Hubble’s Law and the Expanding Universe

In the early 1900s, Vesto M. Slipher of the Lowell observatory in Arizona analyzed the spectra of many spiral nebulaeNebulae . He found that most of them mysteriously had spectral linesSpectral lines that were red-shifted, indicating that they were moving away from the EarthEarth , some at speeds of up to 1000 km/s. Motion at high speed is not uncommon in the cosmos—the Sun, for example, orbits around the center of our Galaxy at 240 km/s.