The Story of How the Universe's Size was Determined

It was into this fiery climate of the 1920s that the Protestant-raised Hubble, adorned with the cape, cane, and British accent he had adopted while at Oxford, returned after the war. He arrived at the Carnegie Institution of Washington-funded Mount Wilson Observatory outside Pasadena, California, insisting on being called "Major Hubble."^'' Looking through the great Hooker telescope—at one hundred and one inches in diameter and weighing more than one hundred tons it was by far the largest and most powerful scientific instrument in the world—Hubble was able to view the universe with a light-gathering capacity of more than two hundred thousand human eyes.

What he saw changed humanity's view of the universe forever—and would further roil the controversy over science's role in defining the origins of creation. A conservative Baconian observer, Hubble p photographed a small blinking star in the Andromeda nebula that he identntified as a Cepheid variable. This observation would become iconic in its poower.

had in 1912 shown somethina remarkable about Ceoheid vari^^^- -*—had in 1912 shown something remarkable about Cepheid variable stars: The longer their period, the brighter they appeared to be. This made sense. Stars were very faint, and to probe deeply, astronomers began to mount cameras to their telescopes and take very-long-exposure photographs on glass plates so they could capture light from stars that were too faint to see with the human eye. A blinking star that has a longer "on" period deposits more light on the plate than one with a shorter period.

Leavitt's brilliant observation was so selt-evident as to be irrefutable, but no one had noticed it before. The accomplishment was all the more remarkable because Leavitt was not allowed to be part of the scientific staff; she was a "computer"—one of several women hired merely to identify and catalog stars and calculate light curves for the male scientists. She was paid a premium rate of thirty cents per hour, over the usual two bits, because of the high quality of her work.

Danish astronomer Ejnar Hertzsprung seized upon Leavitt's discovery to create a means of measuring intergalactic distances. Using inductive reasoning, Hertzsprung determined in 1913 that if two Cepheid variable stars had similar periods but one was dimmer than the other, it was probably farther away. He found Cepheids close enough to Earth to measure the distance to them using statistical parallax and was able to compipare this with their apparent brightness (their brightness as observed from Earth). The resulting formula allowed scientists to measure the distances to all Cepheid variables. These blinking stars became "standard candles' for measuring distance throughout the heavens.

This was an immense discovery, and in 1915 American astronomer Harlow Shapley, a liberal Democrat, used it and Mount Wilson's sixtyinch telescope to map the Milky Way in three dimensions. Shapley's leasurements expanded the known size of the Milky Way severalfold d showed that the sun was not at the center of the galaxy, as had been thought, but was in fact located in a distant outer arm. This overturned the concept of the centrality of humans yet again, and Shapley was celebrated as the greatest astronomer since Copernicus—a title he tiimself helped promote—for having achieved the "over-throw" of the heliocentric universe

In what would become an important cautionary tale for American science—and, by extension, democracy and much of what ails Amerian politics today—Shapley became blinded to reality by his belief that the Milky Way was the entire universe—or maybe his hubris in wanting to believe that he had mapped the whole shebang. He argued that the spiral nebulae seen in the heavens were simply wisps of gas and clouds of dust within the Milky Way, rather than entire "island universes"—galaxies—of their own, as fellow astronomer Heber Cur:is posited. He debated this point with Curtis at a meeting of the National Academy of Sciences in April of 1920, in an event famously called the Great Debate.

The debate ended in a draw because there wasn't yet enough observational data to draw firm conclusions, but this was partly because Shapley, without realizing it, had stopped using Locke's and Bacon s inductive reasoning to build knowledge from observation and was instead trying to prove his point with a rhetorical argument—an a priori, top-down, Cartesian approach of first principles that had him arguing more like an attorney than a scientist. When his assistant Milton Humason showed Shapley a photographic plate that seemed to indicate the presence of a Cepheid variable in Andromeda, Shapley shook his head and said it wasn't possible. Humason had unimpressive formal credentials—he had been elevated to assistant from a lowly mule driver and had but an eighth-grade education—but it was Shapley's a priori idea that occluded his vision. He took out his handkerchief and wiped the glass plate clean of Humason's grease pencil marks before handing it back.^^ No one realized it at the time, but Shapley's career as a major scientist ended at that moment.

Soon after, Shapley moved on to run the Harvard College Observatory while his rival Edwin Hubble took over the telescope. Adopting th uncredentialed but nevertheless brilliant Humason as his assistant and conservatively adhering to a strict Baconian observational methodology. Hubble identified Cepheids in Andromeda and used them to show that the spiral "nebula," as he called the galaxy, was not part of the Milky Way at all; it was in fact nearly a million light-years distant—more than three times farther away than the diameter of Shapley's entire known universe. The Great Debate was settled, and Hubble became an overnight sensation.

Includes a cautionary tale of Shapely, who helped prove the Sun was not the center of the Universe, but who thought the Milky Way was all the Universe there was without empirical data.