Recall from last week that at the beginning of the 20th century, many astronomers believed that the “spiral nebulae,” what we now call the galaxies, were part of our own island universe, the Milky Way. That is, until Vesto Slipher was called to study the rainbow band of those strange pinwheel-shaped objects.
On this matter, a battle raged. Many astronomers believed that the spirals were stars in the process of formation. They were new stars in our own Milky Way galaxy and thus were relatively close to our own sun and Earth. Their distinctive central bulge was the star. Their swirling spiral arms were the stuff of planets that many astronomers then assumed would naturally form in orbit around those stars.
Other astronomers believed that the spiral nebulae were distant galaxies that resembled our own Milky Way.
The resolution of the question came from an odd confluence of sources – a quirky amateur astronomer, an Indiana farm boy, and the spectrum of light.
Last week, we left the farm boy, newbie astronomer Vesto Slipher, trembling in front of a meeting of American Astronomical Society meeting in 1914.
Percival Lowell, the amateur astronomer, was rich. He also loved to look through telescopes. So he built himself a huge one under the dark skies of Flagstaff, Arizona. Lowell eventually used the telescope to stare night after night at Mars. He dubiously discovered signs of intelligent life there in the form of a complex set of Martian “canals.”
Lowell needed a real astronomer to apply more scientific techniques to his telescope. Thus, in 1901, he hired Vesto Slipher, a graduate student from Indiana University to pass the light from galaxies through a device to break up the light into the rainbow band that astronomers call its spectrum.
Lowell assured the professors at Indiana that the position would only be a temporary one. Slipher did indeed complete his Ph.D. eight years later, but he did it at a distance. He was destined to spend the rest of his life devoted to Lowell’s telescope and the study of the rainbow bands, the spectral lines, it generated.
Scientists have known for some time that the spectrum of a moving object is different from one at rest. If the object is moving away from us, all of the bands of color are shifted over toward the red side of the spectrum. If it is moving closer, they are shifted toward the violet side.
This effect can be easily observed with sound, which exhibits similar characteristics. In 1842, Christian Doppler noticed that if, for example, you are standing by a railroad track and you hear the whistle of an approaching train, the pitch rises (gets shriller) as it comes toward you. It decreases (gets deeper) as it moves away.
The same thing happens with light. As a galaxy gets closer, the “pitch” of its spectrum rises toward the violet end of the spectrum. If it is moving away, its “pitch” decreases toward the red. The larger the shift toward the red end, the faster the object is moving away.
Using Lowell’s telescope, Vesto Slipher was the first to get a faint photographic image of a galactic spectrum.
Between 1912 and 1914, Slipher took the spectra of 14 galaxies. He discovered that most of them were moving away from us at incredible velocities.
The most remarkable case was the Sombrero Galaxy, marked as M104 on most star charts. It was receding at a velocity of over 2½ million miles an hour!
This time of year, many are the nights that I have swept my telescope to that galaxy. It rises in the east on the boundary of the constellations Corvus, the Crow, and the more familiar Virgo.
In a medium-sized telescope, its appearance is typical of a spiral galaxy seen “edge on.”
It looks like a lens seen from its edge — tapering at its ends and bulging outward at its center. Also typical of a galaxy seen edge on is its beautiful “dust lane,” a pencil-thin, dark line that almost cuts the galaxy in half lengthwise. Dust lanes are made up of the dust and hydrogen gas that have not formed into stars, the leftovers of galactic evolution.
Astronomers eventually drew two important conclusions from Slipher’s spectra of M104 and the other galaxies. First, the “spiral nebulae” must be island universes like our Milky Way and must exist at huge distances from it. Thus, the universe is immeasurably larger than our galactic neighborhood. Second, the galaxies are receding from each other at unimaginable velocities. The universe must have been born from a gigantic explosion we now call the “Big Bang.”
Fourteen spectra don’t seem like much. But this simple collection of data has in the last 100 years led us by a complicated route to a completely new understanding of the cosmos.
The astronomers who listened to Slipher give his talk at that conference in 1914 couldn’t have completely understood all the world-shattering implications of the data. But they must have had inkling of what was to come. Those staid scientists rose to their feet and gave Slipher a standing ovation.