This article on Curtis-Shapely debate is a guest article by Ariana Vlad, senior at the International Computers High School of Bucharest, Romania, where she focuses on studying Physics and Mathematics.

A Historical Debate

26 April 1920. The opulent Baird Auditorium in the Smithsonian Museum of Natural History, the beginning of the Curtis-Shapley debate. On one side, the young, aspiring scientist and former journalist Harlow Shapley, ready to make good use of his energy and enthusiasm. On the other side, the world-renowned professor Heber Curtis, known for his oratory skills, fearsome motivation, and comprehensive work on spiral nebulae for over a decade. Their knowledge, thoughts, and ideas clashed against one another, focused on one topic: the extent and boundaries of our (lesser known at the time) Milky Way. How

Curtis-Shapley debate
Shapley (left) and Curtis (right)

The Great Debate was a day-long event: the morning started with each of the scientists’ speeches about their own technical papers, and the actual debate took place later in the evening. Due to lack of proof (as breakthroughs and measurements were not happening as often and as easy as today), the Curtis-Shapley debate wasn’t settled that same day. 

Shapley’s Claims

Shapley believed that the Milky Way contained all the known and unknown celestial objects – that it represented the whole Universe. He argued that galaxies like Andromeda were simply part of our own galaxy. He backed up his claim with a dimensional argument. Looking into former studies, he calculated that if Andromeda were not in the Milky Way, then it should be as big as 100,000,000 light-years. While today’s astronomers are not baffled by such a span, in 1920 that was believed to represent a good argument.

Andromeda galaxy and Curtis-Shapley debate
Andromeda galaxy in night sky

Shapley was also supported by a fellow scientist – Adriaan van Maanen. Van Maanen reported an independent study about the Pinwheel galaxy. His measurements and calculations showed that the celestial object was rotating on a timescale of years. If the Pinwheel galaxy were farther than the Milky Way border and rotating with such a great angular velocity around Earth observers, it would need to have a speed greater than the speed of light. Since van Maanen was a treasured astronomer and it was already known that no object can go over the threshold speed of light, this argument had a great value in this debate. 

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Curtis’ Claims

In Curtis’ vision, Milky Way and Andromeda were independent galaxies, just two of many such bodies. His argument focused mainly on astronomical events called novas, transient phenomena that represent a sudden appearance of a bright, star-like object that fades over several weeks or a few months. For a nova to be created, some exceptional circumstances have to be met. For example, the event will only appear near a white dwarf in a close binary system.

Nova and Curtis' claims
Illustration of a nova (Photograph: K. Ulaczyk / Warsaw University Observatory)

Therefore, one could conclude that by looking into two independent regions in space, one could tell if they have a common past by measuring the number of novas in a given (yet long enough) time. Curtis used this idea, recorded the rate of nova occurrences, and found that there were move novas in Andromeda than in the Milky Way. While he strongly believed in his argument, he noted that if van Maanen’s measurements and observations were correct, Shapley’s assumptions proved correct. 

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Conclusion of the Curtis-Shapley debate

For today’s science enthusiasts, there is no mystery around who is the real “winner” of the debate. That same decade, Edwin Hubble’s work on Cepheid variable stars proved that the Andromeda galaxy lies outside of the Milky Way – the first proof for Curtis’ claim, and an irrevocable one. Thanks to the same astronomer, it is now known that the Milky Way and Andromeda are just two of the more than 2 trillion estimated galaxies. 

The Uniqueness of the Curtis-Shapley debate

The Curtis-Shapley debate is a unique one in the whole scientific history for two reasons. Firstly, not many such debates sparked since the 1920s, as astronomy became more of a collaborative science; contemporary, ideas can be easily shared, measurements are usually precise, and every article is proofread before it becomes public. Secondly, the topic of the debate – and its result – changed the way scientists viewed the cosmos. After Curtis’ victory, astronomers plucked the courage to broaden their horizons and move their observations and measurements farther than the rim of the Milky Way. 

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