and has been observed since A.D. 830: it radiates from a point in the
constellation Perseus. Schiaparelli calculated in 1866 the orbit and
motion of the meteorites producing it, and was surprised to find that
the numbers corresponded exactly with those calculated for one of the
recently observed comets; in other words, a comet was moving in the
path of the meteorites, and at exactly the same speed. At the same time
Schiaparelli gave numbers defining the motions of the meteorites which
would cause the periodic November star-showers.
[Sidenote: Star-showers related to comets.]
59. Immediately afterwards, when the numbers calculated by Oppolzer
for the orbit of the comet discovered by Tempel were published, it was
seen that they were really identical with those already calculated
by Schiaparelli for the orbit of the meteorites of the November
star-shower, and that here again a comet and a swarm of meteorites were
moving in exactly the same path at exactly the same rate.
Almost immediately afterwards it was shown that the radiant points
of the small star-showers of April 20-21 and November 27-28 both
correspond to the orbits of known comets.
It was evident that these could not be accidental coincidences, and
that the comets and the attendant swarms of meteorites are closely
related to each other.
[Sidenote: Comets.]
60. An intimate connection between, if not complete identity of,
meteorites, shooting stars and comets, had indeed long been suspected.
Astronomers were convinced that comets, though occasionally of enormous
size, are always of extremely small mass, since they pass by the
earth and other planets without sensibly disturbing their motions;
the comet of 1770 passed through the system of Jupiter's satellites
without any perceptible action upon them: it has been calculated that
the mass of a small comet may be about eight pounds. Again, the light
of a comet, like that of a cloud or planet, was seen to be partially
polarised: hence part, at least, must be reflected sunlight, for the
plane of polarisation passes through the sun's place. Further, stars
of very small magnitude have been seen not only through the tail, but
even through the nucleus, of a comet without any apparent alteration
of position by refraction: hence it was inferred that a comet is not
a continuous mass, but consists of particles so far distant from each
other that a ray of light may pass through the comet without meeting
a single one of them. Such a constitution likewise accounts for the
absence of phases of the reflected light: for although only half of
each particle will be directly illuminated by the sun, the remaining
half will receive light irregularly reflected from the particles more
distant from the sun.
Among others, Chladni in 1817 had referred to the great similarity in
the motions of comets and meteorites: Olmsted, in 1834, had calculated
the orbit of a comet which would cause the November star-shower; his
results were wrong owing to the assumption that the shower was annual:
Cappocci, in 1842, gave reasons for believing that a meteorite is a
small comet: Reichenbach, in 1858, in a most elaborate paper,[32]
sought to prove that a comet is a swarm of meteorites; that each
chondrule of a meteorite had once been an individual of a cometary
swarm, and owes its rounded shape to frequent collision with its
fellows; that the rest of the stone consists of the broken splinters
thus produced; and that the brecciated aspect of many meteorites
is due to collisions in the denser part or nucleus of a comet. As
already pointed out in Art. 51, later modes of investigation have led
petrologists to reject this method of accounting for the rotundity of
the chondrules.
[Sidenote: Other star-showers.]
61. In addition to the few radiant points which correspond to swarms
moving in orbits identical with those of known comets, there are
numerous radiant points which have not yet been recognised as related
to existing comets, and may possibly be due to swarms produced by the
dispersal of comets along their orbits; indeed, it has been inferred
from observation of shooting stars that on the average there are no
fewer than fifty distinct radiant points, and therefore showers, for
any night of the year. But there are still others of which there is
yet no satisfactory explanation. A cometary swarm is thin, and is
passed through in a few hours; the stars are seen to radiate from the
corresponding point of the sky for only that length of time: but there
are other radiant points which have a duration of several months, and
this is the case notwithstanding the constantly changing direction of
the earth's motion in space.[33] Since the position of the radiant
point in the sky as seen by a terrestrial observer depends not only on
the direction in which the swarm is moving, but also on the velocity
and direction of motion of the observer through space, it is easily
seen that a radiant point having a fixed position during some months
corresponds to something quite distinct from a cometary swarm. It
has been suggested by Mr. W. F. Denning (1899) that in some cases a
long-continued radiant point may really be due, not to a single swarm,
but to successive swarms not physically associated with each other.
On the other hand, Professor H. H. Turner has shown that the average
effect of the earth's attraction on a meteorite passing near it is to
change only the _position_ in our orbit at which we meet the meteorite
(i.e. the time of year), not the relative-direction of motion or the
relative speed; hence, a swarm of such meteorites must be spread
out, in the course of ages, into a succession of rings, all of them
equally inclined to the earth's orbit, but intersecting it at different
places; the radiant point will then be of long duration. Professor
A. S. Herschel[34] made the suggestion that the radiant points of
long duration may have resulted from the passage, in bygone epochs,
of quickly moving streams of cosmical matter through a ring of small
bodies circulating, as satellites, round the earth.
[Sidenote: Daily and yearly maxima of shooting stars.]
62. The rotation of the earth round its axis is such that the part
furthest from the sun, for which it is therefore midnight, is moving
in the same direction as the earth in its orbit; whence, at the part
of the earth most forward in the orbit it is sunrise, and at the part
most backward it is sunset. Thus, as Schiaparelli pointed out, the
meteorites which enter the atmosphere in the first half of the night
are more or less following the earth in its orbit, and have their
velocity relative to the earth diminished by the earth's own motion
of translation; they are thus less likely to produce shooting stars
than those which enter the atmosphere in the second half of the night
and are travelling more or less oppositely to the earth as it moves in
its orbit, and have their relative velocity increased. Hence, if the
directions of flight of meteorites were uniformly distributed in space,
the number of shooting stars hourly visible at one place, a number
which would be constant if the earth were at rest, would gradually vary
during the night, reaching a maximum about 3 A.M.
Also, as the point in space towards which the earth is moving in
its orbit varies in height above the horizon during the year, being
highest in autumn and lowest in spring, the number of shooting stars
hourly visible at one place will gradually vary from night to night,
reaching a maximum in the former season and a minimum in the latter,
if the directions of flight of the meteorites be themselves uniformly
distributed in space.
[Sidenote: The breaking up of comets.]
63. The history of Biela's comet[35] is of great interest as throwing
light on the relationship of comets and swarms of meteorites. Though
already observed in 1772 and in 1806, this comet was not recognised
as periodic till it was seen by Biela in 1826, when its orbit was
determined. On its returns in 1832 and 1845 it was found in its
calculated positions, but in the latter year was seen to be double, a
small comet being visible beside a larger one. Vast changes took place
during the time the companions were visible. The smaller one grew both
in size and brightness, each threw out a tail, the smaller threw out
a second tail, afterwards the larger showed two nuclei and two tails,
then the smaller became the brighter of the two companions; next three
tails were shown by the primary, and three cometary fragments were
visible round its nucleus. On the next return, in 1852, the two comets
were farther apart, one being more than a million miles ahead of the
other. The next favourable return was to be in 1866, and the orbit was
by this time so well known that the positions of the two companions
could be calculated beforehand with great precision; owing to the
changes which had been visibly taking place, the arrival of the comets
was looked forward to with great interest by astronomers. But neither
in 1866, nor on the next occasion in 1872, were they to be seen in
their calculated positions, and a careful examination of the whole sky
failed to lead to their discovery.
The connexion between several comets and meteoritic swarms having
in the meantime been established, it was now surmised that Biela's
comet might have been scattered along part of its path, and that
some evidence of the dispersal might perhaps be obtained on the next
occasion, November 27, 1872, of the passage of the earth across the
comet's orbit. In fact the star-shower of that date, with a radiant
point corresponding to the orbit of Biela's comet, was observed to be
much more dense than usual, the stars shooting across the sky at the
rate of a thousand an hour for several hours.
[Sidenote: Passage of the earth through a comet.]
64. Klinkerfues, a German astronomer, was struck with the idea that if
this star-shower were really due to the passage of the earth through
a moving swarm of meteorites, the latter might possibly be visible
as it departed from our neighbourhood. The swarm having come from
a radiant point in the northern sky, after passing the earth would
need to be sought near the opposite point in the southern sky; he
telegraphed, therefore, to the Madras observatory, asking Pogson, the
astronomer, to search for the swarm in the direction opposite to the
radiant point. The search was successful; on two mornings a small comet
was distinctly seen, and on the second morning it showed a tail with
an apparent length equal to one-fourth the apparent diameter of the
moon. Bad weather came on, and the comet got away without being again
seen. The two Madras observations agree with a motion in the orbit
of Biela's comet, and show that the earth had passed excentrically
through the small comet seen by Pogson. This small comet was probably
a third fragment of Biela's, for it was 200 million miles behind the
calculated position of the first two. From these two observations it
is inferred that a swarm of meteorites, though only manifesting itself
by a star-shower when passing through the earth's atmosphere, at some
distance from us may be visible as a comet by reflected sunlight.
[Sidenote: Fall of a meteorite during a star-shower.]
65. A dense star-shower[36] recurred on the same day of the month
(November 27) in 1885, the principal part being over in six hours. The
hourly number visible at one place at the time of greatest density was
estimated at 75,000. In the densest part of the stream, the average
distance of the individuals from each other was about twenty miles.
During this star-shower a piece of iron weighing about 8 lbs. was seen
to fall at Mazapil in Mexico:[37] in external characters and chemical
composition it is similar to the other meteoric irons: the simultaneity
was probably accidental.
[Sidenote: The reason of its rarity.]
66. It may be asked why, if star-showers are caused by the entry of
solid bodies into our atmosphere from without, there is only one
authentic instance of material being actually seen to fall and being
picked up during such a shower. As it is absolutely beyond question
that star-showers do come from outer space, we can seek an explanation
only in the size or speed of the entering individuals, or in the nature
of their material. A sufficient reason is to be found in the small
size of the individuals; for the meteorites which actually reach the
ground rarely weigh more than a few pounds, and are often quite minute;
a small diminution of the original individual would thus ensure its
complete destruction before the planetary velocity was exhausted: that
the individuals of a swarm are extremely minute follows from the fact
that the total mass of the biggest swarm is small, while the number of
the individuals seems almost infinite.
[Sidenote: Large and small luminous meteors essentially similar.]
67. Between the small silent shooting star visible only with the
telescope and the large detonating meteorite-yielding fireball
there is every gradation; during the star-showers themselves many
fireballs of great size and brilliancy are seen, while the smaller
individuals appear in no way different from the solitary shooting star.
The luminous meteors, large and small, are in the upper atmosphere,
few higher than 100 miles, few lower than 30 miles from the earth's
surface; they all have velocities of the same order of magnitude,
comparable with that of the earth in its orbit; in each there must be
a solid body, as is proved by the long path in the sky, for attendant
gas or vapour would be immediately scattered or burnt; large and small
present similar varieties of colour, and leave similar luminous trails;
examination with the spectroscope teaches us that the light of the
meteors is such as would result from the ignition of such meteorites as
have actually reached the ground. The frequent absence of detonation
may likewise be due in many cases to the small size, or small relative
velocity, of the entering meteorite.
[Sidenote: The light of a comet.]
68. That part of the light of a comet is reflected sunlight is
confirmed by examination with the spectroscope, in which instrument is
seen a feeble continuous spectrum crossed by dark lines, identical with
those afforded by the direct light of the sun. But a comet is also more
or less self-luminous; for, in addition to the continuous spectrum,
there are bright flutings and bright lines to which much attention
has been given. The three ordinary bright flutings were found by Sir
William Huggins in 1868 to be identical with the spectrum obtained
when an electric spark is passed through olefiant gas, and they are
now recognised as due to carbon. The carbon is presumed to be combined
with hydrogen, sometimes also with nitrogen; in the case of comets
approaching very near the sun, the lines of sodium, and others which
have been supposed to be iron-lines, are seen.[38]
[Sidenote: Tait's suggestion.]
69. The discovery made by Schiaparelli proves, as already pointed out,
that there is a relationship between comets and meteoritic swarms;
Schiaparelli himself held the view that a comet and its attendant
swarms are merely of identical origin. In 1869[39] Tait discussed,
from a purely dynamical point of view, the question as to whether
the swarm of meteorites attending a comet may not really be part of
the comet itself; he showed that many cometary characters can be
mechanically explained on the assumption that comets are really swarms
of small meteorites, and pointed out that the self-luminosity may be
produced by the heating of the individuals through collision with each
other.
[Sidenote: Reproduction of the spectrum of a comet.]
70. Flutings exactly identical with those seen in the spectrum of a
comet were obtained by Professor A. W. Wright in 1875[40] on allowing
the electric glow to pass through a heated tube, in which, after the
introduction of fragments of the Iowa meteorite, the gaseous density
had been reduced by an air-pump. The bright lines, too, in the spectrum
of a comet, even when nearest to the sun, are found by Sir Norman
Lockyer to be identical with those yielded when the electric glow is
passed over ordinary meteorites at comparatively low temperatures; and
further, the changes in these lines as the comet approaches and recedes
from the sun are exactly those which take place on variation of the
temperature of the meteorites enclosed in the glow-tubes.
[Sidenote: A comet is perhaps a swarm of meteorites.]
71. From these facts it is inferred that a comet may be in every
instance a swarm of isolated large or minute meteorites, at a not very
high temperature, shining partly by reflected sunlight and partly by
the electric glowing of the gases evolved owing to the action of the
sun's heat on the meteorites: further, some of the heat may be due to
the clashing together of the meteorites, the grouping of which becomes
more and more condensed as the swarm approaches the sun.
The gases driven from the meteorites by the sun's heat would be quite
sufficient in quantity to form the tail of the comet: as pointed out by
Professor Wright, a meteorite like that which fell at Cold Bokkeveld
would furnish 30 cubic miles of gas measured at the pressure of our
own atmosphere, and in space itself this gas would expand to enormous
dimensions owing to the small mass and attraction of the meteoritic
swarm. We are still uncertain, however, as regards the actual physical
condition of the matter composing the tail of a comet.
[Sidenote: Saturn's rings are probably swarms of meteorites.]
72. Clerk-Maxwell proved, as long ago as 1857, that the stability
of the rings which revolve round the planet Saturn is inconsistent
with their being formed of continuous solid or liquid matter; and has
shown, by mechanical reasoning, that they must be revolving clouds of
small separate bodies, like cannon-shot, each moving as a satellite
and almost independent of the rest in its motion: determination of the
motions of the inner and outer parts of the ring-system made with the
help of the spectroscope supports this conclusion.
[Sidenote: Nebulæ.]
73. Reichenbach, in 1858, before the self-luminosity had been proved
by means of the spectroscope, had imagined a nebula to be a cloud of
isolated meteorites, illuminated by some neighbouring sun: Chladni,
long before, had supposed a nebula to be a cloud of phosphorescent
dust. But, in 1864, it was established by Sir William Huggins that
the light is due, not to reflection or phosphorescence, but to
incandescence, for the spectrum consists of bright lines such as are
yielded by glowing gas. Tait,[41] in 1871, suggested that the nebulæ
may be clouds of mutually impinging meteorites, mingled with glowing
gases developed by the impacts; he pointed out that the heat produced
by the clashing of the individuals of such an immense group as a nebula
evidently is would be quite adequate for the production of their light.
Sir Norman Lockyer finds that the bright lines (generally accompanied
by a certain amount of continuous spectrum) which have been observed
in nebular spectra are consistent with this suggestion, and regards
them as closely related to the low temperature lines obtained when
a gentle electric glow is passed over meteorite-fragments in a tube
containing gases given out by them, and of which the density has been
reduced by the air-pump; further, he points out that the nebular
spectrum is identical with that of the comets of 1866 and 1867 when
distant from the sun. According to this suggestion, a nebula and a
comet are of identical constitution, and a comet is merely a nebula
which has become entangled in the solar system. On the other hand, Sir
William Huggins has expressed (1891) the opinion that the spectrum of
the bright-line nebulæ is certainly not such as we should expect to
result from the collision of meteorites like those which have reached
the earth, and that it is suggestive of a high temperature; he points
out that the particles which have just been in collision may be at high
temperatures and yet the average temperature of all the particles may
be low.
[Sidenote: Stars.]
74. The examination and classification of the spectra of the stars has
likewise led to remarkable conclusions. Secchi, following Rutherfurd,
found that the stars could be distributed into classes according to the
characters of their spectra,[42] and his classification has since, with
little modification, been adopted by Vogel and Dunér, by whom several
thousand star-spectra have now been systematically mapped. The first
three classes are characterised by absorption, the fourth by radiation.
In the spectra of Class I the absorption is small and simple, the dark
lines being broad and few; the stars themselves are white: in one
division of this class, represented by Sirius and Vega, the principal
lines are due to hydrogen; in another important division, represented
by beta, gamma, delta, epsilon, zeta Orionis, lines of helium are very
pronounced.
In Class II the dark lines are thinner and more numerous; the stars are
bluish-white to reddish-yellow: to this class belong the Sun, Arcturus,
Capella.
The absorption in Class III manifests itself predominantly as flutings,
though there are also many thin lines: the stars are orange or red:
in one division (a) of this class the darkest part and the sharpest
edge of each fluting is towards the violet end of the spectrum, as in
Betelgeux; in a smaller division (b) the darkest part of each fluting
is towards the red end, as in star 152 Schjellerup; the fluting
absorption of the latter division being due to carbon.
The remaining Class IV is an extremely small one: the spectra are
characterised by bright lines: some of the lines are due to hydrogen,
and others to substances not yet recognised in terrestrial chemistry.
[Sidenote: Supposed cooling of all the stars.]
75. Soon after the classification suggested by Secchi had been
announced, it was surmised that the differences in the stars of the
first three classes might be due, not so much to differences of matter,
as to differences of temperature, and that a very hot star such as,
from its brightness and distance, its small and simple absorption, and
the development of the blue end of its spectrum, Vega is believed to
be, would, on getting older and colder, pass from Class I to Class II,
and thence to one or other of the divisions of Class III.
[Sidenote: New stars.]
76. In 1866 a star of 9th or 10th magnitude burst into greater
brilliancy and nearly reached the intensity of Vega; the spectrum
showed the presence of brilliantly glowing hydrogen. Almost as suddenly
the light went down again, and within a month returned to its original
brightness. Ten years later, another new star of the 3rd or 4th
magnitude appeared at a place in the sky where no star had been noticed
before; its spectrum showed numerous bright lines; gradually, in the
course of a year, it dwindled down to the 10th magnitude, then giving
the telescopic appearance and the spectrum of a nebula. Several other
new stars have since been observed, the most notable being Nova Persei,
which appeared in 1901. In each case, as the star faded, its spectrum
changed into that which is characteristic of the nebulæ.
The appearance of a new star has been generally attributed to the
collision of two bodies in space; Sir Norman Lockyer[43] has pointed
out that the rapidity of the change in the brilliancy, so different
from that of other stars, may be due to the smallness of the mass,
and that such a star may be produced by the collision of two swarms
of widely separated meteorites. He has shown that the changes in the
spectrum as such a star varies in brightness are confirmatory of this
view.
[Sidenote: The heat of the sun.]