernova 2005E — first observed by telescopes in 2005 — don’t match the established criteria for designating it a Type Ia (caused when a white dwarf star or the thermonuclear explosion of an old dead star) or Type II supernova (caused when a young massive star runs out of fuel and implodes). They conclude that we are seeing a new type of supernova. But a team from Japan’s Hiroshima University begs to differ, based on their analysis of a supernova with similar properties (SN 2005cz), arguing that 2005E is more likely to have been caused by the core-collapse scenario.

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Who’s right? We’ll just have to wait for more experimental evidence and analysis to find out. But such debates are nothing new. In fact, this new debate over SN2005E reminded me of one of the most famous academic clashes in astrophysics history: The Shapley-Curtis Debate, a.k.a., “The Great Debate.”

It happened in April 1920, at the annual meeting of the National Academy of Sciences, and it centered in the nature of the spiral nebulae and the size of the universe. The key players were Heber D Curtis, a well-established authority on spiral nebulae, and a promising young (and ambitious!) upstart named Harlow Shapley. This was before Edwin Hubble’s groundbreaking work (which helped resolve the dispute), so it was very much an open question at the time. Wikipedia nicely sums up the gist of the controversy:

Shapley was arguing in favor of the Milky Way as the entirety of the universe. He believed galaxies such as Andromeda and the Spiral Nebulae were simply part of the Milky Way. He could back up this claim by citing relative sizes—if Andromeda was not part of the Milky Way, then its distance must have been on the order of 108 light years—a span most astronomers would not accept. … Curtis … contended that Andromeda and other such nebulae were separate galaxies, or “Island universes”. He showed that there were more novae in Andromeda than in the Milky Way. From this he could ask why there were more novae in one small section of the galaxy than the others. This led to supporting Andromeda as a separate galaxy with its own signature age and rate of novae occurrences. He also cited dark lanes present in other galaxies similar to the dust clouds found in our own galaxy and massive Doppler shifts found in other galaxies.

Shapely and Curtis decided to settle the matter once and for all in a debate held at the Natural History Museum in Washington, DC. According to historical documents on NASA’s Website, the idea of a debate on this subject was proposed by George Ellery Hale, and enthusiastically embraced by Curtis. He was the more senior researcher, and known for his skill at public presentation and effectively arguing his case.

Shapley knew of his rival’s reputation, and was understandably more reluctant to accept the challenge — particularly since, despite his relative youth (mid-30s), he was being considered as a possible director for the Harvard College Observatory to succeed the late Edward C. Pickering. He just didn’t want to risk such a public routing.

Eventually Shapley agreed, and the rest is history. Although by modern standards, it was an odd sort of debate: each man gave a 40-minute presentation and each was allowed one short rebuttal to the others remarks, followed by discussion from the assembled audience of scientists. And according to science historian Michael Hoskin, the published papers of that debate by both men — appearing in the May 1921 Bulletin of the National Research Council — don’t accurately reflect what was actually said at the debate itself. But they do nicely summarize the evidence for each argument, based on the scientific evidence available at the time.

So, who won? Well, it depends. If we’re scoring just on debating skills, Curtis emerged victorious, certainly by his own estimation: in a May 15 letter ti his family, he wrote, “Debate went off fine in Washington and I have been assured that I came out considerably in front.” Several colleagues who were present agreed. And Shapley did not, alas, win the coveted directorship.

But if we’re talking about who won on the scientific merits, the winner is less clear. A few years later, Hubble identified Cepheid variable stars in the Andromeda Galaxy, and he used that to show that the galaxy was even further away than Shapley had first proposed. Score one point for Shapley. And in the 1930s, astronomers discovered the effect of interstellar absorption, which led them to accept that the Milky Way was much larger than previously estimated, and also that the sun was not near its center. Also, better telescope resolutions revealed that there were many other galaxies beyond the Milky Way, and that the spiral nebulae were, in fact, galaxies. Score one point for Curtis. They were both wrong in concluding that interstellar absorption wasn’t important for determining a galaxy’s size, however.

So it was draw, and a very respectable one at that. No doubt resolving the current question of SN 2005E will turn out to be equally nuanced. That’s scientific life at the cutting edge!