Although the universe includes everything, this discussion is limited to the origin of galaxies, stars, and planets. Although some scholars use the term evolution for the processes and changes in the history of the universe, the processes of origin and change in the universe have no counterparts to genetics or natural selection.

The modern understanding of the origin of the universe would have been practically unthinkable to people even a century ago. For one thing, they had no concept of the size of the universe. Astronomers knew that the universe was, for them, incalculably vast. It might have been possible to use triangulation to calculate the distance of a star, using the diameter of Earth's orbit around the Sun as the base of the triangle, but the stars are so far away that the angle opposite the base was effectively zero. Before large modern telescopes, it was not clear whether the nebulae were clouds of gas or clusters of stars (there are many of each). It was not until 1923 that astronomer Edwin Hubble was able to focus on the Andromeda galaxy well enough to discern individual stars. If those stars appeared so small, then the galaxy must be incredibly distant. There was a suspicion that nebulae and stars went through the equivalent of life cycles, but there was no direct observation that showed that anything changed in the universe, except for occasional supernova explosions.

With improved techniques, it became possible to estimate, though not directly measure, the distance of galaxies. Astronomers Henrietta Swan Leavitt and Harlow Shapley, at the Harvard College Observatory, determined that Cepheid variable stars had a very reliable correlation between the periodicity of their variation and their absolute luminosity. The intensity of light decreases with the square of its distance from the observer. When Edwin Hubble looked at the individual stars in the Andromeda galaxy, he found some that changed their luminosity in the same way as a Cepheid variable star. From the correlation calculated by Leavitt and Shapley, Hubble calculated the absolute luminosity; and by comparing the absolute with the observed luminosity, he calculated the distance of the galaxy. The resulting distance could be meaningfully expressed only in light-years, the distance that light can travel in a year, as it travels at 186,000 miles (almost 300,000 km) per second. The Andromeda galaxy was 900,000 light-years away.

Astounding as this discovery was, Hubble's main breakthrough was his discovery that the universe is expanding. Ever since German chemist Joseph Frauenhofer discovered that chemical elements block certain wavelengths of light, astronomers had been able to analyze the chemical composition of stars and planets by studying these lines of darkness in the spectrum of light from a star or planet. Hubble discovered that the Frauenhofer lines displayed a marked redshift--the absorbance lines were further toward the red (long wavelength) end of the spectrum than they should be--and that different stars had different degrees of redshift. When an object is coming toward the observer, waves emitted from the object are shortened; when an object is moving away from the observer, waves emitted from the object are lengthened. This Doppler effect explains why the whistle of a train coming toward an observer has a higher pitch than when the train has passed the observer: Sound consists of waves of air molecules. The redshift of stars and galaxies suggested that they are moving away from the Earth (or, more properly, that the space between them and the Earth is expanding). This suggested an expanding universe. . .

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