In the early 20th century, astronomers marvelled at the abundant and beautiful “spiral nebulae” that flecked the night sky. But they had few clues about the nature of these lovely objects. Were they new solar systems forming within our own Milky Way? Or were they galaxies in their own right, separated from our own galaxy by immense distances?
A hundred years ago, no one knew. But a diligent and unassuming Indiana farm boy made a fundamental discovery that hinted at the nature of the spiral nebulae, and laid the framework for understanding the nature of our universe.
Like many distinguished American astronomers, Vesto Slipher grew up under the inky-dark skies of the rural midwestern U.S. in the late 19th century. One of eleven children, Slipher had a skill for numbers and a head for mechanics. And he had the good fortune and work ethic to advance himself. In 1896 at the age of 21, Slipher headed to Indiana University to take a degree in astronomy and mechanical engineering.
In 1901, Slipher took a position at the new Lowell Observatory near Flagstaff, Arizona. Percival Lowell, the observatory’s founder, promised Slipher’s professor to give the young man a short-term position. But Lowell and Slipher worked well together, with the quiet and unassuming Slipher happy to leave the limelight to the wealthy and flamboyant Lowell. Slipher remained at Lowell observatory for his entire 50-year career.
After arriving in Arizona, Slipher was set to work with the observatory’s spectrograph, an instrument which separated and analyzed the visible wavelengths of light from celestial objects. He struggled for a year with the unfamiliar instrumentation, once even confusing the red and blue ends of the spectrum, which was a very fundamental mistake.
But he kept at it, became expert with the instruments, and learned to take excellent spectra of planetary atmospheres. Slipher measured the rotation periods of the outer planets, detected matter in the space between stars, and discovered a thin layer of sodium atoms in the Earth’s upper atmosphere. Not bad for a young lad just off the farm.
Then at Lowell’s request, Slipher turned the spectrograph to the “spiral nebulae” (which we now call spiral galaxies). A hundred years ago, no one knew the nature of these spiral clouds. Some astronomers believed they were distant “island universes” like our own Milky Way. Others, including Lowell, believed they were spirals of gas within our own galaxy that were coalescing into new solar systems. Lowell directed Slipher to measure the spectra of the light from the outer edges of the spiral nebulae to see if the chemical makeup resembled the outer planets of our own solar system.
It was a tough job, but Slipher used his mechanical acumen to increase the sensitivity of the complex spectrograph, a 450-lb instrument attached to the bottom of the observatory’s 24-inch refractor. By the end of 1912, Slipher recorded a good spectrum of the Andromeda “Nebula” on a photographic plate.
What he found changed our view of the cosmos forever.
Slipher’s spectrum showed the light from Andromeda was “blueshifted”, which meant its light waves were compressed by the Doppler effect to appear more “blue” as a consequence of its motion towards our solar system. A quick calculation revealed that Andromeda was approaching us at 300 km/s (about a million kilometres per hour), far faster than any other object in the Milky Way. This strongly suggested the nebula was not within our galaxy.
Over the next few years, Slipher measured two dozen more spiral “nebulae” and found they were all travelling a fantastic speeds. Some moved towards the Milky Way, but most galaxies showed redshifted spectra which meant they were moving away from us. The “nebula” known as M104, for example, was flying away at nearly 1000 km/s, an astonishing speed.
Slipher presented his results of the speed of 15 galaxies to the American Astronomical Society in 1914, and received a standing ovation (a very rare occurrence at a scientific conference, I can assure you).
But what did Slipher’s results mean? Why were these nebulae flying away at such amazing speeds? It was left to another Midwesterner, an aristocratic and movie-star-handsome former lawyer, to take Slipher’s results and make the greatest astronomical discovery of the last century.
(Come back next week for Part 2 of the story of this astonishing discovery…)