The star at the bend in the handle of the Big Dipper, called Mizar, is of special interest. If one has good eyesight, he will see close to it a faint star. This is Alcor, which is Arabic for The Test. The two stars are also called the Horse and the Rider.
Mizar forms with Alcor what is known as a wide double star. It is, in fact, the widest of all double stars. Many stars in the heavens that appear single to us are separated by the telescope into double or triple or multiple stars. They consist of two or more suns revolving about a common center, known as their center of gravity. Sometimes the suns are so close together that even the most powerful telescope will not separate them. Then a most wonderful little instrument, called the spectroscope, steps in and analyzes the light of the stars and shows which are double and which are single. A star shown to be double by the spectroscope, but not by the telescope, is called a spectroscopic binary star.
Mizar is of historic interest, as being the first double star to be detected with the aid of the telescope. A very small telescope will split Mizar up into two stars. The brighter of the two is a spectroscopic binary star beside, so that it really consists of two suns instead of one, with the distance between the two so small that even the telescope cannot separate them. About this system of three suns which we know as the star Mizar, the faint star Alcor revolves at a distance equal to sixteen thousand times the distance of the earth from the sun.
May—Leo
If we follow the imaginary line drawn through the Pointers in a southerly direction about forty-five degrees, we come to Leo, The Lion, one of the zodiacal constellations. There should be no difficulty in finding the constellation Leo, as its peculiar sickle-shaped group of bright stars makes it distinctive from all other constellations. At the time we have mentioned, that is, the early evening hours, it will lie a little to the southwest of the zenith. Leo is one of the finest constellations and is always associated with the spring months because it is then high in the sky in the evening. Regulus is the beautiful white star which marks the handle of The Sickle, and the heart of Leo; and Denebola is the second-magnitude star in the tail of Leo.
May—Corvus and Crater
Due south of Denebola, about thirty degrees, we find the small star-group known as Crater, The Cup, which is composed of rather faint and inconspicuous stars. Just east of Crater is the group known as Corvus, The Crow, which forms a very characteristic little four-sided figure of stars differing very little from one another in brightness. These two star groups lie far to the south in our latitudes; but if we lived twenty degrees south of the equator, we would find them nearly overhead, at this time of the year. Just south of Corvus and Crater we find Hydra, one of the constellations for April which extends beneath these constellations and also beneath Virgo, one of the June constellations.