After his initial discussion of solar system papers, Dr. Abt described the manner in which Harlow Shapley studied the distances to and directions of a class of objects known as globular star clusters. These are relatively spherical collections of fifty thousand to a few hundred thousand stars. Dr. Shapley found they were distributed spherically around a point in space that he assumed to be the center of the Milky Way Galaxy. This distance was correct within a factor of 2, and Shapley had determined the size of our galaxy and our location within it nearly 100 years ago.

In the early years of this century, astronomers theorized whether spiral "nebulae," as spiral galaxies were then called, were inside or outside of our galaxy. In a famous debate on April 26, 1920, Shapley used strong arguments to reach the incorrect conclusion that spiral nebulae are inside the Milky Way. His opponent, Heber Curtis, had weaker arguments but reached the correct conclusion. The solution to this problem waited a few more years until Edwin Hubble determined the distance to the spiral "nebula" called Messier 31. Using the new 100-inch telescope on Mt. Wilson , he studied a class of variable stars known as Cepheid stars, whose period of variation is directly proportional to their intrinsic or real brightness. The distance Hubble found was more than 5 times greater than Shapley’s determination to the center of the galaxy. This proved that these "nebulae" were actually galaxies external to the Milky Way. The actual distance is now known to be about 2.5 million light years.

The next problem remains unsolved today. Fritz Zwicky studied clusters of galaxies and wondered why they appeared to stay together. To be mutually attracted gravitationally, each must contain a certain amount of mass. However, when all the visible material is accounted for, there is a shortage of mass. Zwicky referred to this shortage as "missing mass." Indeed, Dr. Abt described that studying orbital dynamics of galactic objects has shown that as much as 90% of the necessary mass is missing within galaxies.

Another unsolved problem is that of extra-galactic gamma ray bursts, first discovered in the late 1960s by US military satellites looking for atomic testing on earth. The paper that Dr. Abt described was published in 1973. Recently, however, it has been discovered that these bursts are associated with very distant galaxies. Current thinking is that if these bursts are isotropic, they are the most energetic occurrence in the universe. But if they are directional, like a searchlight, much less energy is being generated in the burst. Exactly what they are is still unknown.

The last major topic that Dr. Abt presented was three important papers that contributed to proving the Big Bang Theory. First was the famous paper that described the red shift, by Hubble and Humason. The second paper, on cosmic microwave background radiation, predicted the observed background temperature that would be left over from the Big Bang today. The third paper, on nuclear synthesis of elements, extrapolated forward from the observed background temperature of the universe to explain the observed distribution of elements in the universe.