Robert Beohm, age 13, of Youngstown, Ohio, for his question:
Where do we look for double stars?
To the naked eye, the night sky is studded with single points of starry light. Each star shines alone, surrounded by vacant spaces of sky or so it seems. Telescopes and spectroscopes reveal otherwise. Thousands are twins. or triplets or multiple groups of stars.
For ages, the star Mizar has been used in a test of long distance vision. It is the middle one of the three stars in the handle of the Big Dipper. As a rule, the night sky glimmers with moonlight or the heavenly view is beclouded by haze. But when conditions are perfect, you may see a dimmer star close beside Mizar. If you pass this test, your distance vision is in fine shape. The small star is Alcor. Mizar is much brighter than its close companion and usually it outshines it so that the two appear to the naked eye as a single star.
Astronomers assumed for centuries that Mizar was a rare pair of double stars. They thought that such stars appeared to be double because they were almost exactly in the same line of vision as seen from the earth. This is true of many stars and we call them optical doubles or line of sight doubles. But the great William Hersahel discovered that true double stars are related to each other by more than line of sight. They are bound together by mutual forces of gravitation and orbit in an ellipse around a common center that is determined by their combined masses.
Herschel named these related pairs binary stars. And for a while Mizar seemed to be a prime example. Later studies, with the help of spectroscopes, revealed that Mizar actually is a related system of six stars. Mizar is a pair of almost equally bright stars that orbit each other every 20 days. But each of these orbiting stars is, in itself, a binary system of two stars. Alcor, the barely visible partner of Mizar, also is a binary system of two related stars.
Mizar, once the prime example of a visual double star turned out to be a whole family. But plenty of true doubles remain, though only a few are visible to the naked eye. One of these can be spotted in Taurus the Bull, another in Lyra and another in Capricornus. A small telescope magnifies your vision by about 60 and with its help you may discover that there is no shortage of binaries and double stars anywhere in the heavens. Thousands of binaries and multiple systems have been charted and astronomers suspect that single stars like our sun may be in the minority.
Many binaries can be observed, even though they are too close to be seen as separate stars. The bright companion dims when its small partner orbits in front of it. A prime example of this sort is Algol in the constellation Perseus. This bright double waxes and wanes about once every three days.
Not all visual binaries can be seen with a naked eye. To an astronomer, a visual binary is any pair that can be viewed as separate stars with a telescope. Algol and such stars that give indirect evidence of their double nature are eclipsing binaries. Other pairs are so close that they can be detected only with the spectroscope. These spectroscopic binaries reveal double, then single lines in the light spectrum as they change their posi¬tions.