Hubble Climbed to the Universe on Shapley’s Ladder
“How Edwin Hubble Expanded the Universe” is a cover story in Astronomy Now magazine. 1/
Good question! How did Edwin Hubble “expand the universe?"
Deborah Shapley is quoted answering this question in a new, clear way in time for the 100th anniversary of Hubble’s great find. The ‘ladder’ Shapley devised to discover the huge size of our galaxy in 1914-1918 was critical to Hubble’s proof there exist galaxies beyond our own - the modern universe.
Hubble laid out his steps in a paper read to fellow astronomers on January 1, 1925. This news hit the astronomy world like a thunderbolt. And, as Hubble continued locating Cepheids and other bright stars at huge distances, using Shapley’s system, the debate over “island universes” ended.
But Hubble could not have done this without Shapley’s effective application of Henrietta Swan Leavitt’s period-luminosity relation and Ejnar Hertzsprung’s 1913 attempt to apply it.
Deborah is quoted likening “Shapley’s deductive steps to the rungs of a ladder: first he created a brightness-distance curve for various Cepheids” including some in globular clusters. Then he compared these with globular clusters having Cepheids and RR Lyrae variables. Then he used RR Lyrae variables in fainter globular clusters to probe further; then he stepped farther using very bright stars in fainter clusters. His final rung used the diameters of the remaining clusters as a proxy for distance; thus his ladder extended to all known globular clusters and mapped the galaxy.
“It was an ingenious mode of attack and one Edwin Hubble would employ to make his breakthrough discovery a few years later,” writes Ron Voller, the author of the article. 2/ Using the new 100-inch telescope atop Mount Wilson, Hubble could scan farther into space than anyone ever had. He used Shapley’s ladder of measurement to verify Cepheids at distances 1 million light years away. He then probed further, to nebulae with very bright stars, and yet farther to using their diameters - the principles Shapley had pioneered.
Hubble depended on Shapley’s feedback, as shown in their correspondence. Once Hubble showed that ‘island universes’ existed, ironically Shapley was the most resistant, the article says. Shapley defined the galaxy he had found as the only universe. He disputed the “island universe” theory during the Great Debate of 1920, also described in the article.
“Shapley resists then bows to the evidence” the article goes on. On this post, Deborah describes his pivot. Her slides tell rest of the story: Shapley made mapping galaxies the core of his research for the rest of his career. Deborah proposed the rungs-ladder analogy to explain Shapley's work and Hubble's reliance on it in "Shapley v Hubble: Rivalry and Dependence" a Cosmos Club lecture in April. Visualization of the rungs will appear on the forthcoming Science page, along with his other breakthroughs such as his discovery of the Fornax and Sculptor galaxies and the galactic halo.
Shapley's Birthday Celebrated Online
November 2 was the anniversary of Shapley's birth in 1885. He was raised on his father’s wheat farm in southwest Missouri. He was taught in a one-room schoolhouse by his sister Lillian. He graduated in astronomy at the University of Missouri (1910 AB, 1911 MS) and Princeton (1914 PhD). Shapley was hired by George Ellery Hale at Mount Wilson Observatory. There he measured distances to globular clusters; these led him to propose a new model of the Milky Way galaxy as a watch-shaped flat disc with a dense center (see Plaskett’s 1935 graphic below). Shapley became a famous lecturer, author and public resister of the McCarthy right wing. From 1953 he promoted the certainty of exoplanetary life.
His Harvard colleague Owen Gingerich wrote that Shapley was revered by many "while even his detractors conceded he was one of the most stimulating figures in twentieth-century science.” 3/
The 139th anniversary of Shapley’s birth was honored on two popular Facebook sites: Starlit Nights by Brian Jones and Friends of HIstorical Observatories by Lawrence Klaes. Some of the images posted are below.
Image credits: Harlow Shapley ca 1940, Frank Hogg, AIP/ESVA; One-Room Schoolhouse, AIP/ESVA; Plaskett Lecture (1935) Fig. 1 Shapley Galaxy Model; Photo Milky Way and Magellanic Clouds, by Trevor Dobson CC BY-NCC-ND.2.O.
Notes
1/ Astronomy Now is “the UK’s biggest astronomy magazine.” It can be read on tablets and phones as part AN’s multimedia hub. Read this article by flipping pages of the November issue at https://shop.astronomynow.com/product/an-november-2024/
All issues are at https://astronomynow.com/magazine/
2/ Ron Voller is the author of The Muleskinner and the Stars (2015), and Hubble, Humason and the Big Bang (2021). His projects are at http://ronvoller.com. For his podcast Bang! Goes the Universe he interviewed Deborah about her grandmother Martha Shapley. See harlowshapley.org.
3/ Gingerich, Owen, Shapley, Harlow, Dictionary of Scientific Biography, Vol. 12, pp 345-352, Scribners, NY, 1975.