THE MAGELLANIC CLOUDS—ZODIACAL LIGHT—STAR GROUPS.—Amédée Guillemin
When we look on the region of the celestial vault which surrounds the South Pole, we can not help being struck with the contrast presented by the small quantity of stars which it contains, with the brilliant zone which borders the Milky Way, from Orion and Argo to the Centaur, passing by the Southern Cross. One solitary star of the first magnitude, Achernar, more distant from the pole than are the beautiful stars of the Centaur and of the Cross, shines in this part of the sky.
But even this circumstance renders the singular aspect of the two nebulous spots, which seem two detached pieces of the great galactic zone, still more striking. These half-stellar, half-nebulous systems, unequal in magnitude and brightness, but easily seen with the naked eye on a clear, moonless night, are situated, one, the larger and more brilliant, between the pole and Canopus, in the constellation of Doradus; the other, the smaller and less brilliant, ordinarily visible during the full moon, in Hydrus, between Achernar and the pole.
Both are known by astronomers and navigators under the name of “Cape Clouds,” or again, “Magellanic Clouds.” And, to distinguish them, we have again the Great Cloud (Nebecula Major) and the Small Cloud (Nebecula Minor).
The Clouds of Magellan are distinguished from all other nebulæ by their great apparent dimensions, and by their physical structure; this last character distinguishes them from most of the branches and offshoots of the Milky Way, with which, we may also add, they do not appear connected in any way.
The Great Cloud extends over a space which embraces not less than forty-two square degrees—about two hundred times the apparent surface of the lunar disk. The Small Cloud occupies in extent four times less than the other; according to Humboldt, it is surrounded “with a kind of desert,” where, it is true, shines the magnificent stellar cluster of Toucan. If the exterior aspect of these two remarkable nebulæ, and their situation in a celestial region poor in stars, give to the southern sky a peculiar appearance, their real structure makes them one of the wonders of the heavens.
In the Great Cloud, Herschel has counted 582 single stars, among which one only is of the fifth magnitude; six others are of the order immediately inferior, and would doubtless be visible to the naked eye if their light were not effaced by the general glare.
In the Small Cloud, the single stars are proportionally more numerous, since 200 have been counted, among which three are of the sixth magnitude, while it only includes thirty-seven of the nebulæ and seven star-clusters. These immense aggregations, the elements of which are themselves swarms of suns, remind us of the largest, in appearance at least, of all the clusters which the eye contemplates in the depths of the sky—the Milky Way.
In the evenings, about the time of the vernal equinox—in March and April, when in our climate the twilight is of short duration—if we examine the horizon toward the west, a little after sunset, we may perceive a faint light that rises in the form of a cone among the starry constellations.
This is what astronomers call the Zodiacal Light. Those unfamiliar with it, or little accustomed to the ordinary aspect of the sky, might confuse the glimmering either with the Milky Way or with the ordinary twilight, or even with an aurora. But, with a little attention, it is impossible to mistake it.
The triangular form of this luminous cone, its elevation and its inclined position to the horizon, make it a thing apart, and one eminently deserving particular mention.
As the days lengthen, and with them the duration of twilight, the Zodiacal Light disappears; it becomes invisible, at least in our climate. But it may again be seen in the morning, in the east, about the time of the autumnal equinox, in September and October, when the dawn has an equally short duration—again, however, to disappear during the period of long nights and long twilights.
It is needless to add that the sky must be clear and the night moonless for observations of the Zodiacal Light to be possible.
Among the explanations that have been given, the most probable one is that which likens the Zodiacal Light to a flattened nebulous ring surrounding the sun at some distance. It is to be remarked that the direction of the axis of the cone, or of the pyramid, prolonged below the horizon, always passes through the sun.
It was believed at first that this direction precisely coincided with the solar equator; but it seems more certain that it coincides with the plane of the earth’s orbit, or the ecliptic.
Now, what is the nature of this luminous mass? Must it be considered as a zone of vapors thrown off by the sun, when in the process of consolidation, when our central star passed from a nebulous state to that of a condensed fluid sphere? This was the opinion of Laplace.
Another hypothesis, also connected with the first, is that the Zodiacal Light is formed of myriads of solid particles, analogous to the aerolites, possessing a general movement, but traveling separately around the focus of our solar world. The light of the ring would be thus produced by the accumulation of this multitude of brilliant points, reflecting toward us the light borrowed by each of them from the sun.
This explanation accounts for the intensity of the Zodiacal Light at different epochs; it would suffice to admit that the condensation of the particles or the density of the ring is not the same throughout its extent, and that its movement of circulation round the sun presents successively different parts to the earth. In this case, it becomes a question whether this lenticular ring of matter is distinct from the zone of aerolites.
Lastly, some astronomers regard the Zodiacal Light as a vaporous ring which belongs to the earth, surrounding it at some distance. But this is an opinion which appears somewhat wild, and is utterly at variance with observation.
Are the stars that are visible to the naked eye spread orderless on the celestial vault? or is there not between those apparently most closely connected some real or physical connection which requires us to rank them in natural groups?
These questions have been already partly solved by what is known of the double and multiple star systems. Soon, exploring the regions of the sky visible by means of the telescope, we shall have to pass in review a multitude of stellar associations, in which suns are found so compact and so numerous, and the form of the groups so regular, that it is impossible to deny their reciprocal dependence.
But long before the discovery of these islands, these archipelagos as worlds, scattered with such astonishing profusion over the infinite, the naked eye had already distinguished a certain number of groups, the stars composing which were so near together that it was impossible to doubt their physical connection.
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.