Celestial Models by Ptolemy and Copernicus

Currently, most people consider it obvious that the sun is at the center of the solar system, but the sun-centered (heliocentric) concept was slow to evolve. In the 2nd century ad , Claudius Ptolemy proposed a model of the universe with the earth at the center (geocentric). His model (shown left) depicts the earth as stationary with the planets, moon, and sun moving around it in small, circular orbits called epicycles. Ptolemy's system was accepted by astronomers and religious thinkers alike for several hundred years. It was not until the 16th century that Nicolaus Copernicus developed a model for the universe in which the sun was at the center instead of the earth. The new model was rejected by the church, but it gradually gained popular acceptance because it provided better explanations for observed phenomena. Ironically, Copernicus' initial measurements were no more accurate than Ptolemy's, they just made more sense.

 

Hubble Photo of Galaxy M100

In 1924 American astronomer Edwin Hubble showed that fuzzy patches in the sky called “spiral nebulas” were in fact galaxies like our Milky Way. The orbiting telescope named after him, the Hubble Space Telescope, took this picture of a distant galaxy called M100 in 1995.

 

Quasar

Quasars are distant astronomical objects, most of which emit very strong radio waves. This false-color radio map of a quasar was taken by the Very Large Array radio telescope in New Mexico . All the quasars that astronomers have found have been very distant. Their light has taken billions of years to reach the earth, so studying quasars is like studying what the universe looked like billions of years ago. The fact that no quasars exist closer to the earth is evidence that the universe has changed over time.

 

Background Radiation

Even when all other sources of radio waves are eliminated, some static appears on even the most sophisticated radios. Some of this radiation is energy left over from the big bang, the explosion that created the universe. As the map created by the Cosmic Background Explorer satellite shows, the background radiation is not entirely smooth.

 

Models of the Universe

According to the widely accepted theory of the big bang, the universe originated between 10 and 20 billion years ago and has been expanding ever since. Three models are commonly considered to model the future of the universe: a closed model, in which the expansion is finite, and the universe will eventually contract back in upon itself; an open model, in which the universe will continue expanding forever; and a flat model, in which the universe will not collapse upon itself, but also will not get larger and larger forever.

 

Distribution of the Galaxies

This false-color optical map, covering about 4300 square degrees, or 10 percent of the sky, shows the distribution in space of some 2 million galaxies. Galaxies tend to clump together—in this image, black represents areas of empty space and blue represents the galaxies. The clustering of galaxies suggests that much more matter than just the visible stars must exist to hold the cluster together.