A few weeks ago you had a look at the justly famous Sombrero Galaxy in the constellation Virgo. In a small telescope, the galaxy looks like a silver shard, a faint sliver of light with a star-like core obscured from end to end by a sharply defined lane of dust. It’s one of the prettiest galaxies in the night sky, and it was also a favorite of the Indiana-born astronomer Vesto Slipher, who paused to admire it from time to time as he analyzed its light to make one of the great discoveries of the 20th century. Let’s examine a few more photogenic galaxies on Slipher’s observing list from more than a century ago, and understand just what it was that he unexpectedly discovered.
One of eleven children born on an Indiana farm, Vesto Slipher graduated with an astronomy degree and went to work at Lowell Observatory, near Flagstaff, in the Arizona Territory, in 1901. The observatory was founded by Percival Lowell, the black sheep of a famous and wealthy Massachusetts family that included the poet Amy Lowell, his sister, and his brother Abbot Lawrence Lowell, a president of Harvard University. ‘Percy’ Lowell was famous for his passionate interest in the planet Mars, and he commissioned the master optician Alvan Clark to build the observatory’s 24-inch refractor for the study of Mars and the other planets. A modest and hardworking Midwesterner, Slipher got along well with the blue-blood Lowell and, although he was initially employed on a short contract, Slipher was kept on and remained at the observatory for his entire 50-year career.
After arriving in Arizona, Slipher set to work with the observatory’s spectrograph, an instrument to separate and analyze the visible wavelengths of light from celestial objects. He was overwhelmed at first with the professional instrumentation– his work in Indiana mostly involved a tiny 4-inch reflector. Slipher struggled for a year with the 450-pound spectrograph, which was mounted on the end of the long tube of the 24-inch refractor. He once even confused the red and blue ends of the spectrum, a very fundamental mistake.
But he eventually got the hang of it and embarked on a long series of scientific discoveries with his spectrograph that included the rotation periods of the outer planets, the discovery of matter in the space between stars, and the discovery of a thin layer of sodium atoms in the Earth’s upper atmosphere that’s now used by astronomers to produce artificial guide stars using powerful lasers.
Like most good hands-on scientists, Slipher learned how to tease more out of his instrumentation, and at Lowell’s behest he eventually turned the telescope and spectrograph from the bright planets to the relatively faint “white nebulae”, objects which we now call galaxies but which in the early 20th-century were still a mystery.
Getting a spectroscopic measurement of these galaxies was no easy task, however. If you’ve ever looked at a galaxy through a telescope, even a prominent galaxy like the Sombrero, you’ve noticed they are quite dim. And while Slipher had a relatively big 24-inch scope to collect light, it was not the ideal telescope for such work, and much of the light was lost as it passed through the optics of the spectrograph. The remaining light was collected by the relatively insensitive photographic plates of the day. Getting a single spectrum of a galaxy took dozens of hours over many nights of painstaking work.
Despite the challenges, Slipher got results with a year after starting. He started with the Andromeda Nebula, M31, the brightest “white nebula”, and by the end of 1910 he found a shift of hydrogen absorption lines in its spectrum that suggested it was moving towards us at about 200 km/s. He found a similar result with the Triangulum Galaxy, M33.
Then he moved to fainter galaxies, and his list of targets reads like the spring observing list of a modern weekend stargazer with a humble 8″ Dobsonian: the edge-on spiral “nebulae” like the Sombrero (M104), NGC 3115, NGC 4565, and NGC 5866, as well as bright and attractive face-on spirals like M81 and M94. If you’re even a moderately serious stargazer, you’ve probably seen them all.
On the other side of the plane of the Milky Way, in the autumn months, he examined Andromeda as well as NGC 1023 and NGC 7331 (the “Little Andromeda” galaxy).
From their spectra, Slipher discovered that many of these galaxies were travelling at fantastic speeds. Some moved towards the Milky Way, but most showed redshifted spectra, which meant they were moving away from us. The Sombrero Galaxy, as mentioned, and the spindle-shaped NGC 4565 were flying away at 1000 km/s or more, more than 25x the average speed of nearby stars in our own galaxy. Slipher’s results strongly suggested these galaxies were not part of our own Milky Way. They simply moved too fast.
Slipher presented his results of the speed of 15 galaxies to the American Astronomical Society in 1914, and received a standing ovation.
Though no one understood it at the time, Slipher’s discovery of receding galaxies had more profound implications. With his modest instrumentation, he had discovered the first hints that we live in an expanding universe, one that had a beginning from a dense, hot, primordial state billions of years ago.
But to get to this conclusion, much serious study, hard work, and discovery remained. More on this in future articles.