THE STARS—(_continued_).
DOUBLE AND MULTIPLE STARS—COLOURED AND VARIABLE STARS—CLUSTERS,
GROUPS, AND NEBULÆ—THE GALAXY, OR MILKY WAY—HOW TO FIND OUT THE
PRINCIPAL STARS.
Although not very clearly visible to the naked eye, there are in the
sky some pairs of stars very close together apparently; but when these
double stars are examined with a good telescope we find that though we
fancy they are two stars very close, in reality an immense distance
separates them. By Vega, which we have already mentioned, there is
apparently a star, which on examination will be found really to be
two stars. It is also in the constellations of the Lyre, but of much
lower magnitude than Vega. But in some instances there are three or
four stars thus placed together, and the frequency of the occurrence of
this fact establishes the farther fact that these combinations are not
accidental—that the stars are interdependent and physically connected.
[Illustration: Fig. 612.—γ Leonis.]
There are now at least six thousand double stars known,[27] and this
is a very small proportion of the forty millions or so of suns which
are believed to exist in space. But of these six thousand a larger
proportion have been ascertained to be physically connected. More than
six hundred of these pairs are double suns, while again there are other
combinations of three and perhaps more. When two are thus connected we
have what are termed binary systems, and when more are associated they
are called triple and multiple stars. An example of the last-mentioned
class is the small star above mentioned near Vega. It is ε Lyræ, and
is a double of a double. In ordinary telescopes this will not be
perceived, but with a high power the combination will be noticed. The
same phenomenon is observable in one of the stars of Hercules and in
Andromeda.
The revolution of these double suns, or binary systems, has been
closely observed, and Professor Newcomb has given us a list of the
binary systems of short period which are well determined. These are as
follows:—
42 Coronæ 26 years.
ζ Herculis 35 years.
Struve 3,121 40 years.
η Coronæ 40 years.
Sirius 50 years.
ξ Cancri 58 years.
ξ Ursæ Majoris 63 years.
η Coronæ Borealis 67 years.
α Centauri 77 years.
μ Ophiuchi 92 years.
λ Ophiuchi 96 years.
ξ Scorpii 98 years.
It must be borne in mind that although these double stars appear close
together from our standpoint, they may be far apart—one behind the
other in a straight line. When such “pairs” exist they are known as
optical pairs, or optically double stars, as distinguished from the
actually physical “pairs” which revolve round the centre of a system.
In Orion there has been discovered a regular system, and the θ in
Orion, which appears in a common telescope a moderate star, and to
the unaided eye only a speck of light, is really composed of seven
stars—four are set in the form of a trapezium, as figured in the
diagram in the margin by dots and asterisks. Two of these have been
ascertained to possess attendants indicated by dots, and a seventh star
was discovered by Lassell, and Humboldt remarks that in all probability
this apparently tiny star in the constellation Orion constitutes a real
system, for the five smaller stars have the same proper motion as the
principal one.
[Illustration: Fig. 613.—Monocerotis.]
[Illustration: Fig. 614.—Trapezium of Orion (Herschel).]
Thus our imagination almost fails to grasp the infinity of the systems
with our single sun, and with the distant double and even triple suns
round which planets revolve perfectly independent of the other systems,
as we are independent of them possessing heat and light from their
own sun or suns as we receive it from ours, day and night seasons
succeeding each other, and the wondrous varieties of the light produced
by the appearance or withdrawal of a sun or two in the firmament of
those most distant planets. These suns being double or triple would
affect each other; the composition of the light given forth would
produce—as we may assume—varying effects. We know something about the
light of the stars by the spectroscope, and the colours of stars are
due to the vapour which takes away a certain part of the light emitted,
leaving the remainder to descend through the atmosphere to us.
Binary stars are most numerous of the doubles; for instance Castor, η
Coronæ, Rigel, Polaris, Mivac, γ Leonis, γ Virginis, ξ Ursæ Majoris, α
Hercules, 36 Andromedæ, λ Ophiuchi, and π Aquilæ. The illustration in
the margin is Castor (or α Geminorum), the most northerly of the Twins.
The η Coronæ is also figured, as are Polaris (_see_ fig. 610 _ante._),
Boötes, Rigel, and γ Leonis.
The cuts herewith illustrate the relative positions at the periods
named of the “doubles,” and of the revolution of suns around other suns
as mentioned. As a consequence of their proper motion the binary stars
appear to vary in their distances from each other, as in the topmost
of the three cuts on the (opposite) page representing γ Virginis. The
stars have gradually approached each other, and so are the stars in
Castor approximating, and when they have closed, and have appeared
almost as a single star, as they will do, they will take open order
again.
[Illustration: Fig. 615.—η Coronæ.]
[Illustration: Fig. 616.—Boötes.]
[Illustration: Fig. 617.—Castor.]
[Illustration: Fig. 618.—Rigel.]
The shortest time occupied by a double star in its revolution is
thirty-five years, and we have already given some of those which
have been ascertained. We will close this section with a few other
examples. For instance, γ Virginis revolves in one hundred and fifty
years, Castor in two hundred and forty years, 4 Aquarii in three
hundred years, 37 Pegasi in five hundred years. There are numerous
other instances up to a period of three thousand years, and about eight
hundred of these binary systems are known. We have mentioned that there
are two or more suns in the multiple systems. These suns are the cause
of the different colours of the stars.
COLOURS OF THE STARS.
The question of star-colour follows naturally the consideration of
the multiple stars; for although single stars have been observed of a
ruddy colour, there are no instances of a blue or green one unattended
by a companion. This colouring has been attributed to the contrast
between multiple stars, for the colours are frequently complementary;
but investigation has shown that this cannot be the case. For instances
have been known in which, when two are thus associated, and one is
concealed from us, the other is just as bright, and retains its former
colour.
Of course in cases in which colour is apparent to the unaided vision,
only the brightest stars betray colour. Antares, Betelgeuse and
Aldebaran are red (orange) colour. Sirius and Canopus are white.
Arcturus and Capella are yellowish, so is Pollux. Vega is bluish-white.
These appearances are, of course, much more marked when the stars are
examined through the telescope, and telescopic stars—which are stars
unobservable without a glass—are very much coloured, and the multiple
stars give us blue, green, violet, and other tints, besides those
already mentioned.
Again, these coloured stars do not always remain the same colour.
Sirius was once red; Mars was at times white. Spectrum analysis shows
that the colours of many are due to absorption by the vapours of some
of the rays; and the existence of certain vapours may cut off some, and
at other times other vapours may exist and cut off other rays, and so
the colours may be changed. Struve gives the following list of binary
complements of “multiple” stars:—
Pairs consisting of a blue and white principal star 53
“ “ and a light yellow principal star 52
“ “ a yellow or red principal star 52
“ “ a green principal star 16
[Illustration: Fig. 619.—Position of the two stars of γ Virginis.*]
[Illustration: Fig. 620.—Position of the two stars of Castor.*]
[Illustration: Fig. 621.—Position of the two stars of ξ Ursæ Majoris.*]
We need scarcely pursue this question farther, though many ideas
concerning the coloured stars will arise in every thoughtful reader’s
mind. Supposing that every system has its sun or suns, can we fancy
the effects of a green or blue or violet sunlight—a light unmixed? To
employ the words of Sir John Herschel—“It may be more easily suggested
in words than conceived in imagination what variety of illumination two
suns, a red and a green, or a yellow and blue one, must afford to a
planet circulating round either—or what charming contrasts and graceful
vicissitudes a red and a green day, for instance, alternating with a
white one and with darkness, might arise from the presence or absence
of one or other or both above the horizon.”
LOST AND NEW STARS.
We may have perhaps read the “Lost Pleiad,” and wondered what has
become of the star supposed to have dropped out of the cluster so
well known in the constellation Taurus—the Pleiades. There are seven
stars, of which six are visible to the average eye, and the ancients
used to declare that one of the seven sisters (the daughters of Atlas
and Pleione) hid herself because she had married a mortal, while all
her sisters wedded gods. It is not improbable that one of the seven,
formerly distinguishable with the unassisted eye, may have disappeared
or been lost; but it is certain that strong eyesight can see more than
seven now, and in the telescope there are about one hundred.
And it is a fact that some stars whose places have been carefully
marked in the catalogue have subsequently disappeared. Many errors
may have been made, and stars put down where no star existed, so a
succeeding observer has not been able to find the star indicated. But,
on the other hand, we may admit that stars have been lost to sight, and
to compensate us for any such disappearances new stars are frequently
observed, and these are very remarkable phenomena. About 121 B.C.
Hipparchus perceived a new star, which was visible even in the daytime,
and on subsequent occasions others came into existence—viz., in the
years 945, 1264, and 1572. In the last-mentioned year Tycho Brahé
suddenly perceived the new star, which was at first very brilliant.
It grew fainter and fainter, after first gaining in intensity, and
disappeared entirely in 1574; and at other times stars have been seen
which remained only for a short time, and then disappeared.
The star discovered by Tycho Brahé was seen by him when walking across
the fields one night, and he encountered peasants who were gazing at
the new luminary. It was so bright that it threw a shadow from Brahé’s
stick. The new arrival appeared in “Cassiopeia,” under the lady’s
chair, forming, as pictured in the diagram, an irregular square. The
strange star is the largest.
[Illustration: Fig. 622.—Cassiopeia.]
Some stars exhibit extraordinary fluctuations, and one discovered
by Mr. Birmingham in 1866, decreased rapidly and sank away to about
the tenth magnitude, and then got brighter, and again diminished in
splendour. The “Eta” Argûs has also been subjected to many fluctuations
likewise, and such alterations have gained for these luminaries the
name of “Variable Stars.”
In the accompanying little chart there will be perceived two particular
stars, named Algol, “the demon,” and Mira “the wonderful.” The latter
is the most celebrated for its variable qualities, and its cycle of
change occupies nearly one of our years. For a few days it appears very
bright, and then fades away for about three months, to disappear for
five months, and then it reappears again, increasing in brilliancy up
to the second magnitude for another three months or so. Some people
account for these phenomena by stating that the sides of the star being
less luminous present the dark and light portions in rotation; but we
can give no satisfactory explanation of the reason, unless it be caused
by an aggregation of spots upon its surface, like sun-spots on our sun,
or perhaps by eclipse.
[Illustration: Fig. 623.—Star-Map.]
[Illustration: Fig. 624.—Nebulæ in Pegasus.]
[Illustration: Fig. 625.—Dumb bell Nebulæ.]
STAR-CLUSTERS AND NEBULÆ.
NEBULÆ and STAR-CLUSTERS are numerous in the heavens. The most
important are the Great Nebulæ in Orion and in Andromeda. But there are
other very beautiful “patches” of luminous matter or cloud appearances
composed of minute stars invisible to the naked eye. We annex specimens
of the Nebulæ, one or two having been already inserted. There must be
thousands of these star-clouds, and they have been classified by Sir
John Herschel from Sir William’s discoveries as follows:—
(1) Clusters of stars, in which the stars are clearly distinguishable,
divided again into regular and irregular clusters.
(2) Resolvable Nebulæ, which may be separated into distinct stars
under powerful telescopes.
(3) Nebulæ, in which there is no appearance whatever of stars, divided
into classes according to brightness, etc.
(4) Planetary Nebulæ.
(5) Stellar Nebulæ.
(6) Nebulous Stars.
We learn also from the foregoing authority that Nebulæ affect a certain
district; that is, they have, as it were, a preference for it, and are
not distributed in a random manner over the heavens, and are found in
Leo, Leo Minor, Ursa Major, Canes Venatici, Coma, Böotes, and Virgo,
and more sparingly in Aries, Taurus, Orion, Perseus, Draco, Hercules,
Lyra, etc. Nebulæ are found associated with stars, as is the case with
η Argus; these are called nebulous stars, and in the case of this
particular star many very interesting investigations have been made.
The Nebulæ are as equally variable as the stars they surround.
[Illustration: Fig. 626.—Nebulæ in Perseus.]
What is termed the Nebular Hypothesis was put forward by La Place, and
by it he endeavoured to account for the regular development of the
stellar system, which is supposed to have originated from an immense
nebular cloud. This immense mass would rotate and contract, and the
outer portions would separate and develop in rings like Saturn’s
rings. Then the rings break into separate portions, and each portion
condenses into a planet, or the small “bits” travel round the sun
like asteroids, and in this manner various systems were formed. This
theory was considered to be quite exploded when stars were discerned in
nebulæ by the more recent telescopes; but then the spectroscope came
to our aid, and it was discovered that there were some nebulæ which
are simply masses of glowing gas or aggregations of stones which are
dashing against each other in so forcible a manner as to produce heat
and luminosity. Mr. Lockyer appears to favour the latter theory as to
nebulæ.
[Illustration: Fig. 627.—Nebulæ in Canes Venatici.]
Mr. Proctor, however, has put forward a hypothesis that the star or
meteor showers are the original cause of the sidereal system, and this
rain of meteors has fallen for all time, gradually consolidating into
orbs. The fact that the constituents of sun, earth and planets, comets
and meteors being fundamentally the same lends probability to this
hypothesis, which is fully explained by the author.
THE MILKY WAY.
The Galaxy is familiar to all readers, and although visible all the
year round, is perceived more plainly in August, September, and
October, or at the beginning and ending of that period. This zone of
stars was of course well known to the ancients, but it is to Galileo
that we owe the first important information about the Galaxy; he
decided that it was formed of stars. Sir John Herschel investigated the
subject very closely, and to him much of the information concerning the
Milky Way is due.
It is not very distinct in the north, but as it advances from Cepheus
southwards by the Unicorn, it gets clearer, and opens out in Argo, and
descends still south, becoming brighter near the Southern Cross. It
then passes northward again, dividing into two branches, one of which
dies out, and then over Sagittarius, and so on to Cygnus, then to
Casseopeia and the starting-point. The number of stars in the Galaxy is
about 18,000,000.
[Illustration: Fig. 628.—The Milky Way.]
In this wonderful zone of stars the centre of our system, the sun, is
placed. It was supposed to be divided as in the diagram above; the
inner portion being the stars seen in their thickness, and the outer
ring representing the stars viewed in the direction of the length and
breadth. But afterwards, Herschel modified his opinions respecting
the Milky Way, and since his death many astronomers—and Mr. Proctor
more particularly—have devoted considerable time to an examination of
this wonderful zone of stars; which, it must be remembered, is not a
continuous stream; it is a series of luminous patches. On this point
Professor Nichol says:—
“It is only to the most careless glance that the Milky Way appears a
continuous zone. Let the naked eye rest thoughtfully on any part of
it, and if circumstances are favourable, it will stand out rather as
an accumulation of patches and streams of light in every conceivable
variety of form and brightness; now side by side, now heaped on each
other, again spanning across dark spaces ... and at other times darting
off into the neighbouring skies in branches of capricious length and
shape, which gradually thin away and disappear.”
The Milky Way has its greatest breadth in the “Swan,” and in the
“Eagle” constellation it divides itself. In the “Southern Triangle”
the zone is brightest, and in the “Southern Cross” the hole or space,
termed by sailors the “Coal Sack,” is very distinct. It then contracts
and expands, and there is in Argo another gap. Then it is lost for a
space, then it branches out, and soon crosses the Equator, dilates,
contracts, opens out again, and so returns to the “Swan” again.
Philosophers have frequently discoursed upon this phenomenon, but all
statements must remain more or less speculative. From Kepler’s to the
present time astronomers have been considering the Milky Way, and
when the Nebular theory was given up, when the Galaxy was found to be
composed of stars, there was, as we have noticed, the idea of the ring
and the cloven disc. Mr. R. Proctor has likened the Galaxy to a coiled
serpent, and considers the openings in the Milky Way as evidence that
the stratum of stars is limited, and that here we can see beyond it.
In fact, it would appear that it is a very complicated question; and
as the zone itself is complicated “with outlying branches beyond the
range of our most powerful telescopes,” so an actual knowledge of the
Milky Way is beyond us at present. It is composed of most extraordinary
aggregations of stars, which appear not only impossible to count, but
each one to be independent of the other. Thus we must conclude our
rapid survey of the Milky Way, and close with Mr. Proctor’s remark in
his “Universe of Stars.” “The sidereal system,” he says, “is altogether
more complicated, altogether more varied in structure than has hitherto
been supposed. Within one and the same region co-exist stars of many
orders of real magnitude, the greatest being thousands of times larger
than the least. All the Nebulæ hitherto discovered, whether gaseous or
stellar, irregular, planetary, ring-formed, or elliptic, exist within
the limits of the sidereal system. They all form part and parcel of
that wonderful system, whose nearer and brighter parts constitute the
glories of our nocturnal heavens.”
And a little reflection will show how true this is. Not very long ago
in the world’s life the solar system was supposed to consist of one sun
with a few planets wandering around him. Then some more were found,
and they were called “satellites.” For a long time man fancied he had
reached the “ultima thule” of astronomy in these depths; but the whole
idea was changed when it was discovered that beyond Mars there lie the
asteroids and the host of bodies in this solar system which we cannot
do more than allude to. Then when we consider that this “sun” of ours,
which we think so enormous, and which keeps in subjection so many
heavenly bodies, and illuminates them; when we reflect that there are
in space, and visible, stars many times larger than our ruling star,
each a sun, and that our sun would, if put where the great Sirius
glows, be but a speck in the firmament, and his system invisible to
our eyes, we may well wonder at the magnitude of the subject, and the
Infinite Wisdom and Power “that telleth the number of the stars, and
calleth them all by their names.”
HOW TO READ THE SKY.
A few particulars, to enable a reader to identify the most prominent
stars, may be given as starting-points from which some few excursions
into the spangled heavens may be attempted. But the suggestions must
be considered with reference to the ever-varying directions of the
supposed lines in consequence of the daily revolution of the sphere.
We have illustrated this in the cut in the margin, wherein the Lesser
Bear is shown as swinging round the Polar Star in different positions.
Sometimes the lines of direction will be vertical, sometimes inclined,
but all retaining their relative positions.
[Illustration: Fig. 629.—The “Swing” of the Lesser Bear.]
We have already learnt that the “pointers” of the Great Bear indicate
the Polar Star in the Lesser Bear, and we can (roughly) estimate the
distance between the pointers as 5°. This will give us the distance
between the pointers and the Polar Star as 29°. By following an
imaginary line through the two northern stars of the “Waggon” (the
Bear) away from the “horses,” we shall find Capella about 50° away.
[Illustration: Fig. 630.—Diagram of the Pole Star.]
If we pass from the first star next the waggon of “Charles’s Wain” to
the Pole Star, and past it, we shall arrive at an irregular W. This is
Cassiopeia, about as far beyond Polaris as the Bear is below it. When
the latter is low, the former is at the zenith, and so on.
A line drawn from the Pole Star through the end star of the Great
Bear leads to Arcturus. A line taken from Arcturus for about an equal
distance will, with the Pole Star, make a triangle with Vega. The Polar
Star may be called the Apex.
Regulus may be found southwards by drawing a line through the two first
stars of the square in the Bear (opposite the pointers). From Vega,
almost opposite the Pole Star, and through it about twice as far away
on the other side, is Sirius, a brilliant “sun.” Procyon will be found
to the westward of Regulus about 30°. From Procyon to the Pole Star a
line will pass through Pollux and Castor.
Another line from the pole star through the middle of the three
“horses” in the “Wain” will reach Spica Virginis about 70° beyond. So
we can describe a large triangle with Spica, Regulus, and Arcturus, at
the angles. Regulus is the apex, Spica and Arcturus a short base line.
[Illustration: Fig. 631.—Diagram of Sirius, etc.]
From the pole star through Capella, passing between Betelgeux and
Bellatrix, we shall describe a line leading to the three stars of
Orion’s belt. Between it and the Pleiades is Aldebaran.
There are many other stars which could be indicated; but on a fine
evening, if the observer will mark them upon a piece of paper, placing
the pole star in the centre, he will be able to add to his star map
very rapidly.
In the foregoing chapters of Astronomy we have seen how the earth and
other planets move around the sun; we have glanced at the “fixed”
stars and their groups, termed the constellations, and have noted
the planets and their characteristics, with many other interesting
facts. There is yet a great deal to be learnt, and much study will be
required with daily (nightly) observations before the young reader
will obtain success as a student of astronomy; but there is no study
so interesting. We have seen what a very small portion of the universe
is occupied by our solar system, and what a speck our earth is on
the plain of creation. We find ourselves on the border-land of the
incomprehensible, and we are lost in speculations upon the unseen.
FOOTNOTES:
[27] Mr. Burnham has lately given a list of two hundred and fifty-one
new double stars, and in the Astronomical Society’s proceedings there
is a list of ten thousand.