The Day We Found the Universe - Marcia Bartusiak [123]
Hubble did not really “discover” this relation but rather demonstrated an effect already suspected, with data that at last convinced his fellow astronomers. In previous attempts the plotted measurements looked like scattershot across the page. But on Hubble's graph, even though there was still some scatter, the galaxies lined up far more tightly. The sure straight line he was able to draw through his points, a diagram that is now an honored icon in cosmology textbooks, gave everyone confidence in the results.
Hubble actually carried out two separate computations. In one, he calculated his rate of recession using all twenty-four of his galaxies. In the second approach, he figured out a rate of recession when he combined the galaxies into nine groups, according to their distance and direction on the sky. Both methods led to similar outcomes. “For such scanty material, so poorly distributed, the results are fairly definite,” concluded Hubble, almost with surprise.
In a clever move, Hubble didn't include on his historic graph the strongest bit of evidence then on hand. He left that for Humason to convey in a separate paper opportunely placed right before his in the Proceedings of the National Academy of Sciences. Humason's result was the attention grabber, setting up Hubble's paper, published under his name alone, for the fait accompli. After cutting his teeth on a few of Slipher's galaxies, Humason had gone after a fainter, previously unmeasured target, as Hubble directed. It was the galaxy NGC 7619 in the Pegasus constellation. “I agreed to try one exposure,” recalled Humason, who wanted to see whether it was even possible to venture farther out than Slipher. That exposure extended over a few nights, the sparse photons from the dim galaxy hitting the plate for a total of thirty-three hours. It was a lonely enterprise. Humason usually worked within the 100-inch dome with only the light of a tiny red bulb as company. For hours he would keep two crossed hairs—the guiding wires—smack-dab on his galaxy, a smudge of light barely visible through the barrel of the telescope. All this despite the fact, as one observer put it, “that the mountain itself is rolling eastward with the earth at ten times an express train's speed.” For assurance, Humason went back and did the measurement again, this time for forty-five hours.
Edwin Hubble's famous 1929 velocity-distance graph for a sample
of galaxies out to 2 million parsecs (∼6 million light-years),
evidence for what later came to be understood as an expanding
universe (From Proceedings of the National Academy of Sciences 15
[1929]: 172, Figure 1)
But making these observations was only the beginning. Back in the office, the spectrum on each photographic plate had to be put under a microscope and the shift of the spectral lines carefully measured. Humason often used the lines generated by calcium atoms glowing within the galaxy, which stood out sharply. Given Humason's lack of training in mathematics, though, he'd then take his measurements to an observatory “computer” (a person with a slide rule or adding machine), who used a formula to convert the physical measurement into a galaxy velocity. In the case of NGC 7619, Elizabeth Mac-Cormack calculated a final velocity of 3,779 kilometers per second, more than twice Slipher's highest velocity. The success spurred Mount Wilson officials to get Humason a better and faster spectrograph, appreciably reducing his bone-wearying exposure times, and the Kodak company was