Cepheid: A type of variable star used to determine distance. Shapley used a relation between the period of variation of this type of star and its absolute magnitude to calibrate distances to globular clusters during the `Great Debate.' Curtis disputed these distances. Cepheids are used today as reliable distance estimators.
Curtis: Heber D. Curtis. Participant in the `Great Debate' in 1920. Curtis argued that our Galaxy was small, the sun was near the center, and spiral nebulae were other galaxies similar to our own. Both Curtis and Shapley were incorrect on the first point (Curtis guessed too small, Shapley too large), and Curtis was also incorrect on the second point. That Curtis was correct in the last point has changed mankind's view of our place in the universe.
galaxy: Before the resolution of the `Great Debate', this referred to our own Milky Way galaxy. Since the resolution of the `Great Debate', it is taken to mean a large group of stars seen external to our galaxy. Galaxies typically have about 10^11 stars. We also now capitalize Galaxy when referring to our own Milky Way galaxy.
globular cluster: A cluster of about 1 million stars. The distances to globular clusters was debated in 1920. Eventually the distribution on the sky of globular clusters as well as their distance was taken as strong evidence that our Sun is not at the center of our Galaxy.
`Great Debate': The debate between Curtis and Shapley in 1920. The term is sometimes taken to mean the general disagreement between Curtis and Shapley on the nature of our Galaxy and the nature of spiral nebulae, instead of the actual physical debate.
island universe: Before the `Great Debate', this term meant "galaxy." This term was no longer used after the resolution of the `Great Debate.'
Milky Way: The galaxy which encompasses our Sun and solar system. In the `Great Debate' it usually referred to the band of light that runs across the sky and contains most of the visible stars. Today this term is synonymous with the whole of our Galaxy.
magnitude: The brightness of an object on a logarithmic scale. "Apparent magnitude" related to how bright an object appears to be, and "absolute magnitude" relates to how bright an object actually is.
nebulae: Before the `Great Debate', this term referred to any resolved nebulosity observed in the sky. It had also been noticed that a there were different types of nebulae (singular: nebula), one of which had spiral structure and so were referred to as "spiral nebulae." Today "spiral nebulae" are referred to as "spiral galaxies" and "nebulae" refer to large gas and dust clouds that populate a galaxy.
Shapley: Harlow Shapley. Participant in the `Great Debate' in 1920. Shapley argued that our Galaxy was large, our Sun was offset greatly from the center, and he did not endorse spiral nebulae as external galaxies. Both Shapley and Curtis were incorrect on the first point (Shapley guessed too large, Curtis too small) and Shapley was also incorrect on the last point. That Shapley was correct in the second point has changed mankind's view of our place in the universe.
spiral nebulae: This term was used only before the resolution of the `Great Debate.' It referred to resolved nebulous objects seen on the sky that appeared to have spiral structure. Their uniform appearance and locations only out of the plane of the Milky Way caused them to be one of the focal points of the `Great Debate', particularly for Curtis. With the resolution of the `Great Debate' came the understanding that these objects are known as spiral galaxies, and are known to be vast collections of stars not unlike our home Galaxy.
star: A massive ball of gas fusing elements together in its core.
Universe: All that is. In 1920 this term was used also to describe the Milky Way stellar system. With the resolution of the Curtis - Shapley debate came the first generally accepted understanding that our Galaxy was not the only galaxy in the cosmos, so that the universe contains more than the Milky Way stellar system.
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