Roger Young, age 14, of Butler, Pennsylvania, for his question:
What is meant by a variable star?
Most of us can identify a number of big and little stars and we take it for granted that each bright twinkling point of light remains the same in size and brilliance. Closer observation proves this steady state of celestial bodies to be quite unfounded. Many stars are known to vary in several different patterns. The astronomers of ancient Arabia noticed that a certain star changed in brilliance every few nights. Because of its odd behavior they named it Algol, the demon star. This star, still using the name Algol and still varying its brilliance, can be seen in the fall constellation, Perseus. For two and a half days Algol shines bright and steady. Then for a period of nine hours the light of the demon star is dimmed. Algol is merely one type of variable star and there are many others like it. In a telescope it looks like one star, but actually it is a binary pair of stars orbiting around each other.
Algol has a dark companion and at regular intervals it passes in front of its bright partner, eclipsing some of its brilliance. In 1593, a Dutch astronomer noted a bright star near the Pliades. Several years later this star was studied more closely and named Mira, the Wonderful. The brilliance of Mira varies in periods of about 11 months. By 1844, 18 variable stars had been listed and in the next 40 years, more than 200 more were added to the list. Then the study of variable stars began in earnest. Astronomers learned that though some were eclipsing binaries, others, like Mira, were pulsating stars that actually swell and shrink, dim and flare up at regular or irregular intervals. We tend to think that astronomical work of this type should be left to professional astronomers. But in the field of variable stars,much basic work has been done by the AAVSO the American Association of Variable Star Observers. And most AAVSO members are devoted amateur astronomers.
Their observations have made it possible to identify and understand many variable stars, both in our Galaxy and the vast reaches of space beyond. When a likely variable.star is reported, the professional astronomers get busy with telescopes and cameras, spectroscopes and photoelectric cells, thermocouples and an assortment of other specialized equipment. The fluctuation period of the newcomer is timed. Its period may be a few minutes, a few hours or days or even a few years. If it is a pulsating star, measurements are taken of the variabilities in its size and magnitude. The neweomer is then classified in one of more than a dozen different categories of variable stars. One of the earliest variable was to be found was in the constellation Cepheus, near the Big Dipper, and stars of the same type are called Cepheid variables.
They are giant and supergiant stars that blaze and dim on schedule at regular intervals. These stars are yardsticks used to measure celestial distances, even to outer galaxies beyond the Milky Way. Astronomers discovered a definite relationship between the period of variation of a Cepheid star and its absolute magnitude. The apparent magnitude is the brightness of a near or distant star as seen from the earth. The absolute magnitude is its brilliance undimmed by distance. Distance dims at a fixed rate. The period of a Cepheid variable is figured, which enables us to determine its absolute magnitude. Comparing the absolute and apparent magnitudes gives its distance, even if the celestial yardstick happens to be in an outer galaxy.