The Day We Found the Universe - Marcia Bartusiak [44]
Percival Lowell's 1909 letter directing Vesto Slipher to get
a white nebula's spectrum (Lowell Observatory Archives)
It began innocently enough. On February 8, 1909, Lowell in Boston sent a typed letter to Slipher with concise instructions: “Dear Mr. Slipher, I would like to have you take with your red sensitive plates the spectrum of a white nebula—preferably one that has marked centres of condensation.” By “white,” Lowell meant a spiral nebula, which in 1909 was still generally understood to be a new planetary system under construction. In a handwritten footnote at the bottom of his note, Lowell stressed that he wanted “its outer parts.” He longed to see if the chemical elements found at a spiral nebula's edge, as revealed by the fingerprints of its spectral lines, matched the composition of the giant planets situated far from our solar system's center. A connection would mean the spirals could indeed be baby solar systems under way.
Slipher balked at first. “I do not see much hope of our getting the spectrum of a white nebula,” he told Lowell. He knew that it would take at least thirty hours to get just a plain old photograph of the nebula with the observatory's 24-inch telescope. Nebulae were extremely faint through its lens. To get a spectrum, with far less light hitting the photographic plate after its passage through the spectrograph, seemed impossible.
But Slipher had something to prove. Campbell at the Lick Observatory had recently written yet another article critical of the Lowell Observatory. It was the latest volley in the observatories' ongoing war over whose refractor could get the better results. Lowell had earlier asserted that the superior air on Mars Hill allowed his 24-inch refractor to see 173 stars in a given field of the sky, where Lick's 36-incher could see only 161. Slipher, deeply loyal to his astronomical home, wanted to settle the matter once and for all. He was eager to set up a challenge between the two observatories, comparing photos of stars taken at the same time on similar plates, but Lowell nixed the idea. To reclaim some honor, Slipher decided to focus on the difficult task of getting the spiral nebula spectrum. “I have come to the conclusion,” he had written John A. Miller, his former astronomy teacher at Indiana, just a few months earlier, “that where we can defend ourselves…we shall have to do it or otherwise everything we publish will be discredited.”
Though Slipher considered the spectral task hopeless, he persisted and by December 1910 was able to wrench some feeble data from the Great Nebula in Andromeda. “This plate of mine,” he informed Lowell by letter, “seems to me to show faintly peculiarities not commented upon.” He was going to say “to show faintly, perhaps” but had scrawled out the “perhaps.” He was now convinced he had captured something on the spectrum previously unseen by other spectroscopists, such as Scheiner in the 1890s.
By trial and error, coupled with an astute technical mind, Slipher started making improvements to the spectrograph. Instead of using a set of three prisms, which better separated the spectral lines, he decided to use just one. Though this made the spectrum more congested and difficult to read, it vastly increased the amount of light available since there was less glass to absorb the incoming photons. More important, he understood that increasing the speed of the camera was vital and so bought a very fast, commercially available camera lens. The entire spectrograph, which included counterweights to keep the telescope balanced, weighed 450 pounds and resembled an oversize nutcracker attached to the bottom of the telescope. The celestial light, instead of going into the eyepiece, was directed to the prism, which separated the beam into its constituent wavelengths. A small photographic plate was suitably positioned to record the spectral lineup, from red to violet.
Planet studies, reports on the return of Halley's Comet, and administrative duties diverted Slipher's attention for a while. He could not get back to the question of the spiral nebula