The Believing Brain - Michael Shermer [169]
In a marathon light-gathering night in September 1912, Slipher imaged Andromeda for 13.5 hours. The spectrographic plate revealed that there was a displacement of the spectral lines toward the blue end of the spectrum.17 By now astronomers had determined that the shifting of spectral lines toward the blue meant that an object is moving toward us, and if shifted toward the red the object is receding away from us. This is the so-called Doppler effect, discovered by the Austrian physicist Christian Doppler, who noted that waves of light moving toward an observer will be squashed and therefore shifted toward the higher-frequency blue end of the spectrum, and if moving away the waves will be stretched and therefore shifted toward the lower-frequency red end of the spectrum. Andromeda was blue shifted. Really blue shifted—to the tune of three hundred kilometers per second by Slipher’s calculations, which put Andromeda astronomically far beyond the range of motion ever measured of individual stars. How could an object moving this fast be located within the Milky Way?
Additional spectral shifts confirmed Slipher’s initial finding. Nebula M81 was measured at one thousand kilometers per second—three times the speed of Andromeda—and it was moving away from us. By 1914, Slipher had more than a dozen nebular speeds, all within the range measured for Andromeda and M81—about twenty-five times faster than the average stellar velocity—and most receding from us. With these speeds, and the estimated size of the Milky Way, it seemed clear to many astronomers that these nebulae could not be within the Milky Way. The island universe theory was gaining momentum, and the seeds of the expanding universe theory were being sown.
What was needed to close out the debate was a reliable distance measurement, which was created in the early 1900s by Henrietta Swan Leavitt at Harvard, who began her career as a volunteer and worked her way up to being a “computer”—a woman who calculated figures for the all-male staff astronomers. She finally carved out a milestone career in astronomy for her work on Cepheid variable stars, which became the standard distance-measurement objects that Hubble noted on his photographic plate in 1923. Cepheid variables—named for the specimen discovered in the Cepheus the King constellation—vary in brightness over the course of days, weeks, or months, and they do so in a highly predictable manner: the brighter the variable the longer its period. Since Leavitt discovered these Cepheids in the Small Magellanic Cloud—those glowing patches in the southern sky first noted by Ferdinand Magellan during his circumnavigation of the globe—it meant that all the stars within that satellite galaxy were the same distance from us. Their periodicity was a direct measurement of their real luminosity and not an effect of varying distances.
Cepheid variables became the “standard candle” of light-distance measurement. If you have a particular type of candle for which all flames are the same size and brightness, and you discover some to be half as bright or a quarter as bright or an eighth as bright as the standard candle nearby, you can reasonably infer that they are two, four, or eight times as far away. Once the distance to a Cepheid variable could be reliably established through such tried-and-true methods as parallax (how much the background stars shift behind the target stars when comparing images taken from one side of Earth’s orbit to those taken from the other side six months later), then finding Cepheids in nebulae that are X times dimmer means that they are X times farther away. If Cepheid variables could be found inside nebulae at distances much greater than the size of the Milky Way, that would confirm that these stars are located in nebulae well outside of our galaxy and validate the island universe theory.
The “Big Galaxy” Hypothesis and the Mysterious Rotating Nebulae
There was one more line of evidence against the