Astronomy in a nutshell

by Garrett Putman Serviss

The apparent association of nebulæ with stars led to the so-called nebular hypothesis, according to which stars are formed, as already suggested, by the condensation of nebulous matter. In the celebrated form which Laplace gave to this hypothesis, it was concerned specially with the origin of our solar system. He assumed that the sun was once enormously expanded, in a nebulous state, or surrounded with a nebulous cloud, and that as it contracted rings were left off around the periphery of the vast rotating mass. These rings subsequently breaking and condensing into globes, were supposed to have given rise to the planets. It is still believed that the sun and the other stars may have originated from the condensation of nebulæ, but many objections have been found to the form in which Laplace put his hypothesis, and the discovery of the spiral nebulæ has led to other conjectures concerning the way in which the transformation is brought about. But we have not here the space to enter into this discussion, although it is of fascinating interest.

A word more should be said about the use of photography in astronomy. It is hardly going too far to aver that the photographic plate has taken the place of the human retina in recording celestial phenomena, especially among the stars and nebulæ. Not only are the forms of such objects now exclusively recorded by photography, but the spectra of all kinds of celestial objects—sun, stars, nebulæ, etc.—are photographed and afterward studied at leisure. In this way many of the most important discoveries of recent years have been made, including those of variable stars and new stars. Photographic charts of the heavens exist, and by comparing these with others made later, changes which would escape the eye can be detected. Comets are sometimes, and new asteroids almost invariably, discovered by photography. The changes in the spectra of comets and new stars are thus recorded with an accuracy that would be otherwise unattainable. Photographs of the moon excel in accuracy all that can be done by manual drawing, and while photographs of the planets still fail to show many of the fine details visible with telescopes, continual improvements are being made. Many of the great telescopes now in use or in course of construction are intended specially for photographic work.

3. The Constellations. The division of the stars into constellations constitutes the uranography or the “geography of the heavens.” The majority of the constellations are very ancient, and their precise origin is unknown, but those which are invisible from the northern hemisphere have all been named since the great exploring expeditions to the south seas. There are more than sixty constellations now generally recognised. Twelve of these belong to the zodiac, and bear the same names as the zodiacal signs, although the precession of the equinoxes has drifted them out of their original relation to the signs. Many of the constellations are memorials of prehistoric myths, and a large number are connected with the story of the Argonautic expedition and with other famous Greek legends. Thus the constellations form a pictorial scroll of legendary history and mythology, and possess a deep interest independent of the science of astronomy. For their history and for the legends connected with them, the reader who desires a not too detailed résumé, may consult Astronomy with the Naked Eye, and for guidance in finding the constellations, Astronomy with an Opera-glass, or Round the Year with the Stars. The quickest way to learn the constellations is to engage the aid of some one who knows them already, and can point them out in the sky. The next best way is to use star charts, or a star-finder or planisphere.

A considerable number of the brighter and more important stars are known by individual names, such as Sirius, Canopus, Achernar, Arcturus, Vega, Rigel, Betelgeuse, Procyon, Spica, Aldebaran, Regulus, Altair, and Fomalhaut. Astronomers usually designate the principal stars of each constellation by the letters of the Greek alphabet, α, β, γ, etc., the brightest star in the constellation bearing the name of the first letter, the next brightest that of the second letter, and so on.

The constellations are very irregular in outline, and their borders are only fixed with sufficient definiteness to avoid the inclusion of stars catalogued as belonging to one, within the limits of another. In all cases the names come from some fancied resemblance of the figures formed by the principal stars of the constellation to a man, woman, animal, or other object. In only a few cases are these resemblances very striking.

The most useful constellations for the beginner are those surrounding the north celestial pole, and we give a little circular chart showing their characteristic stars. The names of the months running round the circle indicate the times of the year when these constellations are to be seen on or near the meridian in the north. Turn the chart so that the particular month is at the bottom, and suppose yourself to be facing northward. The hour when the observation is supposed to be made is, in every case, about 9 o'clock in the evening, and the date is about the first of the month. The top of the chart represents the sky a little below the zenith in the north, and the bottom represents the horizon in the north.

Fig. 18. The North Circumpolar Stars.

The apparent yearly revolution of the heavens, resulting from the motion of the earth in its orbit, causes the constellations to move westward in a circle round the pole, at the rate of about 30° per month. But the daily rotation of the earth on its axis causes a similar westward motion of the heavens, at the rate of about 30° for every two hours. From this it results that on the same night, after an interval of two hours, you will see the constellations occupying the place that they will have, at the original hour of observation, one month later. Thus, if you observe their positions at 9 P.M. on the first of January, and then turn the chart so as to bring February at the bottom, you will see the constellations around the north pole of the heavens placed as they will be at 11 P.M. on the first of January.