To the east of Vega lies Cygnus, or the Swan, a well-known northern constellation. It may be recognized by the long cross formed by its principal stars, Alpha, Beta, Gamma, Delta, and Epsilon; Alpha, or Deneb, being the most northern and brightest, and Beta the most southern and faintest of the five.
To the southeast of Cassiopeia’s Chair lies the constellation Perseus, distinguished by its well-known festoon, or curve, of stars. South of this lies the constellation Taurus or the Bull, which contains the well-known groups or clusters, the Pleiades and the Hyades. The Pleiades form perhaps the most remarkable group of stars in the heavens, and are easily found, when above the horizon. To ordinary eyesight the cluster consists of six stars. Some persons gifted with exceptionally keen eyesight have, however, seen eleven or twelve. A map of the Pleiades made in the sixteenth century shows eleven stars very correctly. This was drawn, of course, from observations made with a measuring instrument, but without the aid of a telescope. The observer (I think it was Möstlin, Kepler’s tutor) must have possessed wonderfully sharp eyesight. The Hyades form a V-shaped figure, and contain the bright reddish star Aldebaran.
South of Taurus and Gemini will be found the splendid constellation of Orion, perhaps the most brilliant group of stars visible in either hemisphere. A remarkable quadrilateral figure is formed by its four stars, Betelgeuse (Alpha) and Gamma[18] on the north, and Rigel (Beta) and Kappa on the south. Of these Betelgeuse and Rigel are bright stars of the first magnitude. Betelgeuse is distinctly reddish and also slightly variable in its light. Rigel is a beautiful white star. In the middle of the quadrilateral are three stars of the second magnitude, nearly in a straight line, known as Delta, Epsilon, and Zeta, Delta being the northern of the three. These form Orion’s “belt.” South of these are three faint stars, also in a straight line, forming the “sword” of Orion. Surrounding the central star of the “sword” is “the great nebula of Orion,” one of the finest objects in the heavens. It is barely visible to the naked eye, but may be seen with a good opera-glass.
To the southeast of Orion will be found Sirius, the brightest star in the heavens. It is the chief star of the constellation Canis Major, or the Great Dog, and has been well termed “the monarch of the skies,” from its great brilliancy.
The bright star Regulus, referred to above, is situated in a remarkable group of stars shaped like a sickle, and known as “the Sickle in Leo.” Regulus lies at the extremity of the handle. Leo is well placed for observation in April and May.
The famous group called the Southern Cross forms a conspicuous object in the southern heavens. It has formed a subject of interest since the earliest ages of antiquity. Its component stars, are, however, not so brilliant as some suppose, the two brightest being between the first and second magnitudes, the next of the second, and one between the third and fourth magnitudes. Near the Southern Cross are two bright stars known as Alpha[19] and Beta of the Centaur.
Among the stars are many objects known as “double stars.” These consist of two stars very close together, but which appear to the naked eye only as single stars. Some are triple, and even quadruple. Of these double stars there are now about 10,000 known to astronomers, but they are only visible with a telescope. Some, indeed, are so close that the highest powers of the very largest telescopes are necessary to see them as anything but single stars. Of the naked-eye stars there are, however, some apparently so close that they present very much the appearance of real double stars as seen in a telescope. These, although not recognized by astronomers as double stars, have been termed “naked-eye doubles.” Houzeau found that the brighter the stars are the easier it is to separate them; and that for small stars, about 15′ of arc, or half the moon’s apparent diameter, is about the limit below which the naked eye can not see a faint star double.
Fig. 15.—Constellation of the Great Bear
Of the “naked-eye doubles,” perhaps the most remarkable is Mizar, the middle star in the “tail” of the Great Bear. Close to it is a small star, sometimes called “Jack on the Middle Horse.” It was known to the ancient astronomers as Alcor, or “the test,” as it was then considered a test of excellent eyesight. Whether it has really brightened seems doubtful, but at present it is perhaps visible to ordinary eyesight. Some, however, fail to see it, while to others with keener vision it seems as plain as the proverbial “pike-staff.” The star Alpha Capricorni consists of two stars which, although closer than Mizar and Alcor, are more equal in brightness, and may be easily seen with the naked eye on a clear night. Nu Sagittarii may also be seen double in this way. Theta Tauri, in the Hyades, is another object which some eyes can see distinctly double; also Kappa Tauri, a little to the north of the Hyades; Omicron Cygni, a little to the west of Alpha Cygni (Deneb), is another example. On a very fine night two stars may be seen in Iota Orionis, the most southern star in the “sword.” Near Gamma Leonis, one of the brightest stars in the “sickle,” is a star of the sixth magnitude, which some can see without optical aid.