Science in Christian Perspective

ASTRONOMY
H. Harold Hartzler

New Information on the age and size of the universe
Owen Gingerich

The roots of this change go back over 30 years, when Harlow Shapley first used the period-luminosity relation of Cepheid variable stars to determine the distance of remote objects. This relationship was de rived from the Magellanic Clouds, in which the stars can all be considered roughly the same distance. Here it was found that the brighter Cepheid variable stars had the longest periods. Shapley's contribution consisted of finding the zero-point on the graph; by statistically determining the distance of similar Cepheids in our own galaxy, he was able to establish the distance to the Clouds as about 80,000 light-years.

This work resulted in several anomalies. Perhaps the most striking was the fact that our own Milky Way galaxy appeared to be twice the size of any other in spite of vigorous searches to find another giant galaxy. In addition, the globular clusters associated with the Andromeda galaxy were much smaller than those of our own system.

The new result which resolves these anomalies is the first important contribution of the 200" Hale telescope at Palomar Mountain. From our present knowledge of galactic structure, it seemed inevitable that large numbers of cluster-type Cepheids, (those with periods under a day) should be found in the nucleus of the Andromeda Galaxy. The light gathering power of the 200" was just sufficient to find these stars, assuming that the distance of 750,000 light-years was correct. When all attempts failed, it became apparent that the distance to this spiral had been underestimated. The work of Walter Baade and his student Allan Sandage at Mt. Palomar and Mt. Wilson showed that the error was close to a factor of two, that is, the Andromeda Galaxy was actually about 1,500,000 light-years away.

Why did the error come about? Probably because Shapley and the other astronomers assumed that the short period cluster-type variables were the same general series as the "~lassical" Cepheids with periods of from 3 to 20 days. The period-luminosity relation was forced into a single continuous curve. Baade has shown now that actually two stellar populations exist. The cluster-type Cepheids belong to Population 11, associated with globular clusters and the nuclei of spiral galaxies, while the classical Cepheids belong to Population 1, associated with spiral arms and the Magallanic Clouds. Related in a continuous curve with the cluster-type variables are the W Virginis stars, which can be differentiated from classical Cephelds of the same period by the appearance of the light curve. In other words, the cluster-type variables and W Virginis stars of Population II form a continuous period-luminosity relation, but the classical Cepheids are brighter for a given period and form their own separate relation.

The method of finding the age of the universe by computing the recession of the distant galaxies backwards till they reach a common point is affected since the galaxies are now twice the distance formerly believed. The rate of recession of course remains the same. Earlier Hubble found 1.8 x 10⁹, years. Hubble was misled by assuming an absolute magnitude of galaxies which is too bright, and hence he misjudged their distances. Recent work shows that there are many dwarf galaxies, and that the bright spirals shown in astronomy textbooks are actually the exceptions. This alone would correct the figure to about 2.5 x 10⁹ years, while the zero-point correction doubles this to 5 x 10⁹' years, a figure in good agreement with the best radioactive dating of the rocks.