Slipher’s measurements helped us understand our universe


By Tom Burns - Stargazing



Recall from a couple of weeks ago the story of astronomer Vesto Slipher.

Slipher was a farm boy from the very tiny rural town of Mulberry, Illinois. He had a genius for mathematics that led him in 1896 to college at Indiana University where he studied astronomy and mechanical engineering.

That combination of skills was a perfect match for Percival Lowell, a rich dilettante who had built a massive telescope under the dark and beautiful sky near Flagstaff, Arizona.

Lowell also had a large spectroscope, an instrument designed to break up the light of an astronomical object into its component rainbow band of colors, called the star’s spectrum.

Lowell didn’t know how to get the thing to work in combination with his telescope. He was also somewhat unsure of how to exploit its scientific capabilities.

He needed someone who would work on the cheap with both astronomical knowledge and mechanical abilities.

Graduate-student Vesto Slipher fit the bill. So in 1901, Lowell hired Slipher for a “temporary” position at the observatory. His job was to tinker with the spectroscope and eventually to take some spectra with it.

After some effort, Slipher at last got the spectroscope to work properly. He then measured the periods of rotation of the outer planets and detected the thin, nearly undetectable clouds of dust and gas in the “empty” space among the stars. He was also the first person to detect a thin layer of sodium atoms in Earth’s upper atmosphere.

In his spare time, he finally completed his doctoral degree in 1909, some 13 years after he first attended Indiana University.

Lowell was particularly interested in the “spiral nebulae,” spinning whirlpools of what looked like glowing gas. Some astronomers thought they were stars in the process of formation. That idea is called the “solar-nebula” theory.

Others thought they were “island universes,” collections of billions of stars much like our own Milky Way galaxy.

Lowell assigned Slipher the job of breaking up the light from the spiral nebulae into their component rainbow bands to prove the solar-nebula theory, of which Lowell was a strong proponent.

Slipher began the tedious work of taking spectra of the spiral nebulae in 1912.

Among other things, a glowing object’s spectrum tells an astronomer how fast the object is moving away from or toward Earth.

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 (i.e., low-frequency) end of the spectrum. If it is moving closer, they are shifted toward the blue (high frequency) 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. Go try it yourself. OOOOO-EEEEEEE-OOOOOO is what you’ll hear.

The same Doppler shift happens with light. As a galaxy gets closer, the “pitch” of its spectrum rises toward the blue 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. He was also among the first to realize the significance of the red and blue shifts.

Between 1912 and 1914, Slipher took the spectra of 15 galaxies. He dutifully calculated their velocities by measuring their red and blue shifts. He discovered that all but one of them was seriously red shifted. The magnitude of those shifts meant that they were moving away from us at incredible velocities.

One galaxy in particular — the Sombrero near the constellation Corvus, the Crow — was moving away from our planet at the ungodly speed of 2.5 million miles per hour, the highest known speed of any object known at the time.

That simply could not be if the nebulae were mere gas clouds in our Milky Way galaxy. They must be far away, and if they were so far, they must be huge enough and bright enough for us to see them at all.

Under the hot glare of Slipher’s observations and the further work of astronomers over the next 15 years, the solar-nebula hypothesis melted slowly away.

We know now that galaxies are indeed island universes, vast disk- or egg-shaped collections of hundreds of billions of stars. They are separated from each other by brain-melting distances. Thanks in large part to Slipher, we are left with a universe so immense that we are small indeed by comparison.

Even more significantly, most of the galaxies appear to be flying away from each other. The universe is expanding. Trace that expansion back in time, and the universe appears to have exploded from an incredibly dense and incredibly small source.

From Slipher’s painstaking work came the germ of what we have come to call the Big Bang theory. Slipher had done more than increase the size of the universe. He had given us a hint its beginning.

Slipher went on to a distinguished, 50-year career at Lowell Observatory. Upon the Lowell’s death, he became its acting director in 1916 and its director in 1926. He held that position until his retirement in 1952.

Among his accomplishments was to hire another farm boy, 23-year-old Clyde Tombaugh, in 1929. Just one year later, Tombaugh discovered Pluto. To do so, he meticulously examined hundreds of star-field images that he had spent many long hours photographing himself.

Both astronomers are prime examples of the value of spending one’s early days on the farm, where long hours of often-tedious work are necessary to produce a bountiful harvest.

If you have a telescope, go out some spring night and find the Sombrero Galaxy. Ponder, if you will, how small we are. Curse, if you wish, the tiny smudge you see. Personally, I prefer to think of Vesto Slipher and how one human can capture in the tiny, colored bands of a galactic rainbow the vastness and majesty of the universe.

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By Tom Burns

Stargazing

Tom Burns is director of the Perkins Observatory in Delaware.

Tom Burns is director of the Perkins Observatory in Delaware.