Expanding Universe

The great astronomer, Edwin Hubble, revolutionized our understanding of the universe not once, but twice. First, he discovered that galaxies are not just clouds within the Milky Way, but that they are whole galaxies unto themselves. And then, as an encore, he went on to discover that all of these galaxies are moving rapidly away from one another. This suggested to Hubble that our entire universe is expanding. Watch the NOVA clip Hubble’s Expanding Universe for a summary of Hubble’s discoveries.
As we saw previously, there are several ways that astronomers can measure the distances to galaxies. One of the primary science missions of the Hubble Space Telescope was to detect and observe cepheid variables in distant galaxies (just as Edwin Hubble had done) in order to precisely measure their distances.
Read 26.5 The Expanding Universe to discover the evidence for the expanding universe, including the redshift of distant galaxies, Hubble’s law, and the concept of the Hubble constant.
To measure the speed of the galaxy, we have to measure its redshift - the amount that the color of the galaxy has changed due to the Doppler Effect. Watch the video Measuring the Speed of a Galaxy for an explanation of how a galaxy’s speed can be measured by carefully examining its spectrum.