Spectrum of the Andromeda Galaxy

Slipher First Discovered Red Shifts in the Spectra of Galaxies

Vesto M. Slipher was the first astronomer to measure the spectrum of an external galaxy. His work led to Hubble’s discovery that the universe is expanding.

Andromeda Galaxy

In 1912 Vesto M. Slipher finally managed to measure the spectrum of the Andromeda galaxy. Continuing the work, he measured spectra of other galaxies. In August 1914 Slipher announced at a meeting of the American Astronomical Society, held in Evanston, Illinois, that he had measured the spectra of thirteen galaxies. By the mid 1920s, he had measured 45 of these spectra.

At that time galaxies were called spiral nebulae, and their nature was a raging controversy. Were they indeed external galaxies like our own Milky Way or were they nebulae, similar to the Orion Nebula, that happened to have a spiral shape? At that time we did yet not know that our Milky Way galaxy had a spiral shape.

Slipher’s spectra, the first spectra of these spiral nebulae, showed something very intriguing. With the exception of the Andromeda Galaxy, now known to be the closest spiral galaxy to the Milky Way, they were all moving away from us.

What did this observation mean? Slipher and the other astronomers of the time didn’t know. They must however have realized that Slipher’s observation presaged something very important. According to astronomical legend, Slipher’s presentation led to the first and one of the very few standing ovations in the history of the American Astronomical Society. Scientific papers, unlike rock concerts, seldom lead to standing ovations.

Doppler Effect

How did Slipher know the galaxies were moving away from us? Listen to a train whistle or ambulance siren. You can tell if it is moving towards or away from you by the pitch. If the source of the sound is moving towards you, the sound waves are squeezed together. The squeezing produces a shorter wavelength and therefore a higher frequency. In sound the frequency determines the pitch (as in the sense of a musical note). If the sound is moving away the sound waves are stretched out. The stretching produces a longer wavelength, lower frequency, and lower pitched sound. This effect is called the Doppler effect, and it works for light as well as sound.

Light from a galaxy that is moving away from us is stretched out. This Doppler shift to longer wavelengths is called a red shift. A galaxy moving towards us has the light waves squeezed, producing a blue shift towards shorter wavelengths. To measure the red or blue shift, we look at the spectrum for known spectral lines that are shifted towards longer or shorter wavelengths compared to what we see in a lab on Earth. We can tell from a galaxy’s spectrum if it is moving away or towards us and how fast it is moving by using the Doppler effect.

The Expanding Universe

A few years after Slipher’s first spectra, as World War I brewed, Einstein brewed his general theory of relativity. Among other things, this theory predicted that the universe is expanding. Einstein thought nonsense. He added a cosmological constant to his theory so that it would not predict the universe is expanding. This constant had absolutely no theoretical or observational justification other than to prevent the prediction that the universe was expanding. It was a fudge inserted solely to make the theory correspond to Einstein’s preconceived notions. Einstein later called this fudge the biggest blunder of his life.

In the 1920s, Edwin Hubble first proved that galaxies were indeed external galaxies when he observed Cepheid variable stars in the Andromeda galaxy. Hubble continued to take spectra of galaxies and from their red shifts, discovered that the universe is indeed expanding.

Astronomers finally understood why Slipher’s observations were important. They presaged the discovery that the universe is expanding and ultimately the big bang theory.

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