Such, for example, is the group of the Pleiades. Such, again, are the groups known under the names of the Hyades, of Præsepe, and of Berenice’s Hair. All are visible to the naked eye, and good eyes distinguish without difficulty the principal stars of the first-named groups. The Pleiades are situated in the constellation of the Bull, which we can distinguish so easily to the northwest of Orion and Aldebaran.
Of about eighty stars which form the group of the Pleiades, six are visible without the help of telescopes. Formerly, the Latin poet tells us, seven were counted, which may be held to prove that one of them is variable, and has diminished in brightness, or else has disappeared.
The most brilliant, Alcyone, is of the third magnitude; Electra and Atlas are of the fourth; Merope, Maïa, and Taygete of the fifth. Three others again have received particular names, although they are below the limit of ordinary vision; these are Pleione, Celeno, and Asterope, from the sixth to the eighth magnitude. All the others are only visible by the aid of a telescope; but with an ordinary glass it is possible to distinguish a large number. The Pleiades are known under the name of the Hen-coop, doubtless because Alcyone appears in the group as a hen surrounded with her chickens.
The Hyades, which are near the Pleiades, form a less numerous and more scattered group. The bright light of Aldebaran, which is, as is known, of the first magnitude, renders them more difficult to distinguish with the naked eye.
They appear in the rainy season. Hence their name of Hyades, from the Greek word which signifies to rain.
The connection of the stars which compose this group is not so striking as in the case of the Pleiades. Nevertheless, it seems difficult to admit that they are quite independent of each other’s attraction. In examining the position of these two groups in the vicinity of the Milky Way, and observing that both are situated in the prolongation of a branch of the great zone, we are almost entitled to consider them as two clusters of stars, belonging to the immense stellar stratum which surrounds us, and in the midst of which the sun himself is placed.
In Berenice’s Hair, most of the stars are visible to the naked eye, and are perfectly distinguished in the sky, a little to the east of the Lion. No very brilliant star in the vicinity inconveniences the eye by effacing their light.
The next group is situated in the Crab, and is known under the name of Præsepe: it is visible to the unassisted sight; but it is impossible to distinguish the separate stars without the help of a telescope. Nevertheless, an instrument of moderate power easily separates them.
The groups which we have just described form a transition between the stars scattered over the celestial vault and the more condensed clusters, the undefined aspect of which caused them formerly to be designated under the general name of nebulæ.
Doubtless, if we could place ourselves in space, and contemplate from a sufficiently distant standpoint the whole of the stars which appear to us isolated, we should see them condensed into one or several distinct groups, analogous to those of the Pleiades; while, were we to penetrate into the midst of one of those compact clusters, we should see the stars of which it is formed separated and scattered over the celestial vault in such a way as to give it the aspect of our own heavens.