Harlow Shapley	Heber Curtis

The 1920 Shapley-Curtis Debate sheds a bright light on the commonly held cosmology of the nineteenth century – a time when astronomers thought very differently about the structure and form of the Universe than we do today. Harlow Shapley and Heber Curtis were two prominent astronomers who had spent their careers studying the Milky Way galaxy.

• Harlow Shapley believed that everything in the universe was located within our own galaxy. He thought that the Galaxy was quite large – 300,000 light-years in diameter – and that the sun was not at the center. Shapley also argued that spiral nebulae (spiral-shaped patches of light observed through telescopes) were nearby clouds of gas located well within the Milky Way.

  • Shapley based his ideas on the size of the Milky Way on observations of globular star clusters. The distance to one prominent globular cluster, M13, had been determined. Shapley assumed that all globular clusters were roughly the same size, and so used their apparent size on the sky to determine their actual distances (the more distant an object is, the smaller it will appear to us). He found that the globular clusters formed a loose halo around the flat, disk-shaped body of the rest of the galaxy. In the same way, he determined that the galaxy was 300,000 light-years in diameter and that the sun was located 50,000 light-years away from the center.

    This agrees roughly with our current view of the Galaxy: the Milky Way has a diameter of 100,000 light-years, and the Sun lies 25,000 light-years away from the center, in the disk component.

  • Shapley did not believe that spiral nebulae lay beyond the Milky Way. He thought that they were located within the boundaries of the Galaxy. One popular theory was that they were late-forming solar systems, in the process of formation.

    In 1908, Harvard astronomer Henrietta Swan Leavitt had studied photographs of the Magellanic Clouds (the two galaxies closest to the Milky Way). They showed that the Magellanic Clouds were made up of clumps of individual stars, some of which were Cepheid variable stars. Cepheid variables are a type of star which changes its luminosity regularly, pulsing from faint (and cool) to bright (and hot) over a period of one to one hundred days. Repeated observations of such a star will show it waxing and waning in brightness, as shown in the light curve below.

    [NMSU, N. Vogt]

    Leavitt found that images of the Cepheids in the Magellanic clouds showed a distinct pattern: the longer the period of the luminosity cycle, the brighter the star. Since all of the Cepheid stars in the Magellanic clouds lay together at the same distance from the Earth, this meant that the longer period Cepheids were intrinsically more luminous stars.

    [NMSU, N. Vogt]

    The trick then became to determine the distance to one (any one!) Cepheid variable star, in order to set a zero point to this relation between intrinsic luminosity and period. With this known, the true distance to any Cepheid could be determined: measure the period of the luminosity variation, observe the apparent luminosity, and calculate the distance at which the star must lie in order for it to appear this bright. The problem was that there were no Cepheid stars near enough to the Earth that parallax could be used to determine their distances. Shapley calibrated the period-luminosity relationship for Cepheids in a rough fashion, and estimated the distance to the Magellanic Clouds. He found that the distance was less than the diameter of the Milky Way, and concluded that extragalactic nebulae were part of the Milky Way.

    Shapley had additional evidence that spiral nebulae were nearby objects. Adriaan van Maanen had measured a rotational speed of 0.02 seconds of arc per year for M101; coupled with an angular size (extent) on the sky of half a degree, this meant that M101 would have had to rotate faster than the speed of light if it lay beyond the Milky Way! Furthermore, the high recessional velocities of spiral nebulae implied that they must be close enough to the Milky Way to be influenced by it.

    Spiral nebulae are now know to lie far beyond the Galaxy. Maanen's measurements of proper motion for M101 were shown later to be simply incorrect, when re-evaluated by other astronomers. The high recessional velocities are now interpreted as support for an expanding universe, where all galaxies are racing away from each other as space itself expands.

• Heber Curtis believed the sun was nearly in the center of a much smaller galaxy – only 30,000 light years in diameter. He argued that the spiral nebulae were island universes – external galaxies located beyond the Milky Way.

  • Curtis agreed with Shapley that the globular clusters fell outside the disk of the Galaxy, but he did not agree with Shapley's estimates of the actual distances to the clusters. Curtis held that they were much closer, and that the Galaxy itself was thus much smaller.

  • Curtis set forth the theory that spiral nebulae were objects similar in form to the Milky Way. He showed evidence that the optical spectrum of a spiral nebula was indistinguishable from the spectrum of the Galaxy. They held a similar appearance, with a central belt of obscuring material (the galaxy disk). Curtis believed that these objects were large collections of stars, comparable in size and located far beyond the Milky Way's boundaries.

    Curtis was correct to claim that spiral nebulae are separate from the Milky Way. The question was settled in 1923 when Edwin Hubble, using the new Mt. Wilson 100-inch telescope, detected Cepheid variables in the great nebulae in Andromeda (M31). Using Shapley's calibration of the period-luminosity relation, Hubble concluded that M31 was at a distance of 1.2 million light-years – far beyond the outer limits of the Milky Way. As observations accumulated, it gradually became clear which fuzzy patches belonged to the Galaxy, and which were completely separate groups of stars. The spiral nebulae became known as extragalactic nebulae, then as island universes, and finally just as other galaxies.

  The Mount Wilson 100-inch telescope	The Andromeda Galaxy (M31), shown next to the Moon for scale. [NOAO/AURA/NSF]

• Who was right, and who was wrong? The answer is that both men were correct on some points, and both men were in error on some points. The elusive spiral nebulae were shown to be external galaxies, as Curtis had argued. However, Shapley was right that the Sun lies far from the center of the Milky Way, and his estimate of the total size was much closer to the currently accepted value.