Lesser Lion, and is usually recognised by the figure of a larger and
more perfect dipper which constitutes the hinder part of the animal.
This has also seven stars, four in the body of the Dipper and three in
the handle. All these are stars of much celebrity. The two in the
western side of the Dipper, Alpha and Beta, are called _Pointers_, on
account of their always being in a right line with the pole-star, and
therefore affording an easy mode of finding that. The first star in the
tail, next the body, is named _Alioth_, and the second, _Mizar_. The
head of the Great Bear lies far to the westward of the Pointers, and is
composed of numerous small stars; and the feet are severally composed of
two small stars very near to each other.
_Draco_ (_the Dragon_) winds round between the Great and the Little
Bear; and, commencing with the tail, between the Pointers and the
pole-star, it is easily traced by a succession of bright stars extending
from west to east. Passing under Ursa Minor, it returns westward, and
terminates in a triangle which forms the head of Draco, near the feet of
Hercules, northwest of Lyra. _Cepheus_ lies eastward of the breast of
the Dragon, but has no stars above the third magnitude.
_Cassiopeia_ is known by the figure of a _chair_, composed of four stars
which form the legs, and two which form the back. This constellation
lies between Perseus and Cepheus, in the Milky Way.
_Cygnus_ (_the Swan_) is situated also in the Milky Way, some distance
southwest of Cassiopeia, towards the Eagle. Three bright stars, which
lie along the Milky Way, form the body and neck of the Swan, and two
others, in a line with the middle one of the three, one above and one
below, constitute the wings. This constellation is among the few that
exhibit some resemblance to the animals whose names they bear.
_Lyra_ (_the Lyre_) is directly west of the Swan, and is easily
distinguished by a beautiful white star of the first magnitude, _Alpha
Lyræ_.
The _Southern Constellations_ are comparatively few in number. I shall
notice only the Whale, Orion, the Greater and Lesser Dog, Hydra, and the
Crow.
_Cetus_ (_the Whale_) is distinguished rather for its extent than its
brilliancy, reaching as it does through forty degrees of longitude,
while none of its stars, except one, are above the third magnitude.
_Menkar_ (_Alpha Ceti_) in the mouth, is a star of the second
magnitude; and several other bright stars, directly south of Aries, make
the head and neck of the Whale. _Mira_, (_Omicron Ceti_,) in the neck of
the Whale, is a variable star.
_Orion_ is one of the largest and most beautiful of the constellations,
lying southeast of Taurus. A cluster of small stars forms the head; two
large stars, _Betalgeus_ of the first and _Bellatrix_ of the second
magnitude, make the shoulders; three more bright stars compose the
buckler, and three the sword; and _Rigel_, another star of the first
magnitude, makes one of the feet. In this constellation there are
seventy stars plainly visible to the naked eye, including two of the
first magnitude, four of the second, and three of the third.
_Canis Major_ lies southeast of Orion, and is distinguished chiefly by
its containing the largest of the fixed stars, _Sirius_.
_Canis Minor_, a little north of the equator, between Canis Major and
Gemini, is a small constellation, consisting chiefly of two stars, of
which, _Procyon_ is of the first magnitude.
_Hydra_ has its head near Procyon, consisting of a number of stars of
ordinary brightness. About fifteen degrees southeast of the head is a
star of the second magnitude, forming the heart, (_Cor Hydræ_;) and
eastward of this is a long succession of stars of the fourth and fifth
magnitudes, composing the body and tail, and reaching a few degrees
south of Spica Virginis.
_Corvus_ (_the Crow_) is represented as standing on the tail of Hydra.
It consists of small stars, only three of which are as large as the
third magnitude.
In assigning the places of individual stars, I have not aimed at great
precision; but such a knowledge as you will acquire of the
constellations and larger stars, by nothing more even than you can
obtain from the foregoing sketch, will not only add greatly to the
interest with which you will ever afterwards look at the starry heavens,
but it will enable you to locate any phenomenon that may present itself
in the nocturnal sky, and to understand the position of any object that
may be described, by assigning its true place among the stars; although
I hope you will go much further than this mere outline, in cultivating
an actual acquaintance with the stars. Leaving, now, these great
divisions of the bodies of the firmament, let us ascend to the next
order of stars, composing CLUSTERS.
In various parts of the nocturnal heavens are seen large groups which,
either by the naked eye, or by the aid of the smallest telescope, are
perceived to consist of a great number of small stars. Such are the
Pleiades, Coma Berenices, and Præsepe, or the Bee-hive, in Cancer. The
_Pleiades_, or Seven Stars, as they are called, in the neck of Taurus,
is the most conspicuous cluster. When we look _directly_ at this group,
we cannot distinguish more than six stars; but by turning the eye
_sideways_ upon it, we discover that there are many more; for it is a
remarkable fact that indirect vision is far more delicate than direct.
Thus we can see the zodiacal light or a comet's tail much more
distinctly and better defined, if we fix one eye on a part of the
heavens at some distance and turn the other eye obliquely upon the
object, than we can by looking directly towards it. Telescopes show the
Pleiades to contain fifty or sixty stars, crowded together, and
apparently insulated from the other parts of the heavens. _Coma
Berenices_ has fewer stars, but they are of a larger class than those
which compose the Pleiades. The _Bee-hive_, or Nebula of Cancer, as it
is called, is one of the finest objects of this kind for a small
telescope, being by its aid converted into a rich congeries of shining
points. The head of Orion affords an example of another cluster, though
less remarkable than those already mentioned. These clusters are
pleasing objects to the telescope; and since a common spyglass will
serve to give a distinct view of most of them, every one may have the
power of taking the view. But we pass, now, to the third order of stars,
which present themselves much more obscurely to the gaze of the
astronomer, and require large instruments for the full developement
of their wonderful organization. These are the NEBULÆ.
[Illustration Figures 70, 71, 72, 73. CLUSTERS OF STARS AND NEBULÆ.]
Nebulæ are faint misty appearances which are dimly seen among the stars,
resembling comets, or a speck of fog. They are usually resolved by the
telescope into myriads of small stars; though in some instances, no
powers of the telescope have been found sufficient thus to resolve them.
The _Galaxy_ or Milky Way, presents a continued succession of large
nebulas. The telescope reveals to us innumerable objects of this kind.
Sir William Herschel has given catalogues of two thousand nebulæ, and
has shown that the nebulous matter is distributed through the immensity
of space in quantities inconceivably great, and in separate parcels, of
all shapes and sizes, and of all degrees of brightness between a mere
milky appearance and the condensed light of a fixed star. In fact, more
distinct nebulæ have been hunted out by the aid of telescopes than the
whole number of stars visible to the naked eye in a clear Winter's
night. Their appearances are extremely diversified. In many of them we
can easily distinguish the individual stars; in those apparently more
remote, the interval between the stars diminishes, until it becomes
quite imperceptible; and in their faintest aspect they dwindle to points
so minute, as to be appropriately denominated _star-dust_. Beyond this,
no stars are distinctly visible, but only streaks or patches of milky
light. The diagram facing page 379 represents a magnificent nebula in
the Galaxy. In objects so distant as the fixed stars, any apparent
interval must denote an immense space; and just imagine yourself
situated any where within the grand assemblage of stars, and a firmament
would expand itself over your head like that of our evening sky, only a
thousand times more rich and splendid.
Many of the nebulæ exhibit a tendency towards a globular form, and
indicate a rapid condensation towards the centre. This characteristic is
exhibited in the forms represented in Figs. 70 and 71. We have here two
specimens of nebulæ of the nearer class, where the stars are easily
discriminated. In Figs. 72 and 73 we have examples of two others of the
remoter kind, one of which is of the variety called _star-dust_. These
wonderful objects, however, are not confined to the spherical form, but
exhibit great varieties of figure. Sometimes they appear as ovals;
sometimes they are shaped like a fan; and the unresolvable kind often
affect the most fantastic forms. The opposite diagram, Fig. 74, as well
as the preceding, affords a specimen of these varieties, as given in
Professor Nichols's 'Architecture of the Heavens,' where they are
faithfully copied from the papers of Herschel, in the 'Philosophical
Transactions.'
[Illustration Figure 74. VARIOUS FORMS OF NEBULÆ.]
Sir John Herschel has recently returned from a residence of five years
at the Cape of Good Hope, with the express view of exploring the hidden
treasures of the southern hemisphere. The kinds of nebulæ are in general
similar to those of the northern hemisphere, and the forms are equally
various and singular. The _Magellan Clouds_, two remarkable objects seen
among the stars of that hemisphere, and celebrated among navigators,
appeared to the great telescope of Herschel (as we are informed by
Professor Nichols) no longer as simple milky spots, or permanent light
flocculi of cloud, as they appear to the unassisted eye, but shone with
inconceivable splendor. The _Nubecula Major_, as the larger object is
called, is a congeries of clusters of stars, of irregular form, globular
clusters and nebulæ of various magnitudes and degrees of condensation,
among which is interspersed a large portion of irresolvable nebulous
matter, which may be, and probably is, star-dust, but which the power of
the twenty-feet telescope shows only as a general illumination of the
field of view, forming a bright ground on which the other objects are
scattered. The _Nubecula Minor_ (the lesser cloud) exhibited appearances
similar, though inferior in degree.
[Illustration Figure 75. A NEBULA IN THE MILKY WAY.]
It is a grand idea, first conceived by Sir William Herschel, and
generally adopted by astronomers, that the whole Galaxy, or Milky Way,
is nothing else than a nebula, and appears so extended, merely because
it happens to be that particular nebula to which we belong. According to
this view, our sun, with his attendant planets and comets, constitutes
but a single star of the Galaxy, and our firmament of stars, or visible
heavens, is composed of the stars of _our_ nebula alone. An inhabitant
of any of the other nebulæ would see spreading over him a firmament
equally spacious, and in some cases inconceivably more brilliant.
It is an exalted spectacle to travel over the Galaxy in a clear night,
with a powerful telescope, with the heart full of the idea that every
star is a world. Sir William Herschel, by counting the stars in a single
field of his telescope, estimated that fifty thousand had passed under
his review in a zone two degrees in breadth, during a single hour's
observation. Notwithstanding the apparent contiguity of the stars which
crowd the Galaxy, it is certain that their mutual distances must be
inconceivably great.
It is with some reluctance that I leave, for the present, this fairy
land of astronomy; but I must not omit, before bringing these Letters to
a conclusion, to tell you something respecting other curious and
interesting objects to be found among the stars.
VARIABLE STARS are those which undergo a periodical change of
brightness. One of the most remarkable is the star _Mira_, in the Whale,
(_Omicron Ceti_.) It appears once in eleven months, remains at its
greatest brightness about a fortnight, being then, on some occasions,
equal to a star of the second magnitude. It then decreases about three
months, until it becomes completely invisible, and remains so about five
months, when it again becomes visible, and continues increasing during
the remaining three months of its period.
Another very remarkable variable star is _Algol_, (_Beta Persei_.) It is
usually visible as a star of the second magnitude, and continues such
for two days and fourteen hours, when it suddenly begins to diminish in
splendor, and in about three and a half hours is reduced to the fourth
magnitude. It then begins again to increase, and in three and a half
hours more is restored to its usual brightness, going through all its
changes in less than three days. This remarkable law of variation
appears strongly to suggest the revolution round it of some opaque body,
which, when interposed between us and Algol, cuts off a large portion of
its light. "It is," says Sir J. Herschel, "an indication of a high
degree of activity in regions where, but for such evidences, we might
conclude all lifeless. Our sun requires almost nine times this period to
perform a revolution on its axis. On the other hand, the periodic time
of an opaque revolving body, sufficiently large, which would produce a
similar temporary obscuration of the sun, seen from a fixed star, would
be less than fourteen hours." The duration of these periods is extremely
various. While that of Beta Persei, above mentioned, is less than three
days, others are more than a year; and others, many years.
TEMPORARY STARS are new stars, which have appeared suddenly in the
firmament, and, after a certain interval, as suddenly disappeared, and
returned no more. It was the appearance of a new star of this kind, one
hundred and twenty-five years before the Christian era, that prompted
Hipparchus to draw up a catalogue of the stars, the first on record.
Such, also, was the star which suddenly shone out, A.D. 389, in the
Eagle, as bright as Venus, and, after remaining three weeks, disappeared
entirely. At other periods, at distant intervals, similar phenomena have
presented themselves. Thus the appearance of a star in 1572 was so
sudden, that Tycho Brahe, returning home one day, was surprised to find
a collection of country people gazing at a star which he was sure did
not exist half an hour before. It was then as bright as Sirius, and
continued to increase until it surpassed Jupiter when brightest, and was
visible at mid-day. In a month it began to diminish; and, in three
months afterwards, it had entirely disappeared. It has been supposed by
some that, in a few instances, the same star has returned, constituting
one of the periodical or variable stars of a long period. Moreover, on a
careful reexamination of the heavens, and a comparison of catalogues,
many stars are now discovered to be missing.
DOUBLE STARS are those which appear single to the naked eye, but are
resolved into two by the telescope; or, if not visible to the naked eye,
are seen in the telescope so close together as to be recognised as
objects of this class. Sometimes, three or more stars are found in this
near connexion, constituting triple, or multiple stars. Castor, for
example, when seen by the naked eye, appears as a single star, but in a
telescope even of moderate powers, it is resolved into two stars, of
between the third and fourth magnitudes, within five seconds of each
other. These two stars are nearly of equal size; but more commonly, one
is exceedingly small in comparison with the other, resembling a
satellite near its primary, although in distance, in light, and in other
characteristics, each has all the attributes of a star, and the
combination, therefore, cannot be that of a planet with a satellite. In
most instances, also, the distance between these objects is much less
than five seconds; and, in many cases, it is less than one second. The
extreme closeness, together with the exceeding minuteness, of most of
the double stars, requires the best telescopes united with the most
acute powers of observation. Indeed, certain of these objects are
regarded as the severest _tests_ both of the excellence of the
instruments and of the skill of the observer. The diagram on page 382,
Fig. 76, represents four double stars, as seen with appropriate
magnifiers. No. 1, exhibits Epsilon Bootis with a power of three hundred
and fifty; No. 2, Rigel, with a power of one hundred and thirty; No. 3,
the Pole-star, with a power of one hundred; and No. 4, Castor, with a
power of three hundred.
Our knowledge of the double stars almost commenced with Sir William
Herschel, about the year 1780. At the time he began his search for them,
he was acquainted with only _four_. Within five years he discovered
nearly _seven hundred_ double stars, and during his life, he observed no
less than twenty-four hundred. In his Memoirs, published in the
Philosophical Transactions, he gave most accurate measurements of the
distances between the two stars, and of the angle which a line joining
the two formed with a circle parallel to the equator. These data would
enable him, or at least posterity, to judge whether these minute bodies
ever change their position with respect to each other. Since 1821, these
researches have been prosecuted, with great zeal and industry, by Sir
James South and Sir John Herschel, in England; while Professor Struve,
of Dorpat, with the celebrated telescope of Fraunhofer, has published,
from his own observations, a catalogue of three thousand double stars,
the determination of which involved the distinct and most minute
inspection of at least one hundred and twenty thousand stars. Sir John
Herschel, in his recent survey of the southern hemisphere, is said to
have added to the catalogue of double stars nearly three thousand more.
[Illustration Fig. 76.]
Two circumstances add a high degree of interest to the phenomena of
double stars: the first is, that a few of them, at least, are found to
have a revolution around each other; the second, that they are supposed
to afford the means of ascertaining the parallax of the fixed stars. But
I must defer these topics till my next Letter.
LETTER XXIX.
FIXED STARS CONTINUED.
"O how canst thou renounce the boundless store
Of charms that Nature to her votary yields?
The warbling woodland, the resounding shore,
The pomp of groves, and garniture of fields;
All that the genial ray of morning yields,
And all that echoes to the song of even,
All that the mountain's sheltering bosom shields,
And all the dread magnificence of heaven,--
O how canst thou renounce, and hope to be forgiven!"--_Beattie._
In 1803, Sir William Herschel first determined and announced to the
world, that there exist among the stars separate systems, composed of
two stars revolving about each other in regular orbits. These he
denominated _binary stars_, to distinguish them from other double stars
where no such motion is detected, and whose proximity to each other may
possibly arise from casual juxtaposition, or from one being in the range
of the other. Between fifty and sixty instances of changes, to a greater
or less amount, of the relative positions of double stars, are mentioned
by Sir William Herschel; and a few of them had changed their places so
much, within twenty-five years, and in such order, as to lead him to the
conclusion that they performed revolutions, one around the other, in
regular orbits. These conclusions have been fully confirmed by later
observers; so that it is now considered as fully established, that there
exist among the fixed stars binary systems, in which two stars perform
to each other the office of sun and planet, and that the periods of
revolution of more than one such pair have been ascertained with some
degree of exactness. Immersions and emersions of stars behind each other
have been observed, and real motions among them detected, rapid enough
to become sensible and measurable in very short intervals of time. The
periods of the double stars are very various, ranging, in the case of
those already ascertained, from forty-three years to one thousand.
Their orbits are very small ellipses, only a few seconds in the longest
direction, and more eccentric than those of the planets. A double star
in the Northern Crown (_Eta Coronæ_) has made a complete revolution
since its first discovery, and is now far advanced in its second period;
while a star in the Lion (_Gamma Leonis_) requires twelve hundred years
to complete its circuit.
You may not at once see the reason why these revolutions of one member
of a double star around the other, should be deemed facts of such
extraordinary interest; to you they may appear rather in the light of
astronomical curiosities. But remark, that the revolutions of the binary
stars have assured us of this most interesting fact, that _the law of
gravitation extends to the fixed stars_. Before these discoveries, we
could not decide, except by a feeble analogy, that this law transcended
the bounds of the solar system. Indeed, our belief of the fact rested
more upon our idea of unity of design in the works of the Creator, than
upon any certain proof; but the revolution of one star around another,
in obedience to forces which are proved to be similar to those which
govern the solar system, establishes the grand conclusion, that the law
of gravitation is truly the law of the material universe. "We have the
same evidence," says Sir John Herschel, "of the revolutions of the
binary stars about each other, that we have of those of Saturn and
Uranus about the sun; and the correspondence between their calculated
and observed places, in such elongated ellipses, must be admitted to
carry with it a proof of the prevalence of the Newtonian law of gravity
in their systems, of the very same nature and cogency as that of the
calculated and observed places of comets round the centre of our own
system. But it is not with the revolution of bodies of a cometary or
planetary nature round a solar centre, that we are now concerned; it is
with that of sun around sun, each, perhaps, accompanied with its train
of planets and their satellites, closely shrouded from our view by the
splendor of their respective suns, and crowded into a space, bearing
hardly a greater proportion to the enormous interval which separates
them, than the distances of the satellites of our planets from their
primaries bear to their distances from the sun itself."
Many of the double stars are of different colors; and Sir John Herschel
is of the opinion that there exist in nature suns of different colors.
"It may," says he, "be easier suggested in words than conceived in
imagination, what variety of illumination two suns, a red and a green,
or a yellow and a blue one, must afford to a planet circulating about
either; and what charming contrasts and 'grateful vicissitudes' a red
and a green day, for instance, alternating with a white one and with
darkness, might arise from the presence or absence of one or other or
both above the horizon. Insulated stars of a red color, almost as deep
as that of blood, occur in many parts of the heavens; but no green or
blue star, of any decided hue, has ever been noticed unassociated with a
companion brighter than itself."