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The Galaxies

As one views the heavens on a clear night, the band of light encircling the sky, known as the Milky Way, is strikingly prominent. The telescope resolves some of this cloud of light into individual stars, which suggests that they must be very far away--but how far? And just what is the Milky Way? Three much smaller patches of hazy light are visible to the naked eye. Powerful telescopes have also resolved many stars in these objects, so that they, too, would seem to be vast clouds of very distant stars. They have been named the Large and Small Magellanic Clouds and the Andromeda Galaxy.

Such distant objects are much too far away to have measurable parallaxes, so an indirect method was developed for estimating their distances. The original method used was based upon the properties of the Cepheid variable stars mentioned earlier. Studies of the Cepheid variables in certain star clusters revealed a regular relationship between their periods of variation (one to 100 days) and their relative apparent brightness in any particular cluster. Astronomers then reasoned that if the correct distances to some closer Cepheids could be measured, and if it could be assumed that the period-brightness relationship was universal for Cepheids wherever found, the result would be a known relationship between the period and the absolute brightness. Thus the distance to any Cepheid could be determined by measuring its period of variation and its apparent brightness, then performing a simple calculation.

Unfortunately, no Cepheids are close enough to have measurable parallaxes, so astronomers made statistical studies of the apparent relative motions of the nearer ones. All stars are moving in random directions and speeds, and the apparent motion of any star in one year as viewed from the earth is called the "proper motion" of that star. By measuring the proper motions of some Cepheid stars over a period of years and determining their radial velocities from the red or blue shift of lines in their spectra, their distances were estimated. This information was analyzed statistically to determine the relationship between the period and the absolute brightness of the Cepheids. Thus they could be;used as astronomical distance markers.

The initial studies were made with the assumption that all of the Cepheids had the same intrinsic brightness versus period relation, which is not really the case. In addition, the absorption of their light by intervening dust in our galaxy as swell as other factors were not correctly estimated. This resulted in a major error when in 1929 the first estimate of the size and age of the universe was made based on the red shift of the light from distant galaxies. During the years from 1929 to the 1950s estimated distances to the galaxies increased, but in 1956 these distances suddenly doubled, as did the age of the universe! This resulted from the conclusion that the Cepheids were actually four times brighter than originally thought and thus twice as far away. This rapid expansion of the universe hit the headlines. Since then the universe has again more than doubled in size and age (in the minds of astronomers). Then, in 1982, the size and age suddenly decreased about 20 percent in the minds of some astronomers but increased in the minds of others! So the size and age of the universe is an "iffy" question, a matter of dispute among the experts. The proposed values vary by a factor of two.

The presently accepted scale (or scales) of the universe is (or are) based on a system of measurement which has been built up of successive links of logical inference from astronomical data. The links are as follows:

Average earth-sun distance (1 astronomical unit = 92,960,000 miles measured by triangulation within the solar system)

Stellar Parallax method (Distance is estimated from apparent motion of star caused by earth's orbital motion.)

Calibrate the Main Sequence method (Based on the Hertzprung-Russell diagram relating absolute brightness to temperature of normal stars)

Calibrate the Cepheid method (Absolute brightness is inferred from period.)

Calibrate Brightest Star method (Brightest stars are assumed to have the same absolute brightness in all galaxies.)

Calibrate Galactic Red Shift method (Red shift of light from galaxies is assumed to be caused by recession of galaxies in an expanding universe--to be considered below.)

In spite of the problems and uncertainties in the use of Cepheid variable star and other methods of measuring distances, it would appear their use has provided a roughly valid measuring stick for the nearer parts of the universe. What are the apparent distances to the nearby galaxies? The two Magellanic clouds are considered to lie about 150,000 to 200,000 light years from the sun, the Andromeda Galaxy almost two million light years. Astronomers believe that there are 20 galaxies within two and a half million light years and many thousands within 50 million light years; there may be a billion galaxies within photographic range of the 200-inch Mount Palomar telescope.

The galaxies appear, then, to be vast assemblages of as many as 100 billion or more stars averaging about the same mass as the sun. Those called spiral galaxies, disk-shaped with spiral arms composed of stars and gas, are observed to be rotating around the center of galactic mass. The nearby Andromeda is an example of a spiral galaxy. The Milky Way is thought to be a spiral galaxy of which the solar system is a member. Since the solar system is supposedly located in the disk, along the direction of the disk in our sky the density of stars appears much higher, with the result that we have a band high in star density in our sky. The diameter of the Milky Way galaxy is of the order of 100,000 light years according to present estimates.