The Fixed Stars—Their Number—The Milky Way—Double Stars—Binary
Systems—Their Orbits and Periodic Times—Colours of the Stars—Stars
that have vanished—Variable Stars—Variation in Sun’s Light—Parallax
and Distances of the Fixed Stars—Masses of the Stars—Comparative Light
of the Stars—Proper Motions of the Stars—Apparent Motions of the
Stars—Motion and Velocity of the Sun and Solar System—The Nebulæ—Their
Number—Catalogue of them—Consist of Two Classes—Diffuse
Nebulæ—Definitely formed Nebulæ—Globular Clusters—Splendour of Milky
Way—Distribution of the Nebulæ—The Magellanic Clouds—Nebulæ round η
Argûs—Constitution of Nebulæ, and the Forces that maintain
them—Meteorites and Shooting Stars.
GREAT as the number of comets appears to be, it is absolutely nothing in
comparison of the multitude of the fixed stars. About 2000 only are
visible to the naked eye; but when the heavens are viewed through a
telescope, their number seems to be limited only by the imperfection of
the instrument. The number registered amounts to 200,000; their places
are determined with great precision, and they are formed into a
catalogue, not only for the purpose of ascertaining geographical
positions by the occultations of the brightest among them, but also to
serve as points of reference for marking the places of comets and other
celestial phenomena. Sirius, α Centauri, and Arcturus are the brightest
stars in the heavens; the others are classed according to their apparent
lustre, from the first to the seventeenth magnitudes. Capella, α Lyræ,
Procyon, and twenty or twenty-one more, are of the first magnitude; α
Persei, γ Orionis, α Cygni, and in all fifty or sixty, are of the
second; and of the third there are about 200, such as η Bootis and η
Draconis, the numbers increasing as the magnitude diminishes. Those of
the eighth magnitude are scarcely visible to the naked eye, and it
requires a very good telescope to see stars of the seventeenth. This
sequence is perfectly arbitrary; but Sir John Herschel has ascertained
by actual measurement the comparative lustre of a great many—for
example, he found that the light of a star of the sixth magnitude is 100
times less than that of one of the first magnitude, and that Sirius
would make between three and four hundred of such little stars. Were the
photometric scale completed, it would be of the greatest importance with
regard to the variable stars.
The three or four brightest classes of stars are scattered pretty
equably over the sky, with the exception of a zone or belt following the
course of the great circle passing through ε Orionis and α Crucis, where
they are very numerous, especially in the southern hemisphere. The stars
of all magnitudes visible to the naked eye increase in numbers towards
the borders of the Milky Way, which derives its lustre and name from the
diffused light of myriads of stars; so numerous are they in some parts
of it that more than 50,000 passed through the field of Sir William
Herschel’s telescope in the course of an hour, in a zone only two
degrees broad; in many places they are numerous beyond estimation, and
most of them are extremely small on account of their enormous distances.
The Milky Way, which forms so conspicuous a part of the firmament, is a
vast and somewhat flattened stratum or congeries of stars, encircling
the heavens in a broad band, split through one part of its circumference
into two streams of stars, bearing a strong resemblance to fig. 5, plate
5. It is contorted and broken in some places, and occasionally
lengthened into branches stretching far into space. Its thickness is
small compared with its length and breadth; yet in some places it is
unfathomable even with the best telescopes; in others there is reason to
believe that it is possible to see through it, and even beyond it, in
its own plane. There is a gradual but rapid increase in the crowding of
the stars on each side of the flat stratum towards the centre.
The solar system is deeply though excentrically plunged into this mass
of stars, near that point where the circular stratum splits into two
streams. Sir John Herschel’s description of the stars of the southern
hemisphere shows that the Milky Way is a most magnificent object there.
“The general aspect of the southern circumpolar regions (including in
that expression 60° or 70° of south polar distance) is in a high degree
rich and magnificent, owing to the superior brilliancy and large
development of the Milky Way, which, from the constellation of Orion to
that of Antinous, is a blaze of light, strangely interrupted, however,
with vacant and entirely starless patches, especially in Scorpio, near α
Centauri and the Cross, while to the north it fades away pale and dim,
and is in comparison hardly traceable. I think it is impossible to view
this splendid zone, with the astonishingly rich and evenly distributed
fringe of stars of the 3rd and 4th magnitude, which forms a broad skirt
to its southern border like a vast curtain, without an impression
amounting almost to conviction, that the Milky Way is not a mere
stratum, but annular, or at least that our system is placed within one
of the poorer or almost vacant parts of its general mass, and that
eccentrically, so as to be much nearer to the region about the Cross
than to that diametrically opposite to it.”
Those dark vacuities called “Coal Sacks” by the ancient navigators,
which are so numerous between α Centauri and α Antaris, are among the
most extraordinary phenomena in the southern hemisphere; they are of
intense blackness, though by no means void of extremely small telescopic
stars; the darkness arises from the contrast these nearly vacant spaces
form with the excessive brilliancy of the surrounding part of the Milky
Way, and the sudden sharp transition from light to darkness. The largest
and most conspicuous of them is a pear-shaped vacuity close to the
Southern Cross. That portion of the Milky Way that is split
longitudinally through its centre lies between α Centauri and the
constellation of Cygnus: the two bands are joined here and there by
narrow bridges of condensed stars, stretching across the darker space
between them. In Scorpio and Sagittarius Sir John Herschel describes the
Milky Way as composed of definite clouds of light running into clusters
of extremely minute stars like sand, not strewed evenly as with a sieve,
but as if thrown down by handfuls, and by both hands at once, leaving
dark intervals. In this astonishing profusion the stars are of all
sizes, from the 14th to the 20th magnitude, and even down to nebulosity.
After an interval the same profusion is renewed, the stars being
inconceivably minute and numerous beyond description—they are in
millions and millions. Thus there is great irregularity in their
diffusion as well as magnitude—in some places intensely crowded, in
others the deep blackness of the sky, over which they are thinly
scattered, irresistibly led to believe that in these regions the power
of our telescopes fairly penetrates through the starry stratum, and
beyond it. Sometimes we look through a sheet of stars nearly of the same
size, of no great thickness compared with their distance from us, and
not unfrequently there is a double stratum, one of large stars spread
over another of very small ones.
The most southerly of the two streams of stars which form the Milky Way
in this part of the firmament maintains an unbroken course of extreme
brilliancy, containing some of the finest clusters of stars in the
heavens. One round γ Sagittarii is an intense aggregate of stars, in
some parts of which they are so crowded as to exceed enumeration; at a
very moderate estimate Sir John Herschel thinks this group cannot
contain fewer than a hundred thousand stars. Other two groups between
the constellations of the Shield and Ophiuchus stand out like
promontories of intense brilliancy in the dark space that separates the
starry streams of the Milky Way.
The distance of the fixed stars is too great to admit of their
exhibiting a sensible disc, but they must be spherical if gravitation
pervades all space, as there is every reason to believe it does. With a
powerful telescope the stars are like points of light: their
occultations by the moon are therefore instantaneous. Their twinkling
arises from sudden changes in the refractive power of the air, which
would not be sensible if they had discs like planets. Thus nothing can
be known of their distance from us or from one another by their apparent
diameters. Although from the appearance of the stars no inference can be
drawn as to their distance, yet among the multitudes in the heavens a
few are found near enough to exhibit distinct parallactic motions
arising from the revolution of the earth in its orbit, from whence their
distance from the sun has been computed: α Centauri, the brightest star
in the southern hemisphere, is a very remarkable instance. Professor
Henderson at the Cape of Good Hope determined its parallax to be 1ʺ by a
series of observations on its position at opposite periods of the year,
that is, from opposite points in the earth’s orbit. The result was
afterwards confirmed by Mr. Maclear, who found the amount to be 0ʺ·913.
The difference between the two is wonderfully small, considering the
many unavoidable sources of error in the determination of such minute
quantities (N. 230).
Since no star in the northern hemisphere has so great an amount of
parallax, an arc of 1ʺ is assumed as the parallactic unit. Now radius is
to the sine of 1ʺ as 206,265 is to 1; hence, α Centauri is 206,265 times
more distant from the sun than the sun is from the earth. Light flying
at the rate of 192,000 miles in a second must take 3 years and 83 days
to come to us from that star.
One or two tenths of a second becomes a very great error when the
maximum amount of parallax is only 1ʺ, and on that account, with the
exception of α Centauri, it has been found impracticable to determine
the annual changes in the apparent motions of single stars affected by
precession, nutation, aberration, and the variations of temperature of
the instruments used in observing. However, as two stars in
juxtaposition are equally affected by all of these; the difference in
their motions is independent of them. Of two stars apparently in close
approximation, one may be far behind the other in space. They may seem
near to one another when viewed from the earth in one part of its orbit,
but may separate widely when seen from the earth in another position,
just as two terrestrial objects appear to be one when viewed in the same
straight line, but separate as the observer changes his position. In
this case the stars would not have real, but only apparent motion. One
of them would seem to oscillate annually to and fro in a straight line
on each side of the other, a motion that could not be mistaken for that
of a binary system where one star describes an ellipse about the other;
or if the edge of the orbit be turned towards the earth, where the
oscillations require years for their accomplishment. The only
circumstances that can affect the stars unequally, and which must be
eliminated, are the proper motion of the stars in space, and specific
aberration, a very minute quantity arising from peculiarities in the
star’s light. This method of finding the distances of the fixed stars
was proposed by Galileo and attempted by Dr. Long without success. Sir
William Herschel afterwards applied it to some of the binary groups; and
although he did not find the thing he sought for, it led to the
discovery of the orbital motions of the double stars.
M. Struve was the first to apply this method, and that in a very
difficult case. He perceived that a very small star is close to α Lyræ,
and by a series of most accurate differential measurements from 1835 to
1838 he found that α Lyræ has a parallax of 0ʺ·261, which was afterwards
corroborated by the observations of M. Peters; hence α Lyræ is 789,600
times more distant from the sun than the earth is.
It was natural to suppose that in general the large stars are nearer to
the earth than the small ones; but there is now reason to believe that
some stars, though by no means brilliant, are nearer to us than others
which shine with greater splendour. This is inferred from the
comparative velocity of their proper motions; all the stars have a
general motion of translation, which tends ultimately to mix those of
the different constellations; but none that we know of moves so rapidly
as 61 Cygni, and on that account it was reckoned to be nearer to us than
any other, for an object seems to move more quickly the nearer it is.
Now M. Bessel saw that two minute and probably very remote stars are
very near 61 Cygni, their directions from that star being at right
angles to one another; so that, during the revolution of the earth, one
of these distances was a maximum and the other a minimum alternately
every three months. This alternation, although it indicated a parallax
or difference of parallaxes of only 0ʺ·348, was maintained with such
perfect regularity every three months, that it leaves not a doubt of its
accuracy, which was afterwards confirmed by the observations of M.
Peters at Polkova. It follows from that small parallax that 61 Cygni
must be 592,700 times farther from the earth than the sun is—a distance
that light would not pass over in less than nine years and three months.
Mr. Henderson found the parallax of Sirius, the brightest of all the
stars, to be only 0ʺ·230; it is consequently more distant than 61 Cygni,
though the latter is but of the 6th magnitude.
M. Argelander has calculated that the apparent magnitude of the stars
depends upon their distance. Supposing them all to be of the same size,
the smallest visible in Sir William Herschel’s 20 feet reflecting
telescope, namely those of the 17th magnitude, would be 228 times
farther off than those of the first magnitude; and M. Peters of Polkova
from the annual parallax of thirty-five, seven of which are now very
accurately determined, has ascertained the distance of the nearest of
them to be such, that light flying at the rate of 95 millions of miles
in a second would take 15 years and a half to come from them to the
earth, and that a star of the 17th magnitude might be extinguished for
3541 years before we should be aware of it. (N. 231.)
The great gulfs that separate the stars from the sun, and probably from
one another, no doubt maintain the stability of the stellar system, in
the same manner that in the solar system the distances of the planets
from the sun and the satellites from their primaries are so arranged as
to preserve their mutual disturbances within due limits. The stars
supposed to be nearest the sun are probably in a great zone which
crosses the Milky Way between η Argûs and α Crucis. It comprises the
bright stars of the constellations Orion, Canis Major, the Southern
Cross, Centaur, Lupus, and Scorpio. The axis of the zone is inclined at
an angle of 20° to the medial line, or circle, passing through the
centre of the Milky Way.
A very great number of stars undergo periodical changes of lustre,
varying in some cases from complete extinction to their original
brilliancy, strongly suggesting the idea that they are temporarily
obscured, and sometimes completely hid, by opaque bodies revolving round
them in regular periodic times, as the planets do about the sun.
The star Mira, or ω Ceti, which was first noticed to be periodical by
Fabricius, in 1596, appears about twelve times in eleven years, or in
periods of 331^d 8^h 4^m 16^s; it remains at its greatest brightness
about a fortnight, being then on some occasions equal to a large star of
the second magnitude; then it decreases during about three months, till
it becomes completely invisible to the naked eye, in which state it
remains about five months; after that it continues increasing during the
remainder of its period. Such is the general course of its changes; but
it does not always return to the same degree of brightness, nor increase
and diminish by the same gradations, neither are the successive
intervals of its maxima equal. From the observations and investigations
of M. Argelander, the mean period given is subject to fluctuation,
embracing 88 such periods, and having the effect of gradually
lengthening and shortening alternately those intervals to the extent of
25 days one way and the other. The irregularities in the degree of
brightness attained at the maximum are probably also periodical. For
four years previous to 1676 it did not appear at all; and on October 5,
1839, it exceeded α Ceti, and equalled β Aurigæ, in lustre. These
irregularities may be occasioned by periodical perturbations among
opaque bodies revolving about the star. Algol, or β Persei, is another
very remarkable instance of a variable star. It has the size of a star
of the second magnitude for two days and thirteen and a half hours, and
then suddenly begins to diminish in splendour, till, in about three
hours and a half, it is reduced to the size of a star of the fourth
magnitude; it then begins again to increase, and in three hours and a
half more regains its brightness, going through all these vicissitudes
in 2^d 20^h 48^m 54^s·7. Sir John Herschel and Mr. Goodricke, by
whom the variable nature of this star was discovered in 1782, considered
this to be a case strongly indicative of the revolution of an opaque
body, which, coming between us and Algol, cuts off a large portion of
the light. This star has been constantly observed, and the more recent
observations, compared with the ancient ones, indicated a diminution in
the periodic time. It is even proved that this decrease is not uniformly
progressive, but is actually proceeding with accelerated rapidity,
which, however, will probably not continue, but will by degrees relax,
and then be changed into increase, according to the laws of periodicity,
which, as well as their causes, remain to be discovered. The first
minimum of this star, in 1844, happened on January 3rd, at 4^h 14^m
Greenwich time. γ Hydræ also vanishes and reappears every 494 days. β
Lyræ was discovered to be variable, in 1784, by Mr. Goodricke, and its
period was ascertained by Argelander to be 12^d 21^h 53^m 10^s, in
which time a double maximum and minimum takes place, the two maxima
being nearly equal, but the two minima unequal; besides these
semi-periods, there is a slow aberration of period, which appears to be
periodical itself: from its discovery to 1840 the time was continually
lengthening, but more and more slowly, till, in 1840, it ceased to
increase, and has since been slowly on the decrease.
The stars δ Cephei and η Aquilæ, or Antinoi, were discovered to be
variable in 1784; their respective periods, being 5^d 8^h 47^m 39^s
and 7^d 4^h 13^m 53^s, have since been accurately determined.
Besides these, the variations of between 30 and 40 have been
approximately ascertained, and a great many more among the smaller stars
have been discovered to be variable by Mr. Hind, who has remarked that
many of those stars which continue visible at their minimum appear hazy
and indistinct, as though some cloudy or nebulous medium intervened.
Some of the variable stars are red, and others present successive
changes through blue, yellow, and red. When the brightness is increasing
the star has a blueish tinge, when it is past its maximum lustre it
assumes a yellow tint, and while on its decrease it becomes ruddy with
flashes of bright red light. These changes are very marked in a small
star near the star 77, at the extremity of the south wing of Virgo.
Sir John Herschel, after having described the glory of the starry
heavens, asks, “For what purpose are we to suppose such magnificent
bodies scattered through the abyss of space? Surely not to illuminate
_our_ nights, which an additional moon of the thousandth part the size
of our own would do much better, nor to sparkle as a pageant void of
meaning and reality, and bewilder us with vain conjectures. Useful, it
is true, they are to man as points of exact and permanent reference; but
he must have studied astronomy to little purpose who can suppose man to
be the only object of his Creator’s care, or who does not see in the
vast and wonderful apparatus around us provision for other races of
animated beings. The planets, we have seen, derive their light from the
sun, but that cannot be the case with the stars. These doubtless then
are themselves suns, and may perhaps, each in its sphere, be the
presiding centre round which other planets or bodies, of which we can
form no conception from any analogy offered by our own system, may be
circulating.”
Another circumstance shows how probable it is that dark bodies are
revolving among the stars. The proper motion of Sirius is very
irregular—sometimes it is rapid, and at other times slow; the cause is
ascribed by MM. Bessel and Peters to a dark companion which revolves
with Sirius about their common centre of gravity, and by its attraction
disturbs the equable motion of the star.
Sometimes stars have all at once appeared, shone with a bright light,
and vanished. Several instances of these temporary stars are on record.
A remarkable one occurred in the year 125, which is said to have induced
Hipparchus to form the first catalogue of stars. Another star appeared
suddenly near α Aquilæ in the year 389, which vanished after remaining
for three weeks as bright as Venus. On the 10th of October, 1604, a
brilliant star burst forth in the constellation of Serpentarius, which
continued visible for a year; and on the 11th of November, 1572, a star
all at once shone forth in Cassiopeia, which rapidly increased in
brightness till it surpassed that of Jupiter so much as to be visible at
midday. It began to decrease in December of the same year, and, in
March, 1574, it had entirely disappeared, having exhibited a variety of
tints. It is suspected, however, that this star is periodically variable
and identical with stars which appeared in the years 945 and 1264. A
more recent case occurred in the year 1670, when a new star was
discovered in the head of the Swan, which, after becoming invisible,
reappeared, and, having undergone many variations in light, vanished
after two years, and has never since been seen. On the 28th of April,
1848, Mr. Hind discovered a star of the 5th magnitude in the
constellation Ophiuchus, which was very conspicuous to the naked eye,
and where he was certain no star even so bright as the 9th magnitude had
ever existed, nor was there any record of such a star. From the time of
its discovery it continued to diminish till it became extinct. Its
colour was ruddy, and was thought to undergo remarkable changes,
probably an effect of its low position, as its polar distance was 102°
39ʹ 14ʺ.
Sir John Herschel discovered very singular variations in the star η of
the constellation Argo. It is surrounded by a wonderful nebula, and
between the years 1677 and 1826 it varied twice from the 4th to the 2nd
magnitude; but in the beginning of 1838 it suddenly increased in lustre,
so as to be nearly as bright as α Centauri. Thence it diminished, but
not below the first magnitude till April 1843, when it had again
increased, so as to surpass Canopus, and nearly equal Sirius in
splendour. With regard to this singular phenomenon, Sir John Herschel
observes, that “Temporary stars heretofore recorded have all become
totally extinct. Variable stars, as far as they have been carefully
attended to, have exhibited periodical and regular alternations (in some
degree at least) of splendour and comparative obscurity; but here we
have a star fitfully variable to an astonishing extent, and whose
fluctuations are spread over centuries, apparently in no settled period,
and in no regular progression. What origin can we ascribe to these
sudden flashes and relapses? What conclusions are we to draw as to the
comfort or habitability of a system depending for its supply of light
and heat on so variable a source? Its future career will be a subject of
high physical interest. To this account I will only add, that in the
beginning of 1838 the brightness of this star was so great as materially
to interfere with the observations of that part of the nebula
surrounding it.” Sir John has also discovered that α Orionis is
variable, a circumstance the more remarkable as it is one of the
conspicuous stars of our hemisphere, and yet its changes had never been
remarked. The inferences Sir John draws from the phenomena of variable
stars are too interesting not to be given in his own words. “A periodic
change existing to so great an extent in so large and brilliant a star
as α Orionis cannot fail to awaken attention to the subject, and to
revive the consideration of those speculations respecting the
possibility of a change in the lustre of our sun itself, which were
first put forth by my father. If there be really a community of nature
between the sun and the fixed stars, every proof that we obtain of the
extensive prevalence of such periodical changes in those remote bodies
adds to the probability of finding something of the kind nearer home. If
our sun were ever intrinsically much brighter than at present, the mean
temperature of the surface of our globe would of course be
proportionally greater. I speak now not of periodical, but secular
changes. But the argument is complicated with the consideration of the
possible imperfect transparency of space, which may be due to material
non-luminous particles, diffused irregularly in patches analogous to
nebulæ, but of great extent—to cosmical clouds, in short, of whose
existence we have, I think, some indication in the singular and
apparently capricious phenomena of temporary stars, and perhaps in the
recent extraordinary increase, and hardly less sudden diminution, of η
Argûs.” Mr. Hind has come to the same conclusion with Goodricke and Sir
John Herschel, that the changes in the variable stars are owing to
opaque bodies revolving round them; indeed there are strong reasons to
believe that there are solar systems analogous to our own in the remote
regions of space. Our sun requires nine times the period of Algol to
perform a revolution on its axis, while, on the other hand, the periodic
time of an opaque revolving body, sufficiently large to produce a
similar temporary obscuration of the sun seen from a fixed star, would
be less than fourteen hours.
It is possible that the decrease of light in some of the variable stars
may arise from large spots on their surface, like those occasionally
seen in the radiant fluid masses on the surface of the sun. One of these
spots which was measured by Sir John Herschel on the 20th of March,
1836, with its penumbra, occupied an area of 3780 millions of square
miles; and the black central part of a spot that appeared on the 25th of
May following would have allowed the globe of the earth to drop through
it, leaving a thousand miles clear of contact all around this tremendous
abyss.
All the variable stars on record of which the places are distinctly
indicated have occurred without exception in, or close upon, the borders
of the Milky Way, and that only within the following semicircle, the
preceding having offered no example of the kind.
Many stars have actually disappeared from the heavens. 42 Virginis seems
to be of the number, having been missed by Sir John Herschel on the 9th
of May, 1828, and not again found, though he frequently had occasion to
observe that part of the sky. Mr. Cooper, of the Markree Observatory,
has given a list of fifty stars that are missing since the publication
of his list of stars in 1847. Comparing the present state of the heavens
with more ancient catalogues, a much greater number have disappeared.
Thousands of stars that seem to be only brilliant points of light, when
carefully examined are found to be in reality systems of two or more
suns, many of which are known to revolve about one another. These binary
and multiple systems are very remote, requiring powerful telescopes to
show the stars separately. They are divided into eight classes,
according to the proximity of the two stars. The first class comprises
only such as are less than 1ʺ of space apart; those of the second class
are more apart than 1ʺ and less than 2ʺ, &c. &c. Sometimes the two stars
are of equal magnitude, but more frequently a conspicuous star is
accompanied by a smaller companion. In some cases the conspicuous star
itself is double, as in ζ Cancri, ξ Scorpio, 11 Monocerotis, and 12
Lyncis, which are triple stars. Each of the two stars of ε Lyræ is a
beautiful and close double star; so that which in a common telescope
appears merely to be a double star, is found to be quadruple with a very
excellent instrument. The multiple system of θ Orionis is one of the
most remarkable objects in our hemisphere. To the naked eye and with an
ordinary telescope it seems to be a single star, but it really consists
of four brilliant stars forming a trapezium, and accompanied by two
excessively minute and very close companions, to perceive _both_ of
which is the severest test of a telescope.
The first catalogue of double stars in which their places and relative
positions are given was accomplished by the talent and industry of Sir
William Herschel, who made so many great discoveries, and with whom the
idea of their combination in binary and multiple systems originated; and
that important fact he established by the discovery of a revolving
motion in 50 or 60, and by the determination of the revolution of one
star about the other of Castor or α Geminorum, the largest and finest
double star in the northern hemisphere. He even assigned the approximate
periodic times of this and of several other binary systems. More than
100 stars are now known to be stellar systems. The positions of many
hundreds were measured by Sir John Herschel and Sir James South; and the
catalogue of the double stars in the northern hemisphere, which have
been micrometrically measured, has been increased to more than 6000 by
MM. Bessel, Struve, and British astronomers.
Extensive catalogues of double stars in the southern hemisphere have
been published by the astronomers in our colonial establishments. To
these Sir John Herschel added 1081 during his residence at the Cape of
Good Hope: the angles of position and distances of the stars from one
another he measured, and found that many of them have very rapid orbital
motions. The elliptical elements of the orbits and periodic times of
fifteen have been determined by the most eminent astronomers with
wonderful accuracy, considering the enormous distances and the extreme
delicacy and difficulty of the subject. M. Savary has the merit of
having first determined the elements of the orbit of a double star from
observation. The difficulty of doing so is great, because the nearest
fixed star is 211,000 times farther from the sun than the earth is, and
the orbit itself is only visible with the best telescopes; consequently
a very small error in observation occasions an enormous error in the
determination of quantities at that distance.
In observing the relative position of the stars of a binary system, the
distance between them, and also the angle of position, that is, the
angle which the meridian, or a parallel to the equator, makes with the
line joining the two stars, are measured. The different values of the
angle of position show whether the revolving star moves from east to
west, or the contrary; whether the motion be uniform or variable, and at
what points it is greatest or least. The measures of the distances show
whether the two stars approach or recede from one another. From these
the form and nature of the orbit are determined. Were observations
perfectly accurate, four values of the angle of position, and of the
corresponding distances at given epochs, would be sufficient to assign
the form and position of the curve described by the revolving star;
this, however, scarcely ever happens. The accuracy of each result
depends upon taking the mean of a great number of the best observations,
and eliminating error by mutual comparison. The distances between the
stars are so minute that they cannot be measured with the same accuracy
as the angles of position; therefore, in order to determine the orbit of
a star independently of the distance, it is necessary to assume, as the
most probable hypothesis, that the stars are subject to the law of
gravitation, and consequently that one of the two stars revolves in an
ellipse about the other, supposed to be at rest, though not necessarily
in the focus. A curve is thus constructed graphically by means of the
angles of position and the corresponding times of observation. The
angular velocities of the stars are obtained by drawing tangents to this
curve at stated intervals, whence the apparent distances, or radii
vectores of the revolving star, become known for each angle of position,
because, by the laws of elliptical motion, they are equal to the square
roots of the apparent angular velocities. Now that the angles of
position estimated from a given line, and the corresponding distances of
the two stars, are known, another curve may be drawn, which will
represent on paper the actual orbit of the star projected on the visible
surface of the heavens; so that the elliptical elements of the true
orbit, and its position in space, may be determined by a combined system
of measurements and computation. But, as this orbit has been obtained on
the hypothesis that gravitation prevails in these distant regions, which
could not be known _à priori_, it must be compared with as many
observations as can be obtained, to ascertain how far the computed
ellipse agrees with the curve actually described by the star.
γ Virginis consists of two stars of nearly the same magnitude; they were
so far apart in the beginning and middle of last century, that they were
mentioned by Bradley, and marked in Mayer’s catalogue, as two distinct
stars. Since that time they have been continually approaching each
other, till in January, 1836, one star was seen to eclipse the other, by
Admiral Smyth at his Observatory at Bedford, and by Sir John Herschel at
the Cape of Good Hope. A series of observations since the beginning of
the present century has enabled Sir John to determine the form and
position of the elliptical orbit of the revolving star with
extraordinary truth by the preceding method. According to his
calculation, it came to its perihelion on the 18th of August of the year
1834. Its previous velocity was so great that the revolving star
described an angle of 68° in one year. By the laws of elliptical motion
its angular velocity must diminish till it arrives at its aphelion. The
accuracy with which the motions of the binary systems are measured, and
the skill employed in the deduction of the elliptical elements, are now
so great, that the periodic time of γ Virginis, determined by Sir John
Herschel and Admiral Smyth from their respective observatories, combined
with those of Sir William Herschel, only differ by two years, Sir John
having obtained a period of 182 years, Admiral Smyth that of 180. By the
aid of more numerous observations Mr. Fletcher has found that the true
period is 184·53 years, and that the revolving star passed its
perihelion in 1837. It is by such successive steps that astronomy is
brought to perfection (N. 232).
Some of the double stars have very long periods, such as ς Coronæ, where
the revolving star takes 737 years nearly to accomplish a circuit.
Others again have very short periods, as η Coronæ, ζ Cancri, and ξ Ursæ
Majoris, whose periodic times are 42·500, 58·91, and 58·26 years
respectively: therefore each of these has performed more than one entire
revolution since their motions were observed. ζ Herculis, whose periodic
time is only about 30-1/4 years, has accomplished two complete circuits,
the lesser star having been eclipsed by the greater each time. The first
of these two truly wonderful events, of one sun eclipsing another sun,
was seen by Sir William Herschel in 1782.
The orbits and periodic times of so many of these binary systems having
been determined proves beyond a doubt that sun revolves about sun in the
starry firmament by the same law of gravitation that makes the earth and
planets revolve about the sun (N. 232).
Since the parallax of 61 Cygni and that of α Centauri have been
determined, Sir John Herschel has made the following approximation to
the dimensions of their orbits and masses. The distance between the two
stars of 61 Cygni, that is the radius vector of the revolving star, has
hardly varied from 15ʺ·5 ever since the earliest observations; while in
that time the star has moved through 50°; it is evident therefore that
the orbit must be nearly circular. It is at right angles to the visual
ray, and the periodic time is 514 years. The parallax or radius of the
earth’s orbit as seen from the star is 0ʺ·348, while the radius of the
star’s orbit as seen from the earth is 15ʺ·5; hence the radius of the
star’s orbit is to that of the earth’s orbit as 15ʺ·5 to 0ʺ·348, or
nearly as 45 to 1. So the orbit described by the two stars of 61 Cygni
about one another greatly exceeds that which Neptune describes about the
sun. Since the mean distance of the stars and their periodic time are
given, the sum of the masses of the two stars is computed to be 0·3529,
that of the sun being 1. Thus our sun is not vastly greater nor vastly
less than the stars composing 61 Cygni, which is a small inconspicuous
star to the naked eye, not exceeding the 6th magnitude.
Of all the double stars α Centauri is the most beautiful: it is the
brightest star in the southern hemisphere, equal, if not superior, to
Arcturus in lustre. The distance between the two stars has been
decreasing at the rate of half a second annually since the year 1822,
while the angular motion has undergone very little change, which shows
that the plane of the orbit passes through the earth like the orbits of
44 Boötes, and π Serpentarii; that is to say, the edge of the orbit in
these three stellar systems is presented to the earth, so that the
revolving star seems to move in a straight line, and to oscillate on
each side of its primary. Were this libration owing to parallax, it
would be annual from the revolution of the earth about the sun; but as
years elapse before it amounts to a sensible quantity, it can only arise
from a real orbital motion seen obliquely. In this case five
observations are sufficient for the determination of the orbit, provided
they be exact; but the quantities to be measured are so minute, that it
is only by a very long series of observations that accuracy can be
attained. In 1834 Captain Jacob determined the periodic time of the
revolving star of α Centauri to be 77 years, and the distance between
the two to be 17ʺ·5; and since the decrease is half a second annually,
the distance or radius vector of the revolving star was 12ʺ·5 in the
year 1822; and as Mr. Henderson had determined the parallax or radius of
the earth’s orbit as seen from the star to be ·913, it follows that the
real semi-axis of the revolving star’s orbit is 13-1/2 times greater
than the semi-axis of the earth’s orbit as a minimum. The real
dimensions of the ellipse therefore cannot be so small as the orbit of
Saturn, and may possibly exceed that of Uranus. It is very probable that
an occultation of one of the suns by the other will take place in 1867,
or a very close appulse of the two stars.
Singular anomalies have appeared in the motions of 70 Ophiuchi, which
was discovered to be a binary system by Sir William Herschel in 1779,
and which has since nearly accomplished a revolution. Various orbits
have been computed: those which best represent the angles of position
fail with regard to the distances of the stars from one another, and
_vice versâ_. But it is a very remarkable fact that the errors are
periodical, being for considerable periods of time alternately in excess
and defect. Captain W. S. Jacob, who determined the periodic time of the
revolving star to be 93 years, attributes this anomaly to the disturbing
action of an opaque body revolving round the lesser star. Assuming that
to be the case, and computing, he found that the errors were
considerably diminished both in the angle of position and distance. It
is a subject of the highest interest, and well worthy of the attention
of such astronomers as have the means of making the necessary
observations. Among the triple systems, as ζ Cancri, two of the stars
revolve about one another in 58·9 years; but the motion of the third and
most distant is so slow, that it has only accomplished a tenth part of
its revolution about the other two since the system was discovered.
It appears from the calculations of Mr. Dunlop that ς Eridani
accomplishes a revolution in little more than 30 years. The motion of
Mercury is more rapid than that of any of the planets, being at the rate
of 107,000 miles an hour. The perihelion velocity of the comet of 1680
was 880,000 miles an hour; but, if the two stars of ς Eridani, or of ξ
Ursæ Majoris, be as remote from one another as the nearest fixed star is
from the sun, the velocity of the revolving star must exceed the power
of imagination to conceive. The elliptical motion of the double stars
shows that gravitation is not confined to the planetary motions, but
that systems of suns in the far distant regions of the universe are also
obedient to its laws. The stellar systems present a kind of sidereal
chronometer, by which the chronology of the heavens will be marked out
to future ages by epochs of their own, liable to no fluctuations from
such disturbances as take place in our system. Some stars are apparently
double, though altogether unconnected, one being far behind the other in
space, as α Lyræ, which apparently consists of two stars, one of the
first, the other of the eleventh magnitude. Aldebaran, α Aquilæ, and
Pollux are remarkable instances of these optically double stars. It has
been shown how favourable that circumstance is for ascertaining the
parallax of the nearest of the two. (N. 232.)
The double stars are of various hues: sometimes both stars are of the
same colour, as in α Centauri and 61 Cygni, where the larger stars are
of a bright orange and the smaller ones a deeper tint of the same, but
they most frequently exhibit the contrasted colours. The large star is
generally yellow, orange, or red; and the small star blue, purple, or
green. Sometimes a white star is combined with a blue or a purple, and
more rarely a red and white are united. In many cases these appearances
are due to the influence of contrast on our judgment of colours. For
example, in observing a double star, where the large one is a full ruby
red, or almost blood colour, and the small one a fine green, the latter
loses its colour when the former is hid by the cross wires of the
telescope. That is the case with γ Andromedæ, which is a triple star,
the small one, which appears green, being closely double. ι Cancri is an
instance of a large yellow star and a small one which appears blue by
contrast. Still there are a vast number where the colours are decidedly
different, and suggest the curious idea of two suns, a red and a green,
or a yellow and a blue, so that a planet circulating round one of them
may have the variety of a red day and a green day, a yellow day and a
blue day. Sir John Herschel observes, in one of his papers in the
Philosophical Transactions, as a very remarkable fact, that, although
red stars are common enough, no example of a solitary blue, green, or
purple star has yet been produced.
Sirius is the only star on record whose colour has changed. In the time
of Ptolemy it was red; now it is one of the whitest stars in the
heavens.
M. Struve has found that, out of 596 bright double stars, 375 pairs have
the same intensity of light and colour; 101 pairs have different
intensity, but the same colour; and 120 pairs have the colours of the
two stars decidedly different.
Certain rays, which exist in the sun’s light, are wanting in the spectra
of every coloured star, and probably never existed in the light of these
stars, as there is no reason to believe that they are absorbed by the
stars’ atmosphere, though they may be by the earth’s. There are no
defective rays in the white light of Sirius, Procyon, and others; but
Sir David Brewster found in the spectrum of the orange-coloured light of
ζ Herculis a defective band in the red space, and two or more in the
blue; consequently, the orange colour of the star is owing to a want of
blue rays; for flames in which certain rays are wanting take the colour
of the predominating rays. If the black rays in the solar spectrum were
owing to the absorption of the sun’s atmosphere, the light from the
margin of his disc, having to pass through a greater thickness of it,
would exhibit deeper lines than that which comes from his centre; but,
as no difference is perceptible, it may be inferred that the analogous
bands in the light of the coloured stars are not due to the absorption
of their atmospheres, but that they arise from the different kinds of
combustion by which these bodies are lighted up.
All the ordinary methods fail for finding the parallax when the
distances of the stars are very great. An angle even of one or two
seconds, viewed in the focus of our largest telescopes, does not equal
the thickness of a spider’s thread, which makes it impossible to measure
such minute quantities with any degree of accuracy. In some cases,
however, the binary systems of stars furnish a method of estimating an
angle of even the tenth of a second, which is thirty times more accurate
than by any other means. From them the actual distances of some of the
more remote stars will ultimately be known.
Suppose that one star revolves round another in an orbit which is so
obliquely seen from the earth as to look like an ellipse in a horizontal
position, then it is clear that one-half of the orbit will be nearer to
us than the other half. Now, in consequence of the time which light
takes to travel, we always see the satellite star in a place which it
has already left. Hence, when that star sets out from the point of its
orbit which is nearest to us, its light will take more and more time to
come to us in proportion as the star moves round to the most distant
point in its orbit. On that account the star will appear to us to take
more time in moving through that half of its orbit than it really does.
Exactly the contrary takes place on the other half; for the light will
take less and less time to arrive at the earth in proportion as the star
approaches nearer to us; and therefore it will seem to move through this
half of its orbit in less time than it really does. This circumstance
furnishes the means of finding the absolute breadth of the orbit in
miles, and from that the true distance of the star from the earth. For,
since the greatest and least distances of the satellite star from the
earth differ by the breadth of its orbit, the time which the star takes
to move from the nearest to the remotest point of its orbit is greater
than it ought to be by the whole time its light takes to cross the
orbit, and the period of moving through the other half is exactly as
much less. Hence the difference between the observed times of these two
semi-revolutions of the star is equal to twice the time that its light
employs to cross its orbit; and, as we know the velocity of light, the
diameter of the orbit may be found in miles, and from that its whole
dimensions; for the position of the orbit with regard to us is known by
observation, as well as the place, inclination, and apparent magnitude
of its major axis, or, which is the same thing, the angle under which it
is seen from the earth. Since, then, three things are known in this
great triangle, namely, the base or major axis of the orbit in miles,
the angle opposite to it at the earth, and the angle it makes with the
visual ray, the distance of the satellite star from the earth may be
found by the most simple of calculations. The merit of having first
proposed this very ingenious method of finding the distance of the stars
is due to M. Savary; but, unfortunately, it is not of general
application, as it depends upon the position of the orbit, and a long
time must elapse before observation can furnish data, since the shortest
period of any revolving star that we know of is 30 years. Still the
distances of a vast number of stars may ultimately be made out in this
way; and, as one important discovery almost always leads to another,
their masses may thus be weighed against that of the earth or sun.
The only data employed for finding the mass of the earth, as compared
with that of the sun, are, the angular motion of our globe round the sun
in a second of time, and the distance of the earth from the sun in miles
(N. 233). Now, by observations of the binary systems, we know the
angular velocity of the small star round the great one; and, when we
know the distance between the two stars in miles, it will be easy to
compute how many miles the small star would fall through by the
attraction of the great one in a second of time. A comparison of this
space with the space through which the earth would descend towards the
sun in a second will give the ratio of the mass of the great star to
that of the sun or earth. According to M. Bessel, the weight of the two
stars of 61 Cygni is equal to half the weight of the sun. Little as we
know of the absolute magnitude of the fixed stars, the quantity of light
emitted by many of them shows that they must be much larger than the
sun. Dr. Wollaston determined the approximate ratio which the light of a
wax candle bears to that of the sun, moon, and stars, by comparing their
respective images reflected from small glass globes filled with mercury,
whence a comparison was established between the quantities of light
emitted by the celestial bodies themselves. By this method he found that
the light of α Lyræ is five and a half times greater than that of the
sun. Sir John Herschel reflected the moon’s light _totally_ by a prism,
which, concentrated by a lens, was compared directly with that of α
Centauri. After making allowance for the quantity of the moon’s light
lost in passing through the lens and prism, he found that the mean
quantity of light sent to the earth by a full moon exceeds that sent by
α Centauri in the proportion of 27,408 to 1. Now, Dr. Wollaston found
the proportion of the sun’s light to that of the full moon to be that of
801,072 to 1. Hence, the light sent to us by the sun is to that sent by
α Centauri as about twenty-two thousand millions to one. But, as the
parallax of α Centauri is 1ʺ, it really is two and a half times brighter
than the sun. The light of Sirius is four times that of α Centauri, but
its parallax is only 0ʺ·230: hence it has an intrinsic splendour 63·02
times that of our luminary. It is therefore estimated to be a hundred
times as large; so that, were Sirius in the earth’s place, its surface
would extend 150 times as far as the orbit of the moon. The light of
Sirius, according to the observations of Sir John Herschel, is 324 times
greater than that of a star of the sixth magnitude; if we suppose the
two to be really of the same size, their distances from us must be in
the ratio of 57·3 to 1, because light diminishes as the square of the
distance of the luminous body increases.
So many of the stars have proper motions altogether independent of the
annual rotation of the earth in its orbit, that it may be doubted
whether there be such a thing as a fixed star. Groombridge is the most
rapid known: it has a proper motion of 7ʺ of arc annually; α Centauri
moves at the rate of 3ʺ·58 annually, and 61 Cygni describes a line in
space of 5ʺ·12 in the same time. These motions are probably in curves,
but at the distance of the earth they will appear to be rectilineal for
ages to come. The motion of little more than five seconds of space,
which 61 Cygni describes annually, seems to us to be extremely small;
but at the distance of that star an angle of one second corresponds to
twenty-four millions of millions of miles; consequently the annual
motion of 61 Cygni is 120 millions of millions of miles, and yet, as M.
Arago observes, we call it a fixed star. From the same cause it is
evident that the crowding of the stars in the Milky Way may be apparent
only, and that the stars may be at vast distances from one another, and
no doubt are.
Were the solar system and the whole of the stars visible to us carried
forward in space by a motion common to all, like ships drifting in a
current, it would be impossible for us, moving with the rest, to
ascertain its direction. Sir William Herschel perceived that a great
part of the motions of the stars is only apparent, arising from a real
motion of the sun in a contrary direction. Among many discrepancies he
found that the stars in the northern hemisphere have a general tendency
to move towards a point diametrically opposite to λ Herculis, which he
attributed to a motion of the solar system in a contrary direction. For
it was evident to him, that the stars, from the effects of perspective
alone, would seem to diverge in the direction to which the solar system
was going, and would converge towards the space it had left, and that
there would be a regularity in these apparent motions which would
hereafter be detected. Since Sir William Herschel’s time the proper
motions of the stars have been determined with much greater accuracy,
and many have been added to the list by comparing the ancient and modern
tables of their places; his views have been established by four of the
greatest astronomers of the age, MM. Lundahles, Argelander, Otto Struve,
and Peters, who have clearly proved the motion of the sun from that of
the stars in the northern hemisphere, and Mr. Galloway has come to the
same conclusion from the motions of the stars in the southern hemisphere
(N. 234). The result is, that the sun, accompanied by all his attendant
planets, is moving at the rate of 422,424 miles—or over a space nearly
equal to his own diameter—in the course of a day, and that the motion is
directed towards a point in a line joining the two stars μ and π
Herculis at a quarter of the apparent distance of these two stars,
reckoning from π Herculis. This investigation was founded upon no law
assumed or observed, such as the circulation of all the stars of our
firmament about a common centre, though philosophers have speculated as
to the probability of such a motion in the sun and stars in the plane of
the Milky Way. Should the sun and his stellar companions be moving in a
nearly circular orbit, the centre of motion would be in the plane
passing through the sun perpendicular to the direction of his motion.
The constellations through which that great circle would pass are
Pisces, Australis, Pegasus, Andromeda, Perseus, &c. M. Argelander is of
opinion that the sun’s orbit is nearly in the plane of the Milky Way,
and, therefore, that its centre must probably be in Perseus, while M.
Mädler places it in the Pleiades, which seems to be inadmissible; but
the data are too uncertain at present to admit of any absolute
conclusion as to the sun’s orbit and the general motion of the stellar
firmament: for though the stars in every region of the sky tend towards
a point in Hercules, it is not yet known whether their motions are
uniform or variable, whether the sun’s motion be gradually changing, and
whether the stars form different independent systems, each having its
own centre of attraction, or if all obey one powerful controlling force
which pervades the whole universe. Accurate observations of the places
of a select number of stars of all dimensions in the Milky Way continued
for a series of years would no doubt decide this point.
The proper motion of a star combined with the progressive velocity of
light alters the apparent periodic time of the revolving star of a
binary system. If the orbit of a double star be at right angles to the
visual ray, and both the sun and the star at rest, the periodic time of
the revolving star, say of 10,000 days, would always be the same. But if
the centre of gravity of the star were to recede in a direct line from
the sun with the velocity of one tenth of the radius of the earth’s
orbit in a day, then at the end of 10,000 days it would be more remote
from us by 1000 of such radii—a space light would take 57 days to
traverse: hence, although the periodic time of the star would really be
the same, the completion of its period would only be known to us 57 days
after it had taken place, so that the periodic time would appear to us
to be 10,057 days instead of 10,000. Were the star to approach to the
sun by the same quantity instead of receding, the apparent periodic time
would be diminished by 57 days.
As the sun is only a unit in the stellar system, so the Milky Way, and
all the stars that adorn the firmament of both hemispheres, constitute a
group which is but a unit among the infinite numbers of starry clusters
and nebulæ that are profusely scattered throughout the universe.
By the aid of a good telescope there may be seen on the clear vault of
heaven, between the stars of our own stellar system, and far in the
depths of space, an immense multitude of objects like comets or clouds
of white vapour of all forms and sizes. Some are mixed with stars,
others are entirely formed of them. Many appear as if they were stellar,
but required a telescope of higher power to resolve them, and vast
numbers appear to be matter rarefied in the highest possible degree,
giving no indication of a stellar nature; and these are in every state
of condensation, from a vague film hardly to be discerned to such as
have actually arrived at a solid nucleus of stars. The cloudy appearance
is merely the blending of the rays of innumerable stars which are
themselves invisible from their extreme distance, like parts of the
Milky Way. Sir William Herschel was at first of that opinion, and the
nebulæ that have been resolved by Lord Rosse’s telescope have led
astronomers to believe that such is the case. Yet the tails of comets,
the zodiacal light, and the extensive luminous atmospheres which
encompass many of the stars, show that, in all probability, a
self-luminous phosphorescent material substance in a highly diluted or
gaseous form exists in vast abundance.
The number of the nebulæ, like that of the stars, is only limited by the
imperfection of our instruments, for each improvement in the telescope
only enables us to penetrate a little farther into the infinity of
space—to see a few more of these shadowy existences in the far distance,
and to resolve a few more of those that are comparatively near. Sir
William Herschel examined the nature and determined the position of 2500
nebulæ in the northern hemisphere whose places were computed from his
observations, reduced to a common epoch, and arranged into a catalogue,
in order of right ascension, by his sister, Miss Caroline Herschel, who
added lustre to the name she bore by her eminence in astronomical
knowledge and discovery. Sir John Herschel revised his father’s
observations, and added 800 nebulæ to the catalogue before he went to
the Cape of Good Hope, in order to complete the survey of the heavens.
On his return he published a catalogue of 2049 nebulæ of the southern
hemisphere, of which 500 were previously unknown, with their position in
the heavens. In a work unparalleled for elegance of style, depth of
knowledge, and originality of views, he has given engravings from his
drawings of the most remarkable objects, so that whatever changes may
take place in their form, place, or condensation, will be known by
astronomers of future ages.
Though infinite in variety, the nebulæ are of two distinct classes; one
consists of patches of great dimensions, capriciously irregular,
assuming all the fantastic forms of clouds, now bright, now obscure;
sometimes like vapour flying before the wind; sometimes stretching long
arms into space. Many present an ill-defined surface, in which it is
difficult to say where the centre of the greatest brightness is. Large
portions are resolvable into stars; many have a granulated appearance,
as if they were resolvable; and others probably are not so merely from
the smallness and closeness of the stars, and possibly from their
remoteness, indicating the complex and irregular form the Milky Way
would present if seen from a distance. A wonderful nebula of this kind
is visible to the naked eye in the constellation of Orion; it is of vast
extent, sending branches even into the southern hemisphere; and,
although Lord Rosse’s telescope has resolved much that had hitherto
resisted others, there are parts that still maintain their nebulous
appearance from extreme remoteness, presenting a kind of mottled aspect,
like flocks or wisps of wool, or mackerel sky. There can be no doubt of
its being an unfathomable congeries of stars, which there is reason to
believe has changed its form in some parts within the last fifty years.
Vast multitudes of nebulæ of this kind are so faint as to be with
difficulty discerned at all till they have been for some time in the
field of the telescope, or are just about to quit it. Occasionally they
are so vague, that the eye is conscious of something being present,
without being able to define what it is; but the unchangeableness of its
position convinces the mind that it is a real object—“an image was
before mine eyes, but I could not discern the form thereof.”
No drawing can give an idea of the boundaries of such nebulæ as that of
Orion; even with Lord Rosse’s telescope the edges either fade into a
luminous mist, which becomes more rare till it is imperceptible, or end
in a tissue of faintish flocculi, or in filaments which become finer and
more scattered till they cease to be visible, showing that the real
boundaries have not been seen.
The other class of nebulæ, vastly inferior in size, of definite forms
and great variety of character, are scattered through the remote
heavens, or congregated in a great nebulous district far from the Milky
Way. Many cling to stars like wisps of clouds, others are exactly like
comets with comæ and tails; but the most definite forms are annular and
lenticular nebulæ, nebulous stars, planetary and elliptical nebulæ, and
starry clusters. However, there are two in the northern hemisphere
differing from all of these, which are described by Sir John Herschel as
amazing objects. One in Vulpecula is like an hourglass or dumb bell of
bright matter, surrounded by a thin hazy atmosphere so as to give the
whole an oval form, or the appearance of an oblate spheroid; with a
higher optical power its form is much the same, but the brighter part is
resolved into stars, and the hazy part, though still nebulous, assumes
that mottled appearance which shows that the whole is a stellar system
of the most peculiar structure: it is a phenomenon that bears no
resemblance to any known object. (Fig. 3, plate 8, and fig. 3, plate 9).
The other is indeed most wonderful, and its history shows the gradual
increase in the space-penetrating power of telescopes. To Messier it
appeared merely to be a double nebula with stars; with Sir William
Herschel’s telescope it presented the appearance of a bright round
nebula encompassed at a little distance by a halo or glory, and
accompanied by a companion; while in Sir John Herschel’s 20 feet
reflector it appeared to “consist of a bright round nucleus, surrounded
at a distance by a nebulous ring split through half its circumference,
and having the split portions separated at an angle of 45 degrees each
to the plane of the other.” (Fig. 1, plate 10.) This nebula appeared to
Sir John to “bear a strong similitude to the Milky Way, suggesting the
idea of a brother system bearing a real physical resemblance and strong
analogy of structure to our own.”
This object, which disclosed to Lord Rosse the astonishing phenomenon of
spiral nebulæ seen in his telescope, presents the appearance of the fig.
1 in plate 10, in which the partial division of the limb of the ring
into two branches is at once recognised in the bright convolutions of
the spiral. The outlying nebula is connected by a narrow curved band of
light with the ring; the whole is either resolved into stars, or
evidently might be with a still higher optical power. With regard to the
marvellous nebula in question Lord Rosse observes, that “with each
increase of optical power the structure has become more complicated, and
more unlike anything that could result from any form of a dynamical law
of which we find a counterpart in our system. The connection of the
companion with this great nebula, of which there is not the least doubt,
adds to the difficulty of forming any hypothesis. It is impossible that
such a system could exist without internal movement, to which may be
added a resisting medium; but it cannot be regarded as a case of mere
static equilibrium.” This is by no means the only instance of a spiral
nebula; Lord Rosse has discovered several others: some are easily
seen—others require the highest powers of his telescope. From the
numerous offsets that branch from the Milky Way and run far into space,
it may possibly partake also of the spiral form.
There are seven annular nebulæ in the northern hemisphere, since Lord
Rosse has discovered that five of the planetary nebulæ belong to this
class. One of the finest examples of an annular nebula is to be seen
midway between β and γ Lyræ (fig. 2, plate 9). According to Sir John
Herschel, it is elliptical in the ratio of 4 to 5, and is sharply
defined—the internal opening occupying about half the diameter. This
opening is not entirely dark, but filled with a faint hazy light like
fine gauze stretched over a hoop. Its diameter, if it is as far from us
as 61 Cygni, must be 1300 times greater than the diameter of the earth’s
orbit—dimensions that are most astounding. Lord Rosse’s telescope
resolves this object into stars of extreme minuteness, with filaments of
stars adhering to its edges and a pretty bright star in its interior.
These rings are like hollow shells whose borders seem brighter because
the nebulous substance, whatever it may be, is more condensed to
appearance than the central part. The other annular nebula in the
northern hemisphere described by Sir John Herschel is a small faint
object, and more easily resolvable into stars. One of the annular nebulæ
seen by Lord Rosse is surrounded by a faint external flat ring; another
has ansæ, as if an annular nebulous ring encompassed it and was
foreshortened. Two annular nebulæ have perforations as if the black sky
was seen through openings in the interior haze, for in no instance is
the central opening quite dark.
Some nebulæ are like very elliptical annular systems seen obliquely. If
they be elliptical flat rings, the dark centre may be a real opening;
but should the systems be a series of very long elliptical concentric
shells surrounding a hollow, the dark axis may be merely a line of
comparative darkness.
The connection of the elliptical nebulæ with double stars is mentioned
as very remarkable. In one elliptical nebula whose longer axis is 50ʺ
there are two individuals of a double star each of the 10th magnitude
symmetrically placed rather nearer the vertex of the ellipse than the
foci; in another the stars are unequal, but placed exactly at the
extremities of the major axis, as in plate 8: besides these there are
several other instances.
Double nebulæ are not unfrequent in both hemispheres, exhibiting all the
varieties of distance, position, and relative brightness, with their
counterparts the double stars. The rarity of single nebulæ as large,
faint, and as little condensed in the centre as these, makes it
extremely improbable that two such bodies should be accidentally so near
as to touch, and often in part to overlap each other, as these do. It is
much more likely that they constitute systems; and, if so, it will form
an interesting object of future inquiry to discover whether they possess
orbital motion.
Nebulous stars are beautiful objects, quite different from all the
preceding. They are round or oval, increasing in density towards the
centre. Sometimes the central matter is so vividly and sharply condensed
and defined that the nebula might be taken for a bright star surrounded
by a thin atmosphere. One is a star of the 8th magnitude exactly in the
centre of a round bright atmosphere 25ʺ in diameter; the star is quite
stellar, and not a nucleus: it has not the smallest appearance of being
resolvable. Another nebulous star is ι Orionis, which has a broad
atmosphere in which is a dark cavity not symmetrical with the star, and
a small double star with a similar opening on the edge of the
atmosphere. Lord Rosse observes that these openings appear to be of the
same nature with that within the bright stars in the trapezium of Orion,
the stars being at its edge; and he is convinced that the stars are not
only connected with the nebula, but that they are equidistant with it;
hence, if their parallax can be found, the distance of this nebula would
be determined. The zodiacal light or lenticular shaped luminous haze
surrounding the sun which may be seen extending beyond the orbits of
Mercury and Venus soon after sunset in the months of April and May, or
before dawn in November and December, seems to place our luminary in the
class of nebulous stars. The extensive and delicate atmosphere of these
nebulous stars assumes all degrees of ellipticity, from the circular to
the spindle-shaped ray, or almost the right line.
Planetary nebulæ have exactly the appearance of planets with round or
oval discs, sometimes sharply terminated, at other times hazy and
ill-defined. Their surface, which is blue or blueish white, is equable
or slightly mottled, and their light occasionally rivals that of the
planets in vividness. They are generally attended by minute stars, which
give the idea of accompanying satellites. These nebulæ are of enormous
dimensions. One near γ Aquarii has a sensible diameter of about twenty
seconds, and another presents a diameter of twelve. Sir John Herschel
has computed that, if these objects be as far from us as the stars,
their real magnitude, on the lowest estimation, must be such as would
fill the orbit of Uranus. He concludes that, if they be solid bodies of
a solar nature, their intrinsic splendour must be far inferior to that
of the sun, because a circular portion of the sun’s disc subtending an
angle of twenty seconds would give a light equal to that of a hundred
full moons; while, on the contrary, the objects in question are hardly,
if at all, visible to the naked eye. From the uniformity of the discs of
these planetary nebulæ, and their apparent want of condensation, he
presumes that they may be hollow shells emitting light from their
surface only. The southern hemisphere is very rich in them, where
twenty-eight or twenty-nine have been discovered, some in the midst of a
cluster of stars, with which they form a beautiful contrast. Three are
of a decided blue colour, one Prussian blue, or verditer green, the
other two of a bright sky blue, of great beauty and delicacy. One seems
to belong to the class of double nebulæ or double stellar nebulæ of the
utmost remoteness. Since Lord Rosse’s telescope has shown that five of
the planetary nebulæ are annular, some of those in the southern
hemisphere may ultimately be found to belong to the same class.
Probably nine tenths at least of the nebulous contents of the heavens
consist of spherical or elliptical forms presenting every variety of
elongation and central condensation. Of these a great number have been
resolved into stars, and a great many present that mottled appearance
which renders it certain that an increase of optical power would
decompose them. Those which resist do so on account of the smallness and
closeness of the stars of which they consist.
Elliptical nebulæ are very common; by much the finest may be seen near
the star υ in the girdle of Andromeda. It is visible to the naked eye,
and has frequently been taken for a comet. With a low optical power it
has the spindle-shaped form of fig. 6, plate 5, the brightness being at
first gradually and then rapidly condensed towards the centre, so that
it has been compared to a star shining through horn, but had never
appeared resolvable even with high optical powers till Mr. Bond examined
it at the observatory of Cambridge in the United States. He found that
its brightness extends over 2-1/2 degrees in length, and more than a
degree in breadth, including two small adjacent nebulæ, so that it is
oval. It is strongly and rapidly condensed into a nucleus on its
northern side; and although it was not all resolved, it was seen to be
strewed over with star dust, or extremely minute visible stars, which
leaves not a doubt of its being a starry system. The most remarkable
part of Mr. Bond’s discovery are two very narrow dark lines which extend
along one side of the oval parallel to its major axis. These black
streaks, difficult to distinguish, indicate a stratified structure, and
are not the only instance of that arrangement in nebulæ. Fig. 1, in
plate 9, is from Mr. Bond’s drawing of this nebula.
Multitudes of nebulæ appear to the unassisted eye, or are seen with
ordinary telescopes, like round comets without tails; but when viewed
with powerful instruments they convey the idea of a globular space,
insulated in the heavens and full of stars, constituting a family or
society apart from the rest, subject only to its own internal laws. To
attempt to count the stars in one of these globular clusters, Sir John
Herschel says, would be a vain task; they are not to be reckoned by
hundreds. On a rough computation, it appears that many clusters of this
description must contain ten or twenty thousand stars compacted and
wedged together in a round space, whose apparent area is not more than a
tenth part of that covered by the moon; so that its centre, where the
stars are seen projected on each other, is one blaze of light. If each
of these stars be a sun, and if they be separated by intervals equal to
that which separates our sun from the nearest fixed star, the distance
which renders the whole cluster barely visible to the naked eye must be
so great, that the existence of such a splendid assemblage can only be
known to us by light which must have left it at least a thousand years
ago. These magnificent globular or spheroidal aggregates of stars are so
arranged that the interior strata are more crowded and become more
nearly spherical as they approach the centre. A most splendid object of
this nature may be seen in the constellation Hercules (N. 235).
Of 131 of these magnificent objects in the southern hemisphere, two of
them are pre-eminently splendid. The globular cluster of α Centauri is
beyond comparison the finest of its kind: it is perfectly spherical, and
occupies a quarter of a square degree; the stars in it are literally
innumerable, crowding and densely aggregated towards the centre; and, as
its light is not more to the naked eye than that of a star of the 4th or
5th magnitude, their minuteness is extreme. It has a dark hole in its
centre, with a bridge of stars across,—a circumstance peculiar to this
cluster.
Lacaille’s globular cluster, or 47 Toucani, is completely insulated in a
very dark part of the sky not far from the lesser of the Magellanic
clouds. The stars, which are of the 14th magnitude, immensely numerous,
compressed and white, form three distinct stages round a centre, where
they suddenly change in hue, and form a blaze of rose-coloured light.
One cluster consists of large ruddy stars and small white ones; another
of greater beauty consists of shells or coats of stars of the 11th and
15th magnitude. There are thirty globular clusters of extreme beauty
collected within a circular space of not more than eighteen degrees
radius, which lies in the part of the sky occupied by the constellations
Corona Australis, the body and head of Sagittarius, the tail of Scorpio,
part of Telescopium and Ara. The Milky Way passes diametrically across
the circular area in question, which gives prodigious brilliancy to this
part of the sky. For besides these globular clusters, which all lie in
the starry part, and not in the dark spaces, there are the only two
annular nebulæ known to exist in the southern hemisphere. No part of the
heavens is fuller of objects beautiful and remarkable in themselves, and
rendered still more so by their mode of association, and by the peculiar
features assumed by the Milky Way, which are without a parallel for
richness and magnificence in any other part of the sky. Some of the
globular clusters are so remote that the stars are scarcely
discernible—mere star dust. There is a double globular cluster in the
southern hemisphere of very small dimensions, separated by a minute
interval,—a combination which suggests the idea of a globular cluster
revolving about a very oblate spheroidal one in the plane of the
equator, and in an orbit which is circular, and seen obliquely like the
central nebula itself, with a diameter somewhat more than four times the
latter,—a stupendous system doubtless, but of which the reality can
hardly be supposed improbable.
There appears to be some connexion between ellipticity of form and
difficulty of resolution, for spherical clusters are in general easily
resolved into their component stars, while there is scarcely an instance
of an elliptical cluster yielding except to a very high optical power.
Vast masses of the nebulæ have never been resolved. Lord Rosse’s great
telescope has resolved parts of the nebula of Orion, and various others
which had not yielded to instruments of less power; it enables the
astronomer to penetrate farther into space, and shows objects with
greater clearness, than any other. But, excellent as this instrument is,
thousands of nebulæ are not to be resolved even by it. Those who imagine
that any work of man can resolve all the nebulous matter in the heavens
must have a very limited idea of the extent and sublimity of creation.
Innumerable nebulæ in both hemispheres take the form of clusters of
stars, but are totally different from the globular clusters, inasmuch as
they are of irregular form and follow no uniform law of condensation.
The Pleiades is an instance in our own stellar system; for although only
7 or 8 stars are visible to the naked eye, telescopes show that more
than 200 belong to the group. In the constellation Cancer there is a
luminous spot called the Præsepe or Beehive, which a very low power
resolves into stars; and the constellation Coma Berenices is another
stellar group. Many are of exquisite beauty, as that round α Crucis,
which, though consisting of only a hundred and ten stars, is like a
piece of fancy jewellery, from the colours of the stars, which are
greenish white, green, blueish green, and red. Many of these clusters
contain thousands of stars, and are frequently in the poorer parts of
the sky, as if in the course of ages the stars had been attracted
towards a centre.
The existence of every degree of ellipticity in the nebulæ—from long
lenticular rays to the exact circular form—and of every shade of central
condensation, from the slightest increase of density to apparently a
solid nucleus—may be accounted for by supposing the general
constitutions of those nebulæ to be that of oblate spheroidal masses of
every degree of flatness from the sphere to the disc, and of every
variety in their density and ellipticity towards the centre. It would be
erroneous, however, to imagine that the forms of these systems are
maintained by forces identical with those already described, which
determine the form of a fluid mass in rotation; because, if the nebulæ
be only clusters of separate stars, as in the greater number of cases
there is every reason to believe them to be, no pressure can be
propagated through them. Consequently, since no general rotation of such
a system as one mass can be supposed, it may be conceived to be a
quiescent form, comprising within its limits an indefinite number of
stars, each of which may be moving in an orbit about the common centre
of the whole, in virtue of a law of internal gravitation resulting from
the compound gravitation of all its parts. Sir John Herschel has proved
that the existence of such a system is not inconsistent with the law of
gravitation under certain conditions.
The distribution of the nebulæ over the heavens is even more irregular
than that of the stars. In some places they are so crowded together as
scarcely to allow one to pass through the field of the telescope before
another appears, while in other parts hours elapse without a single
nebula occurring. They are in general only to be seen with the best
telescopes, and are most abundant in a zone whose general direction is
not far from the hour circles 0^h and 12^h, and which crosses the
Milky Way nearly at right angles. Where that nebulous zone passes over
the constellations Virgo, Coma Berenices, and the Great Bear, they are
to be found in multitudes.
The nebulous system is nearly divided into two parts by the Milky Way.
One-third of the whole visible nebulous contents of the heavens forms a
broad irregular mass, interspersed with vacant intervals, which fills
about an eighth of the surface of the northern hemisphere. It occupies
the constellations Leo, Leo Minor, the body, tail, and hind legs of Ursa
Major, the nose of Camelopard, the point of the tail of Draco, Canis
Venatica, Coma Berenices, the preceding leg of Boötes, and the head,
wings, and shoulder of Virgo, which is the richest part. There is a
lesser nebulous region in this hemisphere, but entirely separated from
the preceding, which occupies the chest and wing of Pegasus, the
constellations Pisces and Andromeda. If we could imagine the ring or
zone of the Milky Way to encircle or coincide with the horizon, the
great nebulous mass would form a canopy over head, descending down to a
considerable distance on all sides, chiefly towards the north pole; and
the richest part, which is in Virgo, would then be directly over head in
the north pole of the Milky Way, that is in 12^h 47^m right ascension,
and 64° north polar distance.
With the exception of the Magellanic clouds, there is a much greater
uniformity in the distribution of the nebulæ in the southern hemisphere
than in the northern. They are separated by spaces of vacuity of greater
or less dimensions. One of these barren regions extends for nearly
fifteen degrees all around the south pole, and close on its border; the
lesser of the Magellanic clouds occurs completely insulated; while the
greater Magellanic cloud is in connexion with something approaching to a
zone of connected nebulous patches which extends along the back of
Doradus, through a portion of Horologium and Eridanus, part of Fornix,
and over the paws of Cetus to the equator, where it unites with the
nebulous regions of Pisces.
The Magellanic clouds form two of the most striking features in the
southern hemisphere; both of these nebulæ are visible to the unassisted
eye, being nearly of the same intensity as the brighter portions of the
Milky Way; but the smaller is entirely effaced by moonlight, and the
larger nearly so. They are altogether unconnected with the Milky Way and
with one another. The Nubecula Major is far superior to the Nubecula
Minor in every respect, though they are similar in internal structure.
The former consists of large tracts and ill-defined patches of
irresolvable nebulæ, and nebulosity in every stage of resolution, up to
perfectly resolved stars like the Milky Way; and also of regular and
irregular nebulæ, properly so called; of globular clusters in every
stage of resolvability; and of clustering groups sufficiently insulated
and condensed to come under the designation of clusters of stars. Of
these the nebula known as Lacaille’s 30 Doradus is too remarkable to be
passed over. It is very large, situate within the Nubecula Major, and
consists of an assemblage of nearly circular loops uniting in a centre,
in or near which there is a circular black hole. In short, for the
number and variety of the objects, there is nothing like this cloud.
Within an area of only forty-two square degrees, Sir John Herschel has
determined the places, and registered 278 nebulæ and clusters of stars,
with fifty or sixty in outlying members immediately adjacent. Even the
most crowded parts of the stratum of Virgo, in the wing of that
constellation, or in Coma Berenices, offer nothing approaching to it. It
is evident, from the intermixture of stars and unresolved nebulosity,
which probably might be resolved with a higher optical power, that the
nubeculæ are to be regarded as systems _sui generis_, to which there is
nothing analogous in our hemisphere.
Next to the Magellanic clouds the great nebula round η Argûs is one of
the most wonderful objects of the southern sky. It is situate in that
part of the Milky Way which lies between the Centaur and the body of
Argus, in the midst of one of those rich and brilliant masses, a
succession of which is so curiously contrasted with the profoundly dark
adjacent spaces, and surrounded by one of the most beautiful parts of
the southern heavens. Sir John Herschel says: “It would be impossible,
by verbal description, to give any just idea of the capricious forms and
irregular gradations of light affected by the different branches and
appendages of this nebula. Nor is it easy for language to convey a full
impression of the beauty and sublimity of the spectacle it offers when
viewed in a sweep, ushered in as it is by so glorious and innumerable a
procession of stars, to which it forms a sort of climax, justifying
expressions which, though I find them written in my journal in the
excitement of the moment, would be thought extravagant if transferred to
these pages. In fact, it is impossible for any one, with the least spark
of astronomical enthusiasm about him, to pass soberly in review with a
powerful telescope, and on a fine night, that portion of the southern
sky which is comprised between the 6th and 13th degrees of right
ascension, and from 146° to 149° of north polar distance; such are the
variety and interest of the objects he will encounter, and such the
dazzling richness of the starry ground on which they are represented to
his gaze.” In that portion of the sky there are many fine double
stars—rich starry clusters; the elegant cluster of variously coloured
stars round κ Crucis; a large planetary nebula with a satellite star;
another of a bright blue colour, exquisitely beautiful and unique; and,
lastly, η Argûs itself, the most extraordinary instance of a variable
star in astronomical history.
It frequently occurred, during Sir John Herschel’s survey of the
southern heavens, that some parts of the sky were noted for deeper
blackness than others, and no stars could be seen; it frequently
happened that far from the Milky Way, or any large nebula or cluster of
stars, there were some indications of very remote branches of the Milky
Way, or of an independent sidereal system or systems, bearing a
resemblance to such branches. These were indicated by an exceedingly
delicate and uniform dotting or stippling of the sky by points of light
too small to admit of any one of them being steadily and fixedly viewed,
and too numerous to be counted even if possible to view them. The truth
of this existence was felt at the moment of observation; but the
conviction, though often renewed, was not permanent. The places where
these appearances occurred are given, in order that those who wish to
verify them may have it in their power.
Such is a brief account of a very few of the discoveries contained in
Sir John Herschel’s great work on the Nebulæ and other Phenomena of the
Southern Hemisphere,—a work which will rise in estimation with the lapse
of years. No doubt the form and internal structure of many of these
nebulæ will be changed by telescopes of higher power; but as the places
of the leading phenomena have been determined, the date of that great
work may be regarded as the epoch of nebular time whence the relative
changes that take place in the most distant regions of the universe will
be estimated for ages to come; and in the inimitable writings of the
highly gifted father and son the reader will find these subjects treated
of in a style worthy of it and of them. Of late years the excellence of
the instruments, and still more of the astronomers, in the foreign
observatories, have aided the progress of sidereal astronomy immensely.
Nor has it been cultivated with less success in our home and colonial
establishments: certainly one of the most remarkable features of the
times is the number of private observatories, built and furnished with
the best instruments by private gentlemen, whose zeal has been rewarded
by eminent success in all departments of the science. (N. 236.)
So numerous are the objects which meet our view in the heavens, that we
cannot imagine a point of space where some light would not strike the
eye;—innumerable stars, thousands of double and multiple systems,
clusters in one blaze with their tens of thousands of stars, and the
nebulæ amazing us by the strangeness of their forms and the
incomprehensibility of their nature, till at last, from the limit of our
senses, even these thin and airy phantoms vanish in the distance. If
such remote bodies shone by reflected light, we should be unconscious of
their existence. Each star must then be a sun, and may be presumed to
have its system of planets, satellites, and comets, like our own; and,
for aught we know, myriads of bodies may be wandering in space unseen by
us, of whose nature we can form no idea, and still less of the part they
perform in the economy of the universe. Even in our own system, or at
its farthest limits, minute bodies may be revolving like the telescopic
planets, which are so small that their masses have hitherto been
inappreciable, and there may be many still smaller. Nor is this an
unwarranted presumption; many such do come within the sphere of the
earth’s attraction, are ignited by the velocity with which they pass
through the atmosphere, but leave no residuum. These, which are known as
falling stars and meteors, are periodical; but that is by no means the
case with aërolites, which are also ignited by the sudden condensation
of the air on entering our atmosphere, and are precipitated in solid
masses with such violence on the earth’s surface that they are often
deeply buried in the ground.
The fall of meteoric stones is much more frequent than is generally
believed. Hardly a year passes without some instances occurring; and, if
it be considered that only a small part of the earth is inhabited, it
may be presumed that numbers fall in the ocean, or on the uninhabited
part of the land, unseen by man. They are sometimes of great magnitude;
the volume of several has exceeded that of the planet Ceres, which is
about 70 miles in diameter. One which passed within 25 miles of us was
estimated to weigh about 600,000 tons, and to move with a velocity of
about 20 miles in a second; a fragment of it alone reached the earth.
The obliquity of the descent of meteorites, the peculiar substances they
are composed of, and the explosion accompanying their fall, show that
they are foreign to our system; but whence derived is still a mystery.
Shooting stars and meteors burst from the clear azure sky, and, darting
along the heavens, are extinguished without leaving any residuum except
a vapour-like smoke, and generally without noise. Their parallax shows
them to be very high in the atmosphere, sometimes even beyond its
supposed limit, and the direction of their motion is for the most part
diametrically opposite to the motion of the earth in its orbit. The
astonishing multitudes of shooting stars and fire-balls that have
appeared at stated periods over different parts of the globe, warrant
the conclusion that there is either a nebula or that there are myriads
of bodies revolving in groups round the sun which only become visible
when inflamed by entering our atmosphere.
One of these nebulæ or groups seems to meet the earth in its annual
revolution on the 12th and 13th of November.
On the morning of the 12th of November, 1799, thousands of shooting
stars, mixed with large meteors, illuminated the heavens for many hours
over the whole continent of America, from Brazil to Labrador: it
extended to Greenland, and even Germany. Meteoric showers were seen off
the coast of Spain, and in the Ohio country, on the morning of the 13th
of November, 1831; and during many hours on the morning of the 13th
November, 1832, prodigious multitudes of shooting stars and meteors fell
at Mocha on the Red Sea, in the Atlantic, in Switzerland, and at many
places in England. But by much the most splendid meteoric shower on
record began at nine o’clock in the evening of the 12th of November,
1833, and lasted till sunrise next morning. It extended from Niagara and
the northern lakes of America to the south of Jamaica, and from 61° of
longitude in the Atlantic to 100° of longitude in central Mexico.
Shooting stars and meteors, of the apparent size of Jupiter, Venus, and
even the full moon, darted in myriads towards the horizon, as if every
star in the heavens had started from their spheres. They are described
as having been frequent as flakes of snow in a snow-storm, and to have
been seen with equal brilliancy over the greater part of the continent
of North America.
Those who witnessed this grand spectacle were surprised to see that
every one of the luminous bodies, without exception, moved in lines
which converged in one point in the heavens: none of them started from
that point; but their paths, when traced backwards, met in it like rays
in a focus, and the manner of their fall showed that they descended from
it in nearly parallel straight lines towards the earth.
By far the most extraordinary part of the whole phenomenon is, that this
radiant point was observed to remain stationary near the star γ Leonis
for more than two hours and a half, which proved the source of the
meteoric shower to be altogether independent of the earth’s rotation,
and its parallax showed it to be far above the atmosphere.
As a body could not be actually at rest in that position, the group or
nebula must either have been moving round the earth or the sun. Had it
been moving about the earth, the course of the meteors would have been
tangential to its surface; whereas they fell almost perpendicularly, so
that the earth in its annual revolution must have met with the group.
The bodies or the parts of the nebula that were nearest must have been
attracted towards the earth by its gravity, and, as they were estimated
to move at the rate of fourteen miles in a second, they must have taken
fire on entering our atmosphere, and been consumed in their passage
through it.
As all the circumstances of the phenomena were similar on the same day
and during the same hours in 1832, and as extraordinary flights of
shooting stars were seen at many places both in Europe and America on
the 13th of November, 1834, 1835, and 1836, tending also from a fixed
point in the constellation Leo, it has been conjectured, with much
apparent probability, that this nebula or group of bodies performs its
revolution round the sun in a period of about 182 days, in an elliptical
orbit, whose major axis is 119 millions of miles; and that its aphelion
distance, where it comes in contact with the earth’s atmosphere, is
about 95 millions of miles, or nearly the same with the mean distance of
the earth from the sun. This body must have met with disturbances after
1799, which prevented it from encountering the earth for 32 years, and
it may again deviate from its path from the same cause.
It is now well ascertained that great showers of shooting stars occur
also on the 12th of August, whose point of divergence is β
Camelopardali, so that the earth’s atmosphere comes into contact with a
zone of these small bodies twice in the year. By a systematic series of
observations, MM. Benzenberg and Brand have clearly made out that the
heights at which the falling stars appear and vanish vary from 16 miles
to 140, and their velocities from 18 to 36 miles in a second, velocities
so great as certainly to indicate a planetary revolution round the sun.
As shooting stars are seen almost every night when the sky is clear, Sir
John Lubbock has thought it probable that some of these bodies may have
come so near, that the attraction of the earth has overcome that of the
sun, and caused them to revolve as satellites round it. Should that be
the case, they might shine by the reflected light of the sun, and
suddenly cease to be visible on entering the earth’s shadow. The
splitting of the falling stars like a rocket, and the trains of light,
may be accounted for by supposing the stars to graze the surface of the
shadow before being eclipsed; and the disappearance would be more or
less rapid according to the breadth of the penumbra traversed. The
calculations of M. Petit, Director of the Observatory of Toulouse, not
only render probable the existence of small satellites, but tend to
establish the identity of a body revolving round the earth in three
hours and twenty minutes, at a distance of 5000 miles above its surface.
It is evident that in this case the same satellite would be seen very
often, and a very few would be sufficient to account for their nightly
appearance. It is possible, however, that some shooting stars may belong
to one class, and some to the other, since one group may be revolving
about the sun, and another round the earth. In the case of a satellite
shooting star, geometry furnishes the means of ascertaining its exact
distance from the spectator, or from the centre of the earth, if the
time and place of its disappearance be known with regard to the
neighbouring stars. Since the falling stars are consumed in the
atmosphere, their masses must be small, but it is possible that
occasionally one may be large enough to arrive at the surface of the
earth as an aërolite.