No modern astronomer made a more profound contribution to our understanding of the cosmos than did Edwin Hubble, who first conclusively demonstrated that the universe is expanding. Basing his theory on the observation of the change in spectra of distant galaxies, called red shift, Hubble showed that this is a Doppler effect, or alteration in the wavelength of light, resulting from the rapid motion of celestial objects away from Earth.Hubble's discovery confirmed an equation previously arrived at by Einstein -- who, however, had not trusted his own figures until Hubble proved their accuracy. Hubble supported his theory with careful measurements, but found a rate of expansion (called Hubble's constant) that was far too high because he assumed that other galaxies were much nearer our Milky Way than they are. On the other hand, Hubble's law, relating the distance of a galaxy to its recession velocity, remains essentially unchanged. In 1935 Hubble described his principal observations and conclusions in the Silliman lectures at Yale University. These lectures were published the following year as The Realm of the Nebulae, which quickly became a classic work. It is here reprinted with an introduction by James E. Gunn.
Realm of the Nebulae
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