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16.1 Galaxy Redshifts

In 1912, Vesto Slipher began a project to measure the spectra of a number of what were then called spiral nebulae. It was ten years before Hubble would show that the spiral nebulae were actually distant systems of stars. Slipher worked at Lowell Observatory under the direction of Percival Lowell, whom you may know from his famous speculations about the "canals" on Mars. Slipher was looking for rotation of the nebulae with a new and efficient spectrograph. He hoped to see evidence of rotation using the Doppler effect — blue shifted light from material moving toward us and red shifted light from material moving away from us. He detected these wavelength shifts. Just as in the Milky Way light from the spiral nebulae is spread out over several hundred kilometers per second due to the rotation of the disk. However, Slipher also encountered a big surprise: most of the spiral nebulae were redshifted and moving away from us at incredible speeds.


Astropedia Image 
Redshifted Spectrum. Click here for original source URL

By the early 1920s, Slipher had collected spectra of many spiral nebulae. In addition to the evidence for rotation, Slipher noted that 21 out of the 25 spectra were shifted to red wavelengths by amounts of up to a thousand kilometers per second. The high speeds were surprising. Several of the nebulae were moving at speeds of over a million miles per hour! The predominance of redshifts was even more surprising. If the redshifts were due to the Doppler effect then almost all of the nebulae were flying away from each other and from the Milky Way.

Were the nebulae gas clouds being ejected from the Milky Way? Or were they systems of stars receding from our own galaxy? Edwin Hubble took the crucial next step by measuring distances to the spiral nebulae using Cepheid variable stars. In 1929, Hubble was able to combine Slipher's velocity measurements with his own distance measurements for several dozen galaxies. Galaxies were identified as distant systems of stars, not material being ejected from our own galaxy. This was a profound discovery. The Milky Way was not the universe. The universe contained many stellar systems and Hubble showed it was hundreds of times larger than anyone had imagined.

In considering the universe beyond the Milky Way, astronomers use a distance unit that is more appropriate for the vast distances between galaxies. A mega parsec (Mpc) is a million parsecs or a thousand kilo parsecs — note the use of the same prefix, mega-, that is used throughout the metric system to indicate 106. A mega parsec is 30 billion billion or 3 × 1019 km! A mega parsec corresponds to a look-back time of 3¼ million years. Galaxies within 1 Mpc are moving at speeds of up to 1000 km/s away from us. The only four exceptions — blue shifted galaxies with velocities less than zero — are galaxies bound by the cumulative gravity of the Local Group that happen to be moving toward us. Most importantly, in general, more distant galaxies are receding faster from us. The correlation between radial velocity, or red shift, and distance is one of the foundations of modern cosmology.

Hubble's discovery of a correlation between distance and red shift for galaxies cemented his reputation as one of the giants of science. That is why astronomy's premier observing facility — the Hubble Space Telescope — was named after him. The awareness of the Milky Way as just one among many galaxies gives us a new sense of our place in the universe. The unexceptional nature of our star and our entire galaxy takes the Copernican revolution another step. Physical laws can be applied over large distances. Hubble found that variable stars millions of light years away behave similarly to variable stars near the Sun. Last, scientists must work with limited information to increase knowledge. Faint, fuzzy patches of light, mostly too faint to see with the naked eye, have been shown to be vast new worlds to explore.

The story of the discovery of galaxies illustrates the interconnectedness of science. Some of the shine on Hubble's reputation is due to the force of his personality. He had a supreme self-confidence that often bordered on arrogance. In truth, Hubble's success depended on synthesizing the work of others. Slipher was the first to discover that most galaxies had red shifts, but he had no way of measuring their distances. It was Henrietta Leave itt who first demonstrated the period-luminosity relation for Cepheids, and it was Harlow Shapely who pioneered the use of Cepheids as distance indicators. Hubble benefited from the entrepreneurial energy of George Ellery Hale, who built the large telescopes that he needed to do his work. Hubble was not particularly adept as an observer. He depended heavily on the skills of Milton Humason at the Mount Wilson Observatory. Humason began as a mule driver during the construction of the observatory and he worked his way up from janitor to telescope operator. By the 1940s, he had become an expert observational astronomer and he worked with Hubble to take photographs that were littered with distant galaxies.