John j. Mauceri jr., age 13, of Staten Island, N.Y., for his question:

What is meant by a variable star?

The glittering stars vary their positions in the sky through the night and the changing seasons. They were still on their appointed paths when our earliest ancestors looked up to behold them. Since that remote time there have beeri few variations in their paths and patterns.

The wheeling stars point out a faithful clock and a reliable calendar in the heavens. They parade in orderly constellations that remain constant through the centuries. The big ones seem to remain big and bright, and the dim ones remain dim. Many of the big stars were described and named before the dawn of history, and a star that varied in brightness seems unlikely. Nevertheless, a variable star does just this.

The discovery of variable type stars is one of the most eye opening chapters of astronomy. The idea was astounding, and when these variable stars were checked out it was found that they could be used as heavenly yardsticks to measure stellar distances in our own galaxy and even in remote galaxies across the vast oceans of outer space.

The brightness of a star is measured by magnitude. A star of first magnitude is 2.5 times as bright as a star of second magnitude and so on down a scale. Brilliant Betelguese in Orion is almost first magnitude. It is 100 times brighter than the dimmest star our eyes can see, which is sixth magnitude. A few early astronomers suspected that certain odd stars varied in brightness.

The Arabs noticed a variable star in the fall constellation Perseus. They named it Algol, the demon. In the 16th century a Dutch astronomer found a variable star in the constellation cetus. Later it was named Mira, the wonderful. Over a period of 11 months, Mira ranges in brightness between magnitudes two and 10. Algol brightens and dims every 69 hours.

Then an extra punctual variable star was found in the constellation Cepheid. Big, bright beauties of this type were spotted across the milky way and in distant galaxies beyond. They were called Gepheids, and in 1908 astronomers figured how to use them to fathom the universe. Their variable magnitudes reveal distance, and the Cepheid variable stars are cosmic yardsticks.

During its pulsating period, a star may vary 10 magnitudes in brightness. A variable star seems to expand and contract at regular intervals. Astronomers suspect that a cepheid variable may explode and subside. The cepheid type is a huge star that varies perhaps less than one magnitude, but its short periods are very punctual.