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Title: Stars of the southern skies
Author: Orr, M. A. (Mary Acworth)
Language: English
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STARS OF THE SOUTHERN SKIES
[Illustration: SOUTHERN HEMISPHERE]
STARS OF THE
SOUTHERN SKIES
BY
M. A. ORR
(MRS. JOHN EVERSHED)
AUTHOR OF “AN EASY GUIDE TO SOUTHERN STARS”
AND “DANTE AND THE EARLY ASTRONOMERS”
“All experience is an arch wherethrough
We glimpse the untravelled world whose margin fades
Forever and forever.”
_WITH ILLUSTRATIONS_
LONGMANS, GREEN AND CO.
39 PATERNOSTER ROW, LONDON
FOURTH AVENUE & 30TH STREET, NEW YORK
BOMBAY, CALCUTTA, AND MADRAS 1915
All rights reserved
TO MISS MARY PROCTOR
CONTENTS
CHAP. PAGE
THE GREEK LETTERS x
INTRODUCTION xi
I. THE CONSTELLATIONS OF THE SOUTH: MODERN GROUPS 1
II. THE CONSTELLATIONS OF THE SOUTH: ANCIENT GROUPS 6
III. THE TEN BRIGHTEST STARS OF THE SOUTHERN HEMISPHERE 15
IV. STARS OF DIFFERENT AGES 23
V. SOME NEAR NEIGHBOURS 32
VI. DOUBLE AND MULTIPLE STARS 35
VII. THE ASTONISHING STAR, ETA ARGŪS 40
VIII. MIRA, THE WONDERFUL STAR 46
IX. ECLIPSING STARS 51
X. SHORT-PERIOD VARIABLE STARS 54
XI. IRREGULAR AND DOUBTFUL VARIABLES 57
XII. STAR-CLUSTERS 59
XIII. GLOBULAR STAR-CLUSTERS 66
XIV. NEBULAE 72
XV. OTHER TYPES OF NEBULAE 80
XVI. THE CLOUDS OF MAGELLAN 85
XVII. THE MILKY WAY 88
LIST OF ILLUSTRATIONS
PAGE
MAP OF THE SOUTHERN HEMISPHERE _to face Title_
THE OLD CONSTELLATION FIGURES OF
THE SOUTHERN HEMISPHERE _to face page 6_
THE ARCHER _page 11_
From a Babylonian Boundary Stone
now in the British Museum
THE STAR-CLUSTER, 47 TOUCANI _to face page 68_
From Sir John Herschel’s drawing
THE MILKY WAY IN SCORPIO, LUPUS, AND ARA _to face page 88_
Photographed at Hanover, Cape Colony,
by Bailey and Schultz
THE GREEK LETTERS
α Alpha ν Nu
β Beta ξ Xi
γ Gamma ο Omicron
δ Delta π Pi
ε Epsilon ρ Rho
ζ Zeta σ Sigma
η Eta τ Tau
θ Theta υ Upsilon
ι Iota φ Phi
κ Kappa χ Chi
λ Lambda ψ Psi
μ Mu ω Omega
INTRODUCTION
This book is for those who have learned or are learning to recognise
the southern constellations, but have not read much about astronomy,
and have only an opera-glass or small telescope, or no instrument
but their own eyes, for examining the stars. I assume that they have
studied some simple guide to the constellations like Proctor’s, or my
_Southern Stars_, and that they would like to know something more about
the stars they are looking at—how far off they are, how large, if they
remain always the same, and if there is any connection between the
different parts of this immense universe of stars and nebulae.
I hope that the book will be useful to teachers and parents, helping
them to answer some of the searching questions children put, and
to teach some of the facts of astronomy in the most vivid and
unforgettable way—viz. in connection with special stars and nebulae
which are at the moment being admired. If an opera-glass or field-glass
can be put into the children’s hands, so that they themselves can see
a faint white spot turn into a lovely cluster of stars, it will give
them a share in the thrill of discovery, and help them to understand
what large telescopes can show.
A map of the southern hemisphere is here given for ready reference,
but it would be well to possess a good atlas which gives the letters
and numbers assigned to all bright stars and shows the positions of
the brightest star-clusters and nebulae. Norton’s _Star-Atlas and
Telescopic Handbook_ is an excellent one, small enough to be handy, yet
complete as far as it goes, and up to date, clear, and convenient to
use.[1]
[1] Published at 5_s._ by Gall & Inglis, Edinburgh and London.
My intention is to treat specially of stars and nebulae visible in
southern countries, so I have confined myself almost entirely to those
of the southern hemisphere, though of course many which are north of
the celestial equator can be seen also. My readers will find that the
southern hemisphere possesses the most beautiful part of the Milky Way,
the two brightest stars in the sky, the finest coloured star-cluster
and the largest globular cluster, the brightest double star, the
nearest of the stars, and the brightest of the large gaseous nebulae.
Let us add that the southern hemisphere has been less studied than the
north, and therefore there is an even wider field for amateur workers.
Stars of the Southern Skies
I
THE CONSTELLATIONS OF THE SOUTH: MODERN GROUPS
When we speak of the southern constellations, everyone thinks of the
Southern Cross, and every traveller coming south for the first time
is eager to see it. Some are disappointed because it is small and
irregular, but it is very brilliant, and lies in an extremely rich
region of the Milky Way. Very beautiful, too, is the way in which it is
seen rising on its side and gradually becoming upright as it reaches
its greatest height above the horizon, then sloping again as it glides
westward.
The Cross seems to have been first so named by the Spanish explorer
Amerigo Vespucci, at the beginning of the sixteenth century, and it
was also described in letters by the Florentine Andrea Corsali, who
says it is so beautiful that in his opinion no other constellation in
the sky is worthy to be compared with it. Some think, however, that
the stars had already been recognised as forming a Cross as early as
the thirteenth century, because Dante in his _Purgatory_ speaks of
four stars which glorified the southern sky; but he does not say they
were in the form of a cross, and he tells us that they had never been
seen before by mortal man except by our first parents, whose original
dwelling-place he sets in the southern hemisphere. This negatives the
suggestion that some traveller had described them to him.
About the same time as the Cross, other groups were named by sailors
and travellers in the part of the sky round about the south pole,
for this region had been left a blank by the framers of the ancient
constellations, doubtless because they lived too far north to see it.
Some students of astronomy in the Middle Ages concluded that there
really were no stars in this part, and Ristoro of Arezzo gravely argued
that this proved the absence of any land further south than India and
Ethiopia; for where there are no stars to pour down influences on the
earth no animals can live, and therefore no vegetation is needed for
their food, and no land for it to grow on.
The new constellations were mostly named after strange birds and fishes
seen by explorers in their southern voyages, and they were admitted to
scientific astronomy by Bayer, who made a map of the skies in 1604.
He also introduced the plan of naming each star in a constellation
by a Greek letter, the brightest of each constellation usually being
called Alpha, the next brightest Beta, and so on. One of these
constellations is Grus, the Crane (originally called the Flamingo), and
it is convenient to be familiar with it, because from it we may easily
identify several others. The chief stars of Grus form a striking curve
with a bright star close beside it. This bright star is Alpha Gruis,
and the brightest in the curve is Beta. A line through Alpha and Beta
leads in one direction to the brightest star of the Phoenix, in the
other direction to the brightest of the Indian; a line through Alpha
and the little naked-eye double Delta takes one to Pavo, the Peacock;
a line through Gamma (at one end of the curve) and Alpha goes to the
brightest star in the Toucan. And these are all the modern groups
containing bright stars except the Southern Triangle, near the Pointers
to the Cross (see p. 6), and Columba, the Dove, not far from Sirius,
each of which has one star brighter than third magnitude.[2] We may,
however, also notice Alpha Hydri, the brightest star in the small
Water-Snake, close to the bright star Achernar, and Alpha Doradūs, the
brightest of the Sword-Fish, between Canopus and Achernar.
[2] Stars are classified by astronomers in “magnitudes,” _i.e._ degrees
of brightness, those of first magnitude being the brightest. Stars
below sixth magnitude cannot be seen with the naked eye.
In the eighteenth century the French astronomer Lacaille, who did much
excellent work in the southern hemisphere with a tiny telescope of only
half an inch aperture, had the unhappy idea of filling up the spaces
still left empty with scientific instruments. It is easy to make Birds
of Paradise and Flying Fishes out of the stars, but such things as
telescopes, easels, and sextants do not lend themselves to irregular
groups, and they are very much out of place among the mythical beasts
and heroes which we are accustomed to see on our star-maps. Fortunately
the beginner need not learn to recognise these intruders, for there
are no bright stars in them, although there are many interesting
objects for telescopic study. We need only note that Octans, the
Octant, occupies the region in which the south pole is situated, which
is quite bare of bright stars. The pole itself may be found by drawing
a line from head to foot of the Cross and carrying it on about four
times as far again; or if the Cross is invisible, the pole may be found
near the middle of a line from Canopus to Alpha Pavonis.
II
THE CONSTELLATIONS OF THE SOUTH: ANCIENT GROUPS
Let us turn once more to the Southern Cross to find the ancient
constellations. It is both ancient and modern itself, for its
stars were known to the Greeks of Alexandria, but were included in
the Centaur. The latter, like all those shown in the accompanying
illustration, is of great antiquity, and probably of Babylonian origin.
The two stars which we call the Pointers to the Cross are Alpha and
Beta of the Centaur, and mark his forefeet. Another conspicuous pair,
Delta and Gamma, are on his horse’s body, while his man’s shoulders
are marked by many bright stars, and the head is formed by a little
group. His arm stretches out through bright Eta to Kappa, which is
very close to Beta Lupi. For the Centaur and Lupus form one large
and very brilliant group, and were perhaps connected with the little
constellation of Ara, the Altar, upon which the Centaur seems to have
been imagined as offering the creature which we now know as the Wolf,
though its older name was simply the Beast. Aratus, a Greek poet of
about 300 B.C., to whom we owe the earliest description extant of the
ancient constellations, says of the Centaur:
“His right hand he ever seems to stretch
Before the Altar’s circle. The hand grasps
Another creature, very firmly clutched,
The Wild Beast,—so the men of old it named.”
[Illustration: _The Old Constellation Figures of the Southern
Hemisphere._]
On the other side of the Cross is another splendid constellation, the
Ship Argo, which covers a large space of the sky with many bright
stars, among them Canopus, the brightest in the whole heavens except
Sirius. For convenience this large constellation has been divided into
four—the Keel, the Poop, the Mast, and the Sails.
Sirius is north of Canopus, in the constellation of the Great
Dog—Orion’s Dog, as it is often called, for Orion is a hunter with
two dogs; but the Little Dog is in the northern hemisphere, and Orion
himself has his head in the north and his feet in the south, his famous
belt and sword lying just south of the equator. Thus he is visible all
over the world, and only at the poles would his feet or his head never
rise above the horizon. Beneath his feet is the little Hare, as Aratus
says:
“And ceaselessly beneath Orion’s feet
The Hare is ever chased.”
Four stars of the Hare (α, β, γ, δ), which form a small square, were
called by the Arabs the “Throne of the Giant” (_i.e._ of Orion); or
sometimes “those which quench the thirst of the camel,” in allusion to
the river of the Milky Way which flows close by. Orion is mentioned in
the books of Job and Isaiah, and also by Hesiod and Homer.
Above the Centaur and the Ship Argo stretches the long straggling
constellation of Hydra, the Water-Snake. Its head reaches beyond the
equator into the northern hemisphere, but the beautiful red star
Alphard is in our hemisphere. This Snake is oddly connected with a Crow
and a Cup, which are somehow perched on its back. (I have seen a snake
pursued by an Indian crow, which kept pecking at its tail until the
snake found refuge in a stream.) Crater, the Cup, has no bright stars;
Corvus, the Crow, is an irregular little square which sailors call
Spica’s Spanker, a spanker being a sail of this shape, and two stars of
the four point to Spica, the bright star of the Virgin.
Virgo, the Virgin, is one of the constellations of the Zodiac, that
zone of stars which marks the apparent pathway of sun, moon, and
planets in the sky. All the twelve zodiacal constellations can be
seen in both hemispheres, but those south of the equator, which are
least well seen in northern countries, are of course the best seen
in the south. These are the Scales, Scorpion, Archer, Sea-Goat, and
Water-carrier, with a very small part of the Fishes, and the part of
Virgo which contains her brightest star, Spica, the Ear of Corn held
in her hand. The Romans called her the Goddess of Justice, but these
constellations were invented long before Rome was a great power, and
the ear of corn shows that she was rather a goddess of the fields. They
added that the Scales were her balance wherein she weighed the deeds of
men, but this constellation was of much later origin than the Virgin:
its place was earlier held by the Claws of the Scorpion.[3] In some
old books we find a compromise between the two ideas, for a pair of
tiny scales is hung on the great claws of Scorpio.
[3] Compare Aratus:
“The Virgin and the Claws, the Scorpion,
The Archer and the Goat.”
Scorpio is a magnificent constellation as seen in the south. In England
it merely creeps along the southern horizon in the pale summer skies,
but in southern countries its whole splendid length rises high and
shines, from the bright stars in its head and the ruby Antares on its
back to the sting in its tail, represented by two stars close together.
Close behind Scorpio is Sagittarius, the Archer, also a brilliant
constellation, with his bow strung ready to shoot the Scorpion; he is
a centaur, like the figure near the Southern Cross. He is followed by
Capricornus, a very strange animal, for it has the head and horns of a
goat and the tail of a fish. As a modern writer quaintly says:
“A startling monster’s hybrid form
Your eyes will there assail;
That sign so often dubbed the Goat,
Yet with a fish’s tail.”
Capricorn has very few bright stars. There are two in his goat’s
head, of which the brightest, Alpha, is a beautiful naked-eye
double, and there are two in his fish’s tail. These three figures—a
Scorpion, Centaur-archer, and Capricorn—are carved on old Babylonian
boundary-stones belonging to the second millennium B.C.
Following these is Aquarius, the Water-carrier, his shoulder marked by
the star β, and his pot by four stars (α, γ, ζ, η), from which fall
splashes and streams of faint stars, aptly representing the water which
he is pouring out. He is a familiar figure to those who have lived in
some countries of the East where water is all carried by hand.
[Illustration: THE ARCHER
From a Babylonian Boundary Stone now in the British Museum.]
This is a very watery part of the heavens, for the zodiacal pair
of Fishes follows the Water-carrier (their only bright star in the
northern hemisphere), and there is also a solitary Fish, Piscis
Australis, swimming in the water poured out; while a little further
east is the great Sea-Monster, which belongs to the northern group of
Perseus and Andromeda and her parents Cepheus and Cassiopeia. Only the
head of Cetus, the Sea-Monster, is in the northern hemisphere, but
that contains its brightest star. Between the dim Sea-Monster and the
bright Orion flows the winding River Eridanus: it rises near Orion’s
foot, and now ends very far south in the bright star Achernar; but this
star was not visible to Ptolemy when he drew up his star-catalogue in
Alexandria, and the original Achernar, or Last-of-the-River, seems to
have been what we now call Theta Eridani, which was much brighter some
centuries ago than it is now. Al-Sufi, an Arab writer of the tenth
century A.D., calls it a star of first magnitude.
To complete our survey of the constellations south of the equator we
must add the tip of the Eagle’s wing, the legs of the Unicorn, and
part of Ophiuchus with the Serpent he is strangling as he treads the
Scorpion under foot (a gallant hero, to contend with both these enemies
at once).
It is worth noting that just as the three stars of Orion’s belt mark
the celestial equator in one part of the sky, so the three bright stars
of the Eagle mark it in the opposite part (Altair, with β and γ on
either side); but they are just north of it, and Orion’s belt is just
south. As it is often interesting to know where the ecliptic lies, we
may point out that the following southern stars lie near it: Spica, α
Librae, Antares, α and β Capricorni (and in the north the Pleiades,
Aldebaran, Regulus). It is also convenient to remember the positions
of a few constellations as a guide to right ascension. Thus, Achernar
is in the Ist hour, Canopus and Sirius are in the VIth, the Cross and
Corvus in the XIIth, and the Bow of Sagittarius is in the XVIIIth.[4]
[4] Right ascension in the skies corresponds with longitude on earth,
but is more often reckoned in time than in degrees. For instance, R.A.
1 hour 35 minutes, the right ascension of Achernar, means that this
star will be on the meridian 1 hour 35 minutes later than the “first
point of Aries”—that is, the point at which the equator cuts the
ecliptic at the spring equinox, the fundamental point corresponding
with Greenwich in earthly longitude.
There are more bright stars in the southern hemisphere than in the
north, for a count of all those above fourth magnitude shows that
there are 228 south of the equator against only 164 north. But whereas
the stars in the north are fairly evenly distributed, there is a more
strongly marked tendency in the south to congregate on the Milky
Way, so that, while the tract through Argo and the Cross to Scorpio
and Sagittarius is extraordinarily rich, the part between Orion and
Fomalhaut is comparatively dark and bare, and the regions round the
south pole and north of Argo are also very barren of bright stars. Some
regions, however, which look dull to the eye abound in marvels for the
telescope and camera.
The brilliance and the complex structure of the Milky Way is
undoubtedly what most strikes the northerner travelling south. In
England we have a glimpse of it in Cygnus and Aquila, where this rich
and bright part begins; but there is nothing to equal the brightness or
the mingling of dark and light which we see in Argo and near the Cross,
in Scorpio and in Sagittarius. When this part is about to rise, there
is often a glow on the horizon as if it were dawn.
III
THE TEN BRIGHTEST STARS OF THE SOUTHERN HEMISPHERE
The brighter of the two Pointers to the Cross, Alpha Centauri,
ranks very high among first-magnitude stars, though it is excelled
in brightness by Sirius and Canopus; but its greatest claim to our
interest is that among all the host of stars it is our nearest
neighbour. Yet, should we decide to pay a visit and travel with the
speed of light (11 million miles a minute), it would be four years
and three months before we could reach our destination—so great are
the spaces which separate our sun with his family of planets from the
nearest of his brother suns.
Alpha Centauri is also interesting as a double star. The two components
are nearly equal in brightness, and are seen as a brilliant yellow pair
in quite a small telescope. It was one of the first doubles known,
being discovered as such in 1689; and Herschel calls it a “superb
double star, beyond all comparison the most striking object of the kind
in the heavens.” The two stars he describes as “both of a high ruddy or
orange colour, though that of the smaller is of a somewhat more sombre
and brownish cast.”
All stars are suns, but they vary so much in brightness and size and
quality of light that it is as difficult to match two stars in the
sky as two trees in a wood. Yet one of this pair is an almost exact
counterpart of our sun. The pair is approaching us, and at the same
time revolving one about the other in a period of about eighty years.
The other Pointer, Beta Centauri, is a star very different from our
sun. It belongs to a type called “Orion stars” because so many are
found in that constellation. Instead of shining with a yellow light,
they are blue or bluish-white, and their glowing atmospheres contain
quantities of helium, the gas which was only recently discovered on
earth though it had been known for many years in the sun. Oxygen and
nitrogen and some yet unknown elements are also present.
Alpha Crucis, the brightest star of the Cross, is also a blue Orion
star, and the telescope shows it to be triple. There are two bright
twin stars, and a fainter one which would be just visible to the naked
eye if alone, but being overpowered by the brilliance of the bright
pair can only be seen in a large telescope. Quite a small telescope
will separate the twins.
Achernar, the Last-of-the-River, is also an Orion star, 75 light-years
distant from us—_i.e._ light takes 75 years to come from it to us. The
sun if removed to a third of this distance would be barely visible to
the naked eye.
Rigel, Orion’s foot, is the brightest star of this class. Its intrinsic
brilliancy and its size must be enormous, for its distance is certainly
more than 360 light-years, and may be greatly more. And the distance
is increasing at the rate of 39 miles a second. Rigel is also a double
star, but its companion is very faint. In an 8-inch telescope the
bright star looks white or pale lemon-yellow, contrasting beautifully
with the little companion which is blue.
Spica, the Ear of Corn in the hand of the Virgin, is immeasurably
distant from us, and its companion is invisible even in the most
powerful telescopes. It was discovered by means of the spectroscope,
for, when the light of the star is drawn out into a long
rainbow-coloured ribbon crossed by dark lines, it is found that there
are two ribbons, one bright, the other very faint, and that the lines
of these two spectra draw apart and then come together again once in
every four days, showing that there are two stars close together and
revolving round one another in this short time. The joint mass of the
pair is two and a half times that of the sun.
Spica is one of the first stars whose invisible companion was
discovered in this way, and it is also connected with another
discovery, made nearly twenty centuries earlier by Hipparchus in the
island of Rhodes. He was patiently plotting the positions of all the
visible stars, when he found a slight discrepancy between the places
given to Spica by himself and another Greek astronomer, who had
observed about a hundred and fifty years earlier. Examining into this
led him to discover that all stars change their apparent positions
very slowly, completing a cycle of change in nearly 26,000 years, so
that 240 centuries more must elapse before any astronomer sees Spica
in exactly the same place as where Hipparchus saw her. It remained for
modern astronomers to discover that the apparent change is due to a
slow nodding motion of the Earth’s axis.
Brightest of all stars in the whole sky is Sirius, the Dog-Star. It was
worshipped by the ancient Egyptians, and the day on which it rose just
before the sun was counted as the first day of their year. The Arabs,
when they learned the astronomy of more ancient nations, were forbidden
to adopt their star-worship, hence the saying in the Koran, often
quoted by Arab writers: “The Highest saith, He is lord of Sirius.”
Sirius is moving rapidly through space, not uniformly but with an
oscillating movement, and Bessel in 1844 “founded the astronomy of the
invisible” by showing that these irregularities might be caused by a
dark disturbing companion. Eight years later, Mr. Alvan Clark, wishing
to test a large lens just made by his firm, turned it on Sirius, and
lo! there was the satellite in the position required to explain the
vagaries of Sirius. It is not therefore wholly dark, but it shines with
so feeble a light that, if it were brought as near to us as our own
sun, it would appear only one-hundredth as bright as he is, even though
it is a somewhat more massive body. Sirius itself is only about two
and a half times as massive as our sun, but immensely more brilliant.
It is the typical star of the “Sirian” class, to which belong many
of the brightest stars in the heavens, white stars in whose spectra
broad hydrogen lines form the most striking feature, indicating a very
extensive atmosphere of glowing hydrogen. Bright Sirius and his dim
companion revolve round their common centre of gravity in fifty years.
Fomalhaut, the mouth of the Fish, belongs to the Sirian type of stars,
and is also very brilliant, giving out fourteen and a half times as
much light as our sun. Its distance is 25 light-years.
Canopus, the rudder of the Ship Argo, must be a giant sun, for its
distance is altogether beyond reach of measurement and it is steadily
receding from us, yet it shines as the brightest star in the sky except
Sirius.
In Southern India it is called Agastya, after a Brahmin rishi who
led an early Aryan colony to the south, and before whom the Vindhya
Mountains prostrated themselves as he passed.
Greek astronomers noticed that this star rose only just above the
horizon of Rhodes, but 7½ degrees above it at Alexandria, from which
Poseidonius calculated that the circumference of the whole earth,
_i.e._ 360°, must be 240,000 stadia. This is equal to nearly 23,500
miles, a value surprisingly near the correct figure, considering
how difficult it must have been to measure the distance over the sea
between Rhodes and Alexandria.
Antares, the brightest star in the Scorpion, was so named by the Greeks
because it rivals the red planet Mars (Greek Ares) in colour. It is red
because a dense atmosphere shuts out most of its blue rays. Like most
red stars it is very distant, and its light takes 155 years to reach
us. Nevertheless it shines 2000 times as brightly as our sun would
do at the same distance, hence it must be of an enormous size. This
immense red star is accompanied by a little green satellite, and there
is also a very close companion which can never be seen, but is known to
exist through the shifting of lines in the spectrum, like that of Spica.
What an amazing variety among these ten stars! Though the eye can only
decide that all are much brighter than the average, and that they
differ somewhat in colour among themselves, science tells us that
they vary enormously in many ways. Half belong to the class of blue
Orion stars, others to the white Sirian, yellow solar, red Antarian
classes; more than half are known to be double or multiple, and among
these we find twins, while others have faint companions differing
from themselves in colour. In one case the pair is widely separated,
and revolves in a period of half a century, while another pair is so
close that the circuit is performed in four days. Again, while one
star is the same size as the sun, others are much greater, and their
distances from us vary all the way from four light-years to spaces we
are powerless to plumb.
IV
STARS OF DIFFERENT AGES
What is the meaning of these different types of stars—the blue, the
yellow, and the red? and can a star change its colour and type? Yes,
we believe that all the blue stars, if they follow the normal course,
will in countless ages become red, and though the life of the whole
human race may perhaps not be long enough to witness a single instance,
we feel confident that this process is going on. One walk through a
forest is enough to teach us that a tree grows from seed to sapling,
from maturity to decay, because we see trees around us in all these
stages. So with the stars. Here the process of development is far
more difficult to understand, and we are still ignorant concerning
the birth and death of stars; but it is clear that we see a series of
stages which pass gradually into one another, and that the cause of a
star’s growing old is a gradual loss of heat by radiation. The blue
Orion stars are the brightest and hottest of all those about which we
have spoken, and are generally thought to have reached the climax of a
star’s career. They are great globes of thin gas, much less dense than
water, but glowing hot through and through. The outer layers naturally
part with their heat first, and in cooling they absorb more and more
light from the radiant centre. As the blue rays are always first and
most powerfully affected, the blue tint is soon lost, and the colour
slowly changes through white, yellow, and orange to deep-red. At the
same time changes take place which cause different gases to become most
conspicuous in the star’s spectrum at different stages.
But what are the early stages which precede the brilliance of an Orion
star? Arguments have been brought forward to show that young stars also
are comparatively cool, but even less dense than Orion stars, that as
they grow denser and smaller through gravity, which causes all the
particles of a star to condense towards its centre, they must at first
grow gradually hotter and brighter until a certain point is reached,
after which they will grow cooler and fainter again until they become
quite cold and dark; and so there is a double progression, viz. from
red towards blue, and back again to red, some stars changing in one
direction and some in the other.
On the other hand, the stage which immediately precedes the Orion
type somewhat resembles a peculiar class of nebulae, so the upward
progression of the Orion stars may have been from nebulae through this
stage.
Let us see what these stages are.
At one end of the series we find stars like Gamma in the Sails of the
Ship Argo. When its light is spread out by prisms into a coloured band
it is a wonderful sight, for not only are there dark lines and dark
flutings, but wide bright bands mingle with these and stand out against
the fainter background. We are fortunate to possess γ Velorum in the
southern hemisphere, for it is the only bright star of its class. The
type is known as WOLF-RAYET STARS, from the name of their discoverer.
Only a few are known, and because of their faintness not very much has
been discovered about them, but it is remarkable that all of them lie
near the middle line of the Milky Way.
The bright lines in their spectra and some other features of Wolf-Rayet
stars point (as we have said) to a connection with gaseous nebulae,
and it is possible that they developed from nebulae not very long
ago—as time is reckoned in astronomy. Though this past is doubtful, the
future of these stars is clearly indicated: they are destined to become
Orion stars, for in stars like τ Canis majoris we see an intermediate
stage between the two types.
ORION STARS sometimes have bright lines in their spectra also,
especially the very blue ones found near nebulae, but the most striking
and characteristic feature is the series of dark helium lines, from
which they are often called “helium stars.” The three in Orion’s belt
are typical of this class; several first-magnitude stars belong to it,
as we have seen, and it contains others which are very bright, such as
β and δ Crucis, β Scorpii, α Lupi, α Pavonis, α Gruis, α Sagittarii.
Spica is a distinctly blue Orion star; Rigel and γ Gruis have already
lost the blue tint and are approaching the next stage.
For the Orion class passes by gradual transition into the SIRIAN STARS,
which may be called “hydrogen stars,” from the wide hazy lines of
hydrogen which are the most conspicuous feature in their spectrum. They
are also very hot and bright, though less so than the Orion stars.
Besides Sirius, a large number of bright southern stars belong to this
class, among which we may mention Fomalhaut, γ Centauri, β Carinae,
δ Velorum, and β Pavonis. Among stars visible to the naked eye this
is by far the most numerous class, partly, no doubt, because they are
intrinsically bright and therefore visible at distances where redder
stars could not be seen. This cannot however be the whole explanation,
or the Orion stars would be still more numerous instead of being
comparatively rare.
The blue end of the spectrum begins to be darkened as a star reaches
the stage of sun-like or SOLAR STARS, and although the hydrogen lines
grow narrower and less intense, an immense number of fine lines cross
the bright band and absorb much of the light. α Centauri, as we have
seen, is a replica of the sun, and another star which almost exactly
resembles it, in spectrum if not in mass, is β Hydri. ζ Gruis and one
star of the naked-eye double in Grus, δ¹ Gruis, are also solar stars.
This class may be subdivided into three: the Sirian-solar, which is
typified by Canopus, and also by η Crucis and α Hydri; the solar,
closely resembling the sun; and the red-solar, which tend towards the
red stars. The temperatures apparently are lower and lower in these
three divisions in the order given, and all solar stars are cooler than
those of the preceding classes. Among these red-solar stars is the
beautiful ε Carinae, the foot of the False Cross,[5] which is a rich
reddish orange even to the naked eye, and more brilliantly coloured in
a binocular. Others are α Toucani, α Trianguli australi, ε Crucis (the
little fifth star of the Southern Cross), the two brightest stars of
Indus (α and β), almost all the bright stars of the Phoenix (α, β, δ,
ε), and ε Scorpii which is beautifully coloured. This class is also
extremely numerous among naked-eye stars, although they cannot be so
bright intrinsically as the whiter solar stars: the significance of the
fact is not easy to understand.
[5] The stars ε and ι Carinae, κ and δ Velorum, form a cross much like
the Southern Cross, but less bright, and this is called the False Cross.
The deep-red stage is reached in suns like Antares, whose spectrum
shows not only lines like those in the sun but also a series of broad
bands or flutings which absorb much of its light, and in photographs
a great part of the violet end is wholly cut off. Were it not such
an enormous size, this would be a very dim star. ANTARIAN STARS are
almost all very remote. They are often very faint, and many of them are
unstable in their light, as if fluctuating towards extinction. β and
δ² Gruis and γ Hydri are among them, and Mira, the wonderful variable
star. Compare the ruddy γ in the Cross with pale δ to see the contrast
between stars of this class and the Orion-type. Because of the want of
blue rays in the former it makes so little impression on a photographic
plate compared with α, β, and δ, which are all Orion stars, that one
can scarcely recognise the form of the Cross in a photograph.
The brightest of these red stars are in the northern hemisphere, viz.
Betelgueux in the shoulder of Orion, and Aldebaran, the eye of the
Bull: both are slightly brighter than Antares.
Nearly all the stars we know have a place in this series, though there
are individual peculiarities, but there is one class which seems to lie
outside it. Stars of this class are red and have spectra crossed by
dark bands, but they are unlike the Antarian bands and resemble instead
those seen in the spectra of comets and of candle-flames. They are due
to carbon compounds, so these stars may be called CARBON STARS. Most of
them are extremely remote, and all are so faint that among the very
brightest is U Hydrae, just visible to the naked eye a little east of
Alphard. They are probably aged stars, but no links between them and
the other types have yet been discovered to enable us to place them in
the series.
Strange and interesting discoveries have been made by grouping large
numbers of stars into their classes and comparing the average motions,
distances, &c., of the groups. It is found that the redder stars are on
an average moving more rapidly and in a more random fashion than blue
and white stars. Thus, Wolf-Rayet and Orion stars have a low average
speed, and both are very much more numerous in and near the Milky Way
than elsewhere; Sirian stars are travelling a good deal faster, show a
marked tendency to congregate in two streams, and move chiefly parallel
with the plane of the Milky Way; solar stars, including our own sun,
move more rapidly still and show less preference for the Galaxy and the
two streams: Antarians have the most rapid motions of all, but these
appear to be haphazard, and the stars are scattered all over the sky in
every direction.
These facts are very unexpected and very difficult to explain. It
looks as if the Milky Way were the birthplace of the stars, and that
as they develop they gradually scatter through space; but how are we
to explain the fact that speed and direction of movement differ for
different types? There seems to be no reason why a cooler star should
move more quickly than a hotter one, and none of the theories yet
advanced can be considered final.
V
SOME NEAR NEIGHBOURS
If it is remarked that Sirius is fifty millions of millions of miles
away from us, it is not at once obvious that he is one of our very near
neighbours; but this is equal to 8 light-years, not twice the distance
of α Centauri, our next-door neighbour among the stars. Some faint
stars in the south must also be counted as very close to us: such are
a little star in Cetus, τ Ceti, only 10 light-years away; ε Indi, 11½;
and two in the River Eridanus, ε and δ Eridani, 10½ and 18 respectively.
But the most interesting among these near neighbours of the south is
a little yellow star in Pictor, too faint to be seen without a good
binocular or a telescope, and bearing the very modern name of CZ
5ʰ.243. This stands for Cordoba Zones 5.243 hours, and means that it
was catalogued at Cordoba Observatory in South America and its position
fixed in the fifth hour of Right Ascension. After this it was observed
by Mr. Innes at the Cape, and he was startled to find (like Hipparchus
comparing his work with that of Timocharis) that its position no longer
agreed with that found at Cordoba. “Can this be motion?” he asked, and
found that the star had indeed a larger visible motion across the sky
than any other, not even excepting the famous “runaway star” in Ursa
Major. Apparent motion, however, depends upon distance as well as real
speed, and when the distance of CZ 5ʰ.243 had been calculated it was
found that its real speed, amazing as it is, yet falls slightly short
of that of the northern star. The rates are 163 and 174 miles per
second.
A few other stars are known to move at speeds approaching 100 miles
a second, and one was announced in December 1913 to have a velocity
of 200 miles a second, but runaway stars are rare. They do not seem
to show any preference for special parts of the sky or special
stellar types, and it is impossible to say what causes them to rush
with such headlong haste through space, or what is their goal. The
average rate for a star is about 13 miles a second, but, as we have
seen, it differs with different types, the average speed increasing
progressively from the blue to the red classes.
Another remarkable fact lately discovered is that the different types
of stars are not indiscriminately scattered through space. Our own sun
seems to be surrounded, to a distance of about 100 light-years, by
suns more or less like himself, while the greater number of the red
Antarian stars lie at a much greater distance from us. Most distant
of all, considered as a class, are the bluest of the Orion stars; for
though the distances of individuals vary greatly, the average distance
of these stars from us is more than 500 light-years. Yet they are
often very bright, so this is another proof of their great intrinsic
brilliancy.
VI
DOUBLE AND MULTIPLE STARS
Among the brighter stars at least one in four is double, and I shall
only mention a few which for different reasons are of special interest.
The brightest double in the sky has already been mentioned, α Centauri,
and we have also described another southern pair scarcely less
brilliant, α Crucis, Sirius with his very dim companion, Rigel and
Antares with theirs of contrasting colours. Other fine southern doubles
are:
β Piscis australis, a white star with reddish companion, visible in a
3-inch telescope.
δ Corvi, an unequal distant pair, pale yellow and bluish, easily
separated in a 4½-inch telescope.
σ Scorpii (near Antares), white and blue.
32 Eridani, yellow and blue-green—“magnifici, superbi,” according to
Secchi.
β Capricorni (close to the splendid naked-eye double α Capricorni),
orange-yellow and blue.
γ Crucis, orange-yellow, companion fifth magnitude, rather distant.
γ Leporis, companion crimson. There is also a third faint star, forming
a triple group.
β Capricorni, like Antares, besides its visible companion, has a close
invisible one, only known by the shifting of lines in the spectrum, and
this is not an uncommon case. β Crucis consists similarly of two bright
stars and a spectroscopic companion, and so also θ Eridani. κ Velorum
and α Pavonis have spectroscopic companions only, one revolving in a
period of 116½ days, the other of only 11¾ days, and the period of μ¹
Scorpii is counted in hours!—34 hours 42 minutes. The brief periods of
these spectroscopic binaries[6] are in striking contrast with those of
many visual binaries, such as ζ Sagittarii with a period of 19 years
and γ Centauri with 150 years; and this is what one would expect, since
the stars must be comparatively far apart and their orbits ample for
them to be visible separately. Sometimes the stars of a pair or of a
group are known to be moving together through space, though no movement
of revolution round a common centre has yet been detected, probably
because it is very slow.
[6] A “binary” is a system of two stars which are known to be
comparatively close together and influencing one another’s movements. A
“double star” may be a binary, or the two stars may really be very far
apart and have no connection, merely happening to lie one nearly behind
the other.
It is an interesting fact that most of the spectroscopic binaries,
which are such close and rapid pairs, are found among the blue and
white stars, the numbers steadily decreasing as we pass through the
yellow to the red stars.
A significant fact about visual binaries is that companions which
differ much in colour invariably differ much in brightness also.
This is probably to be explained by supposing that one was from the
beginning much larger than the other, and that there is consequently
a difference in the rapidity with which each runs through its
life-changes. Where the two are alike in colour and spectrum, like the
two solar stars of α Centauri and the two Orion stars of α Crucis, it
is found that they are also nearly equal in mass, so they keep the same
pace and grow old together.
Among multiple stars there are some very remarkable instances in
Scorpio. β Scorpii is a pair of bright stars (easily separated with a
3-inch telescope) with a third fainter companion, and besides these,
one of the bright components is a spectroscopic pair, and the whole
company is travelling together through space. The joint mass of the
spectroscopic pair is twenty-one times as great as that of our sun,
and they revolve about one another in seven days; but a very strange
feature is that _some_ lines in their joint spectrum, due to calcium
gas, behave differently from the rest, and it is thought that these two
revolving stars may be enveloped in a calcium cloud which travels with
them.
ξ Scorpii is a telescopic double which has been watched throughout
a complete revolution of ninety-six years. It was discovered by Sir
William Herschel in 1782. Here also there is a third star, much fainter
and more distant than the brighter companion, and all are travelling
together through space.
ν Scorpii is one of the “double-doubles,” of which a good many are
known, where a star that looks single to the naked eye is seen as a
pair with a telescope, and each of these becomes a pair with higher
powers. It has been described as “perhaps the most beautiful quadruple
in the heavens.” Both pairs journey together through the skies.
σ Orionis, the fourth-magnitude star just below Orion’s belt, separates
very easily into two unequal components. Herschel found each of these
to be triple, and called it a “double-treble.” Later it was found to be
“double-quadruple,” with more stars between the two groups.
In the same way the beautiful naked-eye double star α Capricorni is
seen in good telescopes to consist of two groups of stars, one (α¹)
triple, the other (α²) quadruple.
If a group like this forms a connected system, the motions of the
several stars must be highly complicated.
VII
THE ASTONISHING STAR, ETA ARGŪS
Midway between the Southern Cross and the False Cross there is a
nebula visible to the naked eye, and in it once shone a bright star.
When Halley was observing in the southern hemisphere towards the end
of the seventeenth century, he catalogued it as of fourth magnitude,
but Lacaille and later astronomers marked it as second. Sir John
Herschel first saw it in 1834, when he was at the Cape, and he says
that it remained steady for three years, from 1834 to 1837. On the
16th of December 1837 he began his observations as usual by noting
the brightest stars in the heavens and arranging them in order on a
list, when to his astonishment he saw “a new candidate for distinction
among the very brightest stars of first magnitude” in a part of the
sky where he was quite sure no such brilliant object had been seen
before. He consulted a map and satisfied himself that it was his “old
friend Eta Argūs,” but nearly three times as bright as usual. He made
careful comparisons with other bright stars then visible, and says
that “Fomalhaut and Alpha Gruis were at the time not quite so high,
and Alpha Crucis much lower, but all were fine and clear, and Eta
Argūs would not bear to be lowered to their standard.” It was a little
brighter than Rigel, and the only stars which outshone it were Sirius
and Canopus.
Still it grew brighter, for twelve days later it greatly surpassed
Rigel and could only be compared with α Centauri. After this the
light began to fade, and by April 1838 it was not much brighter than
Aldebaran.
Herschel now returned to England, and therefore he did not see the
still more startling changes of this wonderful star, but he has
recorded what he heard from others. In March 1843 it became much
brighter than Rigel or α Centauri, but its light wavered, and he says:
“We have here an epoch of great interest, a temporary minimum, with
a kind of trepidation or fluttering of light, followed, however, by
another step in advance even yet more extraordinary.” This was in the
following month, April 1843, when Eta became almost equal to Sirius,
the brightest of all stars. It was the highest point reached by this
extraordinary star, and two years later Maclear at the Cape wrote to
the Astronomer-Royal in England: “When you see Sir John Herschel again,
tell him that Eta Argūs has been for some time rather larger than
Canopus, and seems again on the decline.”
Sir John’s concluding remarks seem to indicate something of pained
surprise that a star could behave in so erratic and unaccountable a
fashion: “A strange field of speculation is opened by this phenomenon.
Here we have a star fitfully variable to an astonishing extent, and
whose fluctuations extend over centuries.... What origin are we to
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 uncertain a source?”
Eta Argūs continued to fade, and for many years it has not been visible
to the naked eye. When the present writer looked for it in April 1908
it was beyond the power of the binocular, although seventh-magnitude
stars in the neighbourhood were clearly distinguished and identified.
In a telescope it in no way stands out from the crowd of small stars
scattered over the nebula whose light it once almost blotted out by its
brilliance. Reports now and then arise that Eta is brightening again,
but it always turns out that some neighbour in the throng, a little
brighter than the faded star, has been mistaken for it.
A few other cases are known in which a bright star has appeared where
none had been seen before. It is said that it was the appearance
of such a “new star” in Scorpio in the year 136 B.C. which led
Hipparchus of Rhodes to draw up his famous star-catalogue. In A.D.
1572 “Tycho’s star” blazed out in Cassiopeia, and in 1604 “Kepler’s
star” in Ophiuchus astonished everyone. The old chronicle says: “It
was exactly like one of the stars, except that in the vividness of its
lustre and the quickness of its sparkling it exceeded anything Kepler
had ever seen before. It was every moment changing into some of the
colours of the rainbow, as yellow, orange, purple, and red, though it
was generally white when it was at some distance from the vapours of
the horizon.” This “new star” must have been even brighter than Eta
Argūs,[7] for it outshone Jupiter, and was only surpassed by Venus. It
remained visible for about a year and then vanished.
[7] Now often called Eta Carinae, since Argo has been subdivided (see
p. 7).
Since the skies have been more carefully watched by astronomers
all over the world, and especially since they have been frequently
photographed, quite a considerable number of new stars have been
recorded, and about half a dozen have become visible to the naked
eye. In the twenty-seven years between 1885 and 1912, twenty were
recorded, and half of these were discovered by Mrs. Fleming of Harvard
Observatory from the examination of photographs.
Astronomers are still asking, like Herschel, what is the origin of
these mysterious objects? Are they literally new stars, or is it the
last flare-up of a dying system, or are we witnessing some catastrophe
which only overtakes a few suns among the universe of stars? A
collision between two dark stars, or between a star and a nebula, is a
supposition which naturally suggests itself, and some probability is
lent to this supposition by the fact that nearly every new star has
appeared in or near the Milky Way, where stars throng most thickly;
but there are difficulties in the way of accepting this hypothesis.
There is a strong likeness between all that have been examined
spectroscopically, and in the declining stage they become so distinctly
nebular in type that we seem justified in saying that new stars change
into small gaseous nebulae.
Does this mean that they are dying, or is it the first stage in the
life-history of a star, immediately preceding the not altogether
dissimilar Wolf-Rayet stage? We do not know enough yet about nebulae to
answer this question.
VIII
MIRA, THE WONDERFUL STAR
The head of Cetus, the Sea-Monster, is formed of three stars in a
crooked line (α, γ, δ); and a little beyond them, as far from δ as that
is from α, you may sometimes see another star, marked on the map by the
name “Mira,” which means “Wonderful.” Watch it carefully, and if it is
on the upward grade you will see it slowly brighten until it equals δ,
then γ, and if you are lucky it may even approach α in brightness; and
meanwhile it will pass from red to a clear orange-yellow; then it will
wane once more and gradually be lost to view, though you can follow it
much longer in even a small opera-glass, and you will notice that as it
grows fainter the colour becomes deeper and deeper crimson.
Unlike η Argūs with its one brilliant phase in two centuries, Mira
waxes and wanes once in every eleven months, although there is a
capricious uncertainty in both the period and the brightness which
makes her a most fascinating object to observe. Sometimes the maximum
brightness is several days earlier, sometimes later, than the average;
sometimes she only equals δ, she has been seen to excel α; and no one
can foretell exactly what and when her maximum will be.
Quite a number of other stars have been discovered which behave like
Mira, and anyone who wishes to contribute something to astronomical
research without having to buy large and expensive instruments, or to
study difficult problems, cannot do better than observe some of these
stars, carefully comparing them night after night with stars in the
neighbourhood. Here is a list of a few southern “variable stars of long
period,” as they are called, all of which are easily visible at maximum
brightness with a binocular, and some even without. A map should be
made of the surrounding stars, and a list drawn up of those which are
of the different magnitudes through which the variable passes. Every
fine night the star should be compared with these, and recorded in a
note-book as brighter than one, fainter than others, perhaps equal
to another, and so on, several comparisons being made to check each
other. When the variable passes out of the range of the binocular,
this should be noted. The British Astronomical Association, which has
a branch in Australia, has a Variable-star Section, and anyone who
becomes a member will receive ready help and advice, and may have the
pleasure of feeling that he is doing useful work in astronomy.
Most long-period variables are red stars of the Antarian or of the
carbon star classes with banded spectra, but they differ from ordinary
red stars by occasionally showing bright lines, which indicate an
uprush of intensely hot hydrogen gas in their atmospheres. These bright
lines always appear at times of maximum, and prove that the star
periodically undergoes some physical change: but what is the nature and
the cause of this change? It can scarcely be due to the near approach
of a satellite, because of the irregularity in the time of maximum.
There is a certain resemblance between the way in which the light
waxes and wanes and the waxing and waning in the number of sunspots
on the sun, and spectra of these Mira stars also somewhat resemble
sunspot spectra: can it be that our sun is an incipient variable star
with a period of about eleven years? It is true that sunspots seem
to be cooler rather than hotter regions on the sun, but a time of
maximum spots is also a time of maximum activity; slightly more heat is
actually radiating, the corona is brighter and larger, and the bright
scarlet flames of hydrogen and calcium which we call prominences are
larger and more abundant. Possibly it is a tremendous display of these
on Mira which makes the hydrogen lines bright at maximum.
SOME SOUTHERN VARIABLE STARS OF LONG PERIOD
+------------+----------+----------+---------+----------------------+
| Name. | Maximum | Minimum | Period. | Remarks. |
| |Magnitude.|Magnitude.| | |
+------------+----------+----------+---------+----------------------+
| | | | Days. | |
|Mira | 1.7 | 9.6 | 331½ | Discovered in |
| | | | |1596. Large irregular-|
| | | | |ities in period |
| | | | |and max. brightness. |
| | | | | |
|R Sculptoris| 6.2 | 8.8 | 376½ | A red “carbon |
| | | | |star.” |
| | | | | |
|R Leporis | 6.1 | 9.7 | 436 | Crimson. A |
| | | | |“carbon star.” |
| | | | | Observed from 1852 |
| | | | |to 1883, but very |
| | | | |few observations |
| | | | |published since, so |
| | | | |new work would |
| | | | |be valuable. |
| | | | | |
|T Centauri | 6.5 | 8.0 | 90¼ | Period unusually |
| | | | |short for this |
| | | | |class of variables. |
| | | | | |
|R Centauri | 5.3 | 13.0 | 568 | An Antarian |
| | | | |star. Double max. |
| | | | |and min. The |
| | | | |secondary min. |
| | | | |about mag. 8½. |
| | | | | |
|R Scuti | 4.8 | 7.8 | ? | Bright and faint |
| | | | |minima, usually |
| | | | |alternative. Period |
| | | | |perhaps irregular. |
| | | | | |
|S Sculptoris| 5.8 | 11.8? | 366 | |
|R Horologii | 5.9 | 12.0 | 405 | |
|L² Puppis | 3.4 | 6.2 | 140 | |
| | | |or longer| |
|R Carinae | 4.5 | 10.0 | 309 |{ Antarian stars. |
| | | | |{ Spectra very similar|
|S Carinae | 5.8 | 9.0 | 148 |{ to that of Mira. |
|R Hydrae | 4.0 | 9.8 | 425 | |
|S Virginis | 5.6 | 12.3 | 379½ | |
|R Aquarii | 6.2 | 11.0 | 387 | |
+------------+----------+----------+---------+----------------------+
Against this suggestion we must set the fact that no stars have been
found to link together the slow and slight change represented by the
sunspot period with the rapid and violent change suffered by Mira
variables. Their periods, though never less than three months, are
never more than two years, and the light radiated by Mira at maximum
sometimes amounts to five thousand times as much as at a faint minimum.
Nevertheless, the fact that these perplexing stars are a link in an
unbroken chain of which our sun also forms part suggests that research
on the sun, our nearest star, will some day help us to understand more
about Mira and stars like Mira.
IX
ECLIPSING STARS
There is another type of variables quite distinct from the Mira stars.
These run through smaller light-changes in much shorter periods, also
they change abruptly with clockwork regularity, and the spectrum shows
no bright lines at maximum, indicative of physical change.
The first known was Algol, the Ghoul or Demon Star in Perseus,
and the brightest southern star of the Algol type is δ Librae. It
shines steadily for a little more than two days as a fifth-magnitude
star, then in a few hours drops suddenly to below sixth magnitude,
becoming invisible to the naked eye, and as quickly recovers its usual
brightness. The entire change takes place regularly in less than two
and a half days.
A few bright Algol stars in the south are:
SOUTHERN ALGOL VARIABLES
+---------------+----------+----------+------------+
| Name. | Maximum | Minimum | Period. |
| |Magnitude.|Magnitude.| |
+---------------+----------+----------+------------+
| | | |Days. Hours.|
|RS Sagittarii | 5.9 | 6.3 | 2 9 |
|R Canis majoris| 5.8 | 6.4 | 1 2 |
|δ Librae | 4.8 | 6.2 | 2 7 |
|R Arae | 6.8 | 7.9 | 4 9 |
+---------------+----------+----------+------------+
Altogether nearly a hundred Algol stars are now known, and seventy-four
of these lie in or near the Milky Way.
Unlike the mysterious Mira stars, the variation of Algol stars has been
explained. The sudden drop in the light is a partial eclipse, caused
by a dark or partly dark companion which for a time hides the bright
star from us. When a source of light is coming towards us, the lines in
its spectrum are shifted towards the violet, when going away they are
shifted towards the red, exactly as the whistle of an engine becomes
more shrill when approaching us, and falls to a lower pitch when
going away. In this way it has been discovered that an Algol star is
revolving round an invisible companion, for it alternately approaches
and recedes, and these movements correspond with its light-changes.
It is, in fact, a spectroscopic binary which happens to have an orbit
whose plane lies just in our line of sight, so that at every revolution
one star passes behind the other.
The speed of the star in its orbit can be accurately determined (by
the amount of shift in the spectrum lines) in miles per second, even
when we do not know its distance from us; hence the size of its orbit
can be calculated, since we know the period in which it is completed;
and, further, the size of the orbit gives us the mass of the stars,
for their movements depend upon the attraction they exercise over one
another, and this is proportional to their mass; and so we are able to
picture the system, although the eclipsing star is never seen and the
distance from us may never be known. Here is indeed a triumph of modern
astronomy.
Very curious are the systems thus discovered. Algol stars are
extraordinarily light for their size, their density being always less,
and sometimes immensely less, than that of water, and the companions
are usually extraordinarily close together. In some pairs they seem to
be actually touching. Nearly all are Sirian stars; a few are of Orion
and solar types.
Sometimes the companion star gives light also, instead of being dark,
and then we have a different type of variation. There are two eclipses
in one revolution, each star passing alternately behind the other, but
neither is a very dark eclipse, only a lessening of light. β Lyrae was
the first star of this kind to be discovered; its southern counterpart
is U Scuti, which varies from magnitude 9.1 to 9.6, and runs through
its two maxima and two minima in less than twenty-three hours! It is a
Sirian star.
X
SHORT-PERIOD VARIABLE STARS
Another extremely interesting class of variable stars runs through the
periods as punctually as the Algol stars, but the light is varying up
and down the whole time without any period of steadiness. The length,
too, is often much greater, though not nearly so great as that of the
Mira variables.
Here are a few stars of this class bright enough to be observed with a
binocular, at least at maximum:
SOUTHERN SHORT-PERIOD VARIABLES
+----------------------+----------+----------+-------+
| Name. | Maximum | Minimum |Period.|
| |Magnitude.|Magnitude.| |
+----------------------+----------+----------+-------+
| | | | Days. |
|W Sagittarii | 4.3 | 5.1 | 7½ |
|κ Pavonis | 4.3 | 5.2 | 9 |
|_l_ Carinae | 3.6 | 5.0 | 35½ |
|S Crucis | 6.5 | 7.6 | 4½ |
|V Centaurii | 6.4 | 7.8 | 5½ |
|S Trianguli australi | 6.4 | 7.4 | 6½ |
|S Normae | 6.6 | 7.6 | 9¾ |
+----------------------+----------+----------+-------+
The three first on the list have been discovered to be spectroscopic
binaries, the motions varying with the light, and as this is found to
be the case with all those whose motions are known, there can be little
doubt that all variables of this class are binaries. Their orbits,
however, are somewhat larger than those of the Algol variables, and are
tilted towards us so that neither star can be seen to pass in front
of the other. For the gradual and continual change of light cannot
possibly be caused by an eclipse. Yet it seems clear that it is in some
way connected with the revolution of two stars about one another.
Various suggestions have been made, such as that the revolving star
is unequally bright over its surface, and shows us now its brighter
and now its duller face; that the two pull one another out of shape
when they approach most nearly; that they are permanently elliptical,
and turn to us first the broad and then the narrow face. Any or all
of these may play some part in the variations of the light. But the
most hopeful theory takes into account a very strange fact lately
established, viz. that the variable is at its brightest not when
approaching its fellow but when coming directly toward us, and at its
faintest when receding directly from us.
This theory is that a dark and bright star are involved in a kind
of thin nebulous cloud, and that the bright revolving star has an
enveloping atmosphere. As it moves through the cloud this atmosphere
is continually brushed back from its advancing face, and therefore we
shall see it when coming straight towards us through a less thickness
of atmosphere, and it will look brighter than when it is retreating
from us.
There are grave difficulties in accepting this view, but it receives
some support from the case of β Scorpii, in which, as we saw, a double
star is suspected, for quite a different reason, of being surrounded by
a thin cloud. And the atmosphere supposed to be surrounding the bright
star may resemble the “smoky veil” which we know envelops our own sun,
and causes a considerable absorption of light, for these variables are
solar stars. The resistance of the cloud to the motion of the revolving
star ought in time to shorten its period, and some variables have been
found to be shortening their periods.
Short-period variables are found chiefly in the Milky Way.
XI
IRREGULAR AND DOUBTFUL VARIABLES
Besides these well-marked classes of long-period, short-period, and
Algol variables, there are some stars which seem to vary spasmodically,
remaining sometimes for months or even for years without any change;
and there are others whose variability is suspected but has never been
confirmed. Useful work might be done by amateurs in trying to decide
the status of these doubtful stars.
It must be borne in mind that red stars are notoriously difficult
objects. Two observers comparing one at the same time with the same
star will often come to opposite conclusions, showing how difficult it
is to compare stars which differ much in colour. Whenever possible, red
comparison stars should be selected to avoid this uncertainty.
The second star on the list below has a very peculiar type of
variation, unlike any other except a northern star, R Coronae, and a
third recently discovered. The spectra of RY Sagittarii and R Coronae
are also peculiar and resemble one another.
The following are bright southern variables suspected to vary, or known
to vary without recognised laws:
SOUTHERN IRREGULAR AND SUSPECTED VARIABLES
+--------------+----------+----------+-----------------------------+
| Name. | Maximum | Minimum | Remarks. |
| |Magnitude.|Magnitude.| |
+--------------+----------+----------+-----------------------------+
|Z Sculptoris | 6 | 8 | Period unknown. |
| | | | |
|RY Sagittarii | 6.5 | Fainter | Usually steady, but subject|
| | |than 11.5 | to a sudden drop at |
| | | | irregular intervals. |
| | | | Worth watching. |
| | | | |
|RT Capricorni | 6.5 | About 8 | Period probably irregular. |
| | | | Very few observations |
| | | | published. |
| | | | |
|S Phoenicis |{ 7.2 | 8.7 }| Observers differ. |
| |{ or | or }| Period irregular. |
| |{ 6.8 | 8.4 }| |
| | | | |
|W Ceti | 6.5 | 12 | Period perhaps 366 days |
| | | | |
|T Ceti | 5.4 | 6.9 | Irregular. |
| | | | |
|S Leporis | 6.5 | 7.5 | Irregular. |
| | | | |
|U Hydrae | 4.5 | 6.3 | Irregular. Red |
| | | | “carbon star.” |
| | | | |
|θ Apodis | 5.5 | 6.6 | Period probably irregular. |
| | | | |
|R Apodis | 5.5 | 6.2 | Suspected. |
| | | | |
|T Indi | 7.2 | 8.9 | No regular period found |
+--------------+----------+----------+-----------------------------+
XII
STAR-CLUSTERS
Endlessly varied is the grouping of the stars. We find solitary stars,
twin stars, large stars with small companions, bright stars with
faint companions, stars alike in colour and type, stars in strongly
contrasted colours, one young, the other aged, stars with invisible
companions, couples close enough to touch one another, and couples
so distant that the satellite takes more than a century to revolve
round its primary. One or even both components of a bright pair may be
themselves divisible into a closer pair, sometimes into a whole group
of stars.
More beautiful and wonderful still are the clusters of stars of various
tints and magnitudes, where scores, even hundreds, are gathered
together in what to the eye is a tiny patch, though it may in fact take
light many years to cross from one side of it to the other.
The finest example in the heavens of this kind of cluster is Kappa
Crucis, near β in the Southern Cross.[8] It is just visible to the
naked eye as a small star, and in a binocular the main star is seen
to be surrounded by a number of others; in a telescope it is a
glorious sight. Orange and red stars are easily distinguished in the
brilliant throng, even if we have not Herschel’s eye for colour and
fail to discriminate the “greenish-white, green, red, blue-green,
and ruddy” which made up what he likened to “a superb piece of fancy
jewellery.” He charted over a hundred stars of all magnitudes from
7 to 17. Herschel’s observations at the Cape were made between 1834
and 1838. When Mr. Russell charted the stars of Kappa Crucis at the
Sydney Observatory in 1872, he found 25 that had not been recorded by
Herschel, although the great reflector was much larger than the Sydney
instrument; many of Herschel’s stars had drifted, and five could not be
found at all. If changes so striking as this take place in less than
forty years, “it is evident,” as Russell observed, “that more attention
should be bestowed on clusters.”
[8] It is easy to remember the names of the stars in the Southern
Cross. Begin at the foot, which is obviously the brightest, and count
round the Cross in clockwise direction α, β, γ, δ. κ is beyond β in a
line with γ, β.
Quite a dozen star-clusters of this kind are visible to the naked eye
in the southern hemisphere, the most striking being in Scorpio, where
it shines as a conspicuous silvery spot just beyond the Scorpion’s
tail, midway between κ Scorpii and γ Sagittarii. It is named M 7,
which means the seventh in Messier’s list of clusters and nebulae.
In a binocular it is seen to be a group of very many stars, some
close together, others scattered. Lacaille, with his little half-inch
telescope, counted from 15 to 20 stars, and Herschel, with his large
reflector, estimated the number at 60. At Cordoba Observatory no less
than 139 were catalogued.
M 6, a little north and west of M 7, is also visible to the naked eye
as a nebulous patch, and is a fine cluster. About 50 stars between
magnitudes eight and twelve have been photographed, and there are
doubtless many more of lesser brightness.
Look also just above the naked-eye double ζ in Scorpio, and see what
appears to be a hazy star. A binocular separates this into a number of
small stars, and 150 have been photographed, of magnitudes seven to
twelve. It is known as h 3652 or N.G.C. 6231, the former being the
number in Sir John Herschel’s catalogue, the latter in Dreyer’s New
General Catalogue of star-clusters and nebulae.
A little south of Sirius is a patch of nebulous light which shows as
stars in a binocular. This was registered by Messier in 1764 as a “mass
of small stars,” and is known as M 41. Webb saw the brighter stars
arranged in curves and a ruddy star near the centre. One hundred and
forty-four stars were registered by Gould, of which only five are as
bright as eighth magnitude.
In the neighbourhood of the Cross there are quite a number of large
bright clusters, for they must be large and bright above the average
to be seen by the naked eye. A line passing through the shorter arm
of the False Cross—_i.e._ from δ Velorum to ι Carinae, the naked-eye
double—and continued for an equal distance beyond, leads to a white
oval patch which is plainly visible to the naked eye, and in the
binocular appears like a few stars sparkling on a nebulous background.
With higher powers the background also is resolved into stars, of which
there are some two hundred of the fifteenth magnitude and brighter up
to the eighth. This is N.G.C. 3114.
In the same direction is θ Carinae, a bright Orion-type star with
numerous small stars crowding close to it. This is a very lovely group
in a good binocular. It contains about twenty stars of magnitudes
between three and eight, and with high powers appears as a brilliant,
loosely scattered cluster covering a portion of sky equal in breadth to
twice the sun’s apparent diameter.
Very similar is a cluster close to the bright and richly coloured star
X Carinae. It looks like an elliptical nebula in a binocular, with a
few stars scattered over it. Two hundred have been photographed.
Near ε Carinae, the beautiful ruddy star at the foot of the False
Cross, is yet another most beautiful cluster, which contains about
fifty stars of the ninth magnitude and brighter. It is visible to the
naked eye, but when a telescope is turned upon it the brilliancy is
startling. Radiant stars are scattered all over the field.
Just east of π Puppis, the top of the poop of Argo, is a
fourth-magnitude star C of a bright orange colour, and round it is a
cluster which Gould describes as “extremely impressive to the naked
eye.” Ninety-two stars show on his photograph.
Close to δ Velorum (in the False Cross) is the star ο Velorum. Even to
the naked eye the bright star (3½ mag.) is seen to be surrounded by
a cluster of faint stars, and in a binocular it is a splendid sight.
The number of stars is small, but they are bright, ten being above the
ninth magnitude.
Lastly we may mention a remarkably fine cluster which is visible in
a binocular though not to the naked eye, in the Centaur, among a
stream of small stars between the Cross and η Argūs. Dunlop called it
“a pretty large cluster of stars of mixt magnitudes”; Brisbane, “a
prodigious number of small stars very close together.”[9] There are at
least a hundred above the sixteenth magnitude.
[9] These two astronomers observed at Paramatta, New South Wales, in
the early part of the nineteenth century.
It should be noticed that nearly all these clusters are in the Milky
Way, and the rest are very near it, for this is characteristic of
star-clusters like these, viz. irregular groups containing mixed
magnitudes of stars. In a list of all bright objects of this kind, only
two are found as far as 30° away from the middle line of the Galaxy,
while 89 are within 30° north or south of it. There are besides 38 in
the Clouds of Magellan, which resemble the Milky Way in constitution.
Many clusters are simply unusually dense portions of the Milky Way, and
we may almost say that this type forms part of it and of the Magellan
Clouds.
XIII
GLOBULAR STAR-CLUSTERS
If Kappa Crucis is the finest irregular star-cluster in the sky,
Omega Centauri is undoubtedly the largest and most splendid of all
the globular star-clusters, for its diameter is more than twice the
diameter of the famous northern cluster in Hercules. It is easily
found, being nearly in line with δ and γ Centauri (the two conspicuous
stars just north of the Cross) and a little further from γ than γ
is from δ. The cluster looks to the naked eye just like a tailless
comet, and was mistaken for one by the author when first seen. In a
binocular it is quite round, the soft milky light growing gradually
brighter towards the centre, but without the slightest suggestion of
irregularity, and no appearance of stars. It must have been seen by
the early navigators who named the southern constellations, but it was
first discovered as a star-cluster by Halley in the island of St.
Helena. Most amazing is its appearance in a telescope, for the milky
disc breaks up into thousands of tiny points of light, densely crowded,
all alike, innumerable.
“This most glorious object,” as Herschel calls it, “the noble globular
cluster ω Centauri, beyond all comparison the richest and largest
object of the kind in the heavens,” is evidently quite distinct from
κ Crucis and clusters like those described in the last chapter. In
form it is circular, and the condensation towards the centre suggests
that it is spherical. There are some scattered members of the group
lying outside the bright crowded sphere. The stars are also immensely
more numerous and more closely packed than in the irregular clusters,
their total number being estimated at 10,000, and 6000 have been
actually counted on photographs, and all these in a space which looks
little larger than that occupied by the sun in the sky. Another
striking difference is that, instead of bright and faint stars mingled
together, here they are all nearly alike and very minute. Curiously
enough, it is found that they belong to two magnitudes, and two only,
the thirteenth and fifteenth, and this seems to be a feature of all
globular clusters, as well as the form and the dense crowding of the
stars. Herschel at first thought the stars in ω Centauri “singularly
equal, and distributed with the most exact equality, the condensation
being that of a sphere equally filled.” But he immediately adds:
“Looking attentively, I retract what is said about the equal scattering
and equal sizes of the stars. There are two sizes ... without greater
or less, and the larger stars form rings like lace-work on it.” In
his later notes he is again doubtful, for he thinks that the effect
may be optical, and the larger stars only knots of faint stars; but
photography has settled the question in our day.
Yet another point of difference between globular and irregular clusters
is that the latter often have wisps of nebulosity clinging about them,
but globular clusters are entirely free from it.
[Illustration: THE STAR-CLUSTER 47 TOUCANI
From Sir John Herschel’s drawing]
Smaller than ω Centauri, but even more beautiful in the telescope,
is the cluster 47 Toucani,[10] which to the unaided eye appears like
a fourth-magnitude star near the smaller Cloud of Magellan. The
long curve of Grus followed southwards leads to it. Nearly as many
stars as in ω Centauri, or about 9500, are here massed into a still
smaller space, so the cluster is brighter, and is “compressed to
a blaze of light” at the centre. The two sets of stars, which are
mingled together throughout, are of thirteenth to fifteenth and of
seventeenth magnitudes respectively. Herschel saw the inner denser part
rose-coloured while the outer was white, but the present writer could
not see this nor find anyone to confirm it to-day, possibly because
the refracting telescopes now so often used do not show colour so well
as large reflectors like Herschel’s. A double star of 11th magnitude,
which is conspicuous in Herschel’s drawing, is doubtless far outside
the cluster, and only appears projected against it by perspective.
[10] Also named ξ Toucani.
Near β Aquarii there shines with the light of a sixth-magnitude star
another “magnificent ball of stars” which has been compared to “a heap
of fine sand.” It is named 2 M Aquarii.
Over seventy of these tightly packed balls of stars are known, even
counting only the brightest, and their distribution is rather curious.
A large number (about twenty) occur in the Clouds of Magellan, and more
than half of the seventy are in the Milky Way, not scattered evenly
along its course, but almost if not entirely confined to its southern
part, and chiefly gathered in a great group in its brightest portion,
where it passes through Sagittarius, Ophiuchus, and Aquila. Here they
are mingled with—or perhaps projected against—numerous stars of the
same magnitudes; but many balls are also found outside the Milky Way,
widely scattered, and in these parts of the sky there are relatively
few of the faint-magnitude stars which compose all the globular
clusters. 47 Toucani, for instance, though it is near the small
Magellanic Cloud, stands quite apart from it, isolated in a black sky.
We do not know the distances of any of these balls of stars. Those
which have been examined spectroscopically shine like Canopus—that is,
they are of a type intermediate between Sirius and our sun—but the
chief light comes, of course, from the brighter stars, and it may be
that the fainter stars mingled with them belong to a different type.
A remarkable fact lately discovered is that many globular clusters—but
not all—contain a large number of variable stars. These vary in light
in a period of about a day and have a range of about one magnitude.
They are not of the Algol type, nor quite of the usual “short-period”
types, and it is not yet clear what is the cause of variation, though
it seems probable that “cluster-variables” are double stars.
XIV
NEBULAE
Athwart the False Cross, from δ Velorum to ι Carinae, a line passing
on leads to the round white spot which we found to be a star-cluster.
A little further in the same direction is a larger curved white patch,
bright enough to be visible, once it is familiar, even after the moon
has risen. This is the Great Nebula in Argo, the Keyhole Nebula, in
which Eta Argūs once blazed out. Even a binocular will divide it into
two parts separated by a chasm, and will show the pearly background
powdered over with many small stars.
But even the most powerful telescopes do not resolve this pale
background into stars, as they resolve the star-cluster just mentioned:
it remains a pearly mist, the brighter part strangely broken by dark
rifts, the fainter, beyond the chasm, a tangled skein of long cloudy
streaks reaching out into the darkness and gradually, irregularly,
fading away.
When Herschel found this background unresolvable into stars, he
concluded that it did not form part of the Milky Way, but was at
an immeasurable distance behind, so that here he was looking right
through the Galaxy at a still more distant region of stars, too distant
and faint for his telescope to distinguish them separately. But the
spectroscope has taught us that these cloud-like nebulae, though stars
are often mingled with them, are not formed of stars at all, but of
inchoate masses of faintly luminous gas; and they cluster so thickly in
the Milky Way, generally avoiding other parts of the sky, that it seems
evident that they lie in it and form part of it. They are also found in
great numbers in the Greater Magellanic Cloud.
If the days of the Herschel’s photography had not come to the aid
of astronomers, and Sir John speaks of the feeling of despair which
often almost overcame him when trying, night after night, to draw the
“endless details” of this nebula, so capricious in their forms are
its curving branches and the dark spaces between, so strangely does
its brightness vary in different regions, and so numerous are the
stars scattered over it. With extraordinary patience he succeeded in
cataloguing the positions of over 1200 of these. To compare the present
aspect of the stars with his catalogue would be a laborious task, but
might lead to results of great value.
The curious dark oval rift in the midst of the bright part, which
he compared with a keyhole, he found to be not entirely devoid of
light, a thin nebulous veil covering part of it; and many of the dark
lanes and holes which in small instruments look perfectly black, are
actually filled with faint stars and extremely faint nebulosity. The
whole region near the nebula is exceedingly rich in stars, and also in
star-clusters, as we have already seen. To quote Herschel once again:
“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 conveyed to these pages. In
fact, it is impossible for anyone with the least spark of astronomical
enthusiasm about him to pass soberly in review, with a powerful
telescope and in a fine night, that portion of the southern sky ...
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
presented to his gaze.”
In the constellation of the Sword-fish, on the edge of the Great Cloud
of Magellan, is another nebula, 30 Doradūs, the Great Looped Nebula,
which is even more marvellous in complexity of structure than the
Keyhole Nebula in Argo. No photograph can reproduce, and no words can
describe, the filmy appearance of these nebulae as seen in a telescope.
The Looped Nebula seems to consist entirely of strangely curved and
twisted streamers on a background of dark sky, with a few sparkling
stars of various brightness scattered over it. At the complicated
centre one of the loops forms a nearly perfect figure-of-eight, and
another takes the outline of an eye.
Brightest of all the large gaseous nebulae is the well-known Orion
Nebula, in the sword of the giant. A 3-inch telescope shows the main
features well, the dark bay running into its brightest region, the
row of three brilliant stars and the “trapezium” of four tiny ones
very close together, and the long outlying branches which have such
fantastic curves. Because of its comparative brightness, its entrancing
beauty, and its position where it can be seen from all latitudes, this
nebula has been studied more than any other. The first drawing of it
was made in 1656, the first photograph in 1880. It remains a baffling
mystery still, but a few facts have emerged.
Its distance is immeasurable: it has been guessed at a thousand
light-years. It must, therefore, be inconceivably vast in extent, but
it is probably excessively tenuous, like a comet’s tail, of which a
million miles contain a negligible amount of matter. It is almost
stationary in space, and a careful study of its form since 1758
proves that there has been no visible change, except perhaps in the
relative brightness of some of its parts. Yet a recent spectroscopic
investigation shows that movements are taking place in different
directions within the nebula, and a slow rotation of the whole mass, or
of its brightest portion, is suggested.
It is composed of faintly luminous gas, though whether it glows from
heat or from some other cause we do not know. Photographs of nebulae
are very misleading with regard to brightness: one must remember that
they have often been exposed for many hours. Helium, hydrogen, and
an unknown gas which we call nebulium are mingled together, but not
in equal quantities. In some of the fainter regions of the nebula,
especially on the south and west borders, hydrogen produces a great
deal of the light; in the brightest parts, near the trapezium, the glow
of nebulium is much more prominent.
It is scarcely doubtful that many of the stars which appear to be
involved in the nebula are physically connected with it, especially
since they are of a type frequently found near nebulae, viz. very blue
Orion-type stars with some of their hydrogen lines not dark but bright,
as in the nebula.
The southern hemisphere is rich in nebulae smaller but of the same kind
as these three magnificent objects, the Keyhole, the Looped, and the
Orion Nebulae—that is, large irregular masses of gas, often spangled
with stars—and each has some special beauty of its own; but for most of
them large telescopes are needed to grasp the faint details. There is
a nest of them in the northern part of Sagittarius: a cloudy streak
visible to the naked eye, a little north of the star γ Sagittarii,
represents three nebulae and clusters close together—M 8, M 20, and
M 21. The first is a wonderful combination of a bright scattered
star-cluster and a gaseous nebula, with dark rifts dividing the cloudy
structure. The second is the celebrated Trifid Nebula, less bright
and large, but with even more striking black lanes which split the
principal part into three almost separate portions. Many faint stars
are scattered over it, but as they are scarcely more numerous than in
the surrounding regions, most of them probably are not connected with
the nebula. M 21 is a star-cluster.
Near these, where Sagittarius borders on Aquila,[11] is a small but
very remarkable nebula, known from its shape as the Horseshoe or the
Omega Nebula (M 17). It has a curious mottled appearance, with bright
knots here and there.
[11] On Scutum in maps where this constellation is not included in
Aquila.
And a little further west, near together, are two wonderful nebulae
which surround the two stars Rho Ophiuchi and Nu Scorpii. Professor
Barnard, who has studied and taken exquisite photographs of many
nebulae, considers the first of these the finest in the sky, because
of its dark, winding lanes and the veiling of the stars in places by
partly transparent nebulous matter.
XV
OTHER TYPES OF NEBULAE
The large irregular nebulae described in the last chapter are all more
or less mingled with stars, at least in appearance, and it has been
suggested that they are star-clusters in process of formation, with
larger and brighter masses of filmy nebulosity all about them than at
later stages, for long-exposure photographs reveal some exceedingly
faint nebulosities surrounding Kappa Crucis and the Pleiades and other
fully-developed star-clusters. But this can only be a guess until we
know more about the nature of nebulae. In some regions of the sky we
find vast spaces thinly veiled by nebulosity so faint and transparent
that it seems to have reached the very limit at which matter can exist
and be recognised as such. Thus in the constellation of Orion nearly
all the bright stars are connected together by the vast convolutions
of an exceedingly faint diffused nebula in spiral form, the innermost
curve of which ends in the Great Nebula of the Sword, and the whole
region within is filled with faint light.
Quite distinct from these nebulae are others of perfectly regular form,
very small, purely gaseous, without intermingling of any stars, but
usually with one bright star-like nucleus at the centre. One form is
the ring nebula, of which much the best known is that in the northern
constellation of the Lyre. There are, however, some in the south. In a
large telescope they appear like little golden wedding-rings against
the dark sky background.
Another regular form is the “planetary nebula,” so called because
they look much like planets in large telescopes, being perfectly
round or oval with a sharply-defined edge, and in several cases
there are handle-like appendages, which may possibly be encircling
rings, like the rings of Saturn. These nebulae shine with a peculiar
bluish-green light, the colour of the unknown gas nebulium, of which
they are chiefly composed. In Hydra, south of the star Mu, is one of
the brightest and largest, known as H 27—that is, No. 27 on William
Herschel’s list. It is elliptical and of a lovely bluish colour, with a
bright nucleus exactly in the centre.
By means of these sharply-defined central nuclei it has been found
possible to measure the approaching or receding movements of these
nebulae, and although the one just mentioned is receding from us with a
speed of only 3½ miles a second, their average speed is high, amounting
to 40 or 50 miles a second. One in Sagittarius is receding at more
than 80 miles a second, and another in Lupus attains a speed of over a
hundred.
These are movements comparable with those of stars, but the average
is higher than even for the most rapidly moving class of stars, the
red-solar and Antarians. May we, then, place the planetary nebulae
at the end of our star-series, since we saw that from the blue down
to the red the average movements became faster and faster, and may
we believe that all stars eventually become gaseous nebulae, as “new
stars” seem to do? But we saw that in spectrum these nebulae rather
resemble the stars at the other end of the series, the Wolf-Rayet,
which lead directly to the hottest and brightest of all, the Orion
stars. Planetary nebulae also resemble Wolf-Rayet, Orion, and Sirian
stars, and differ from solar and red stars in that they cluster near
the Milky Way, and are scarcely ever found far from it. Their place in
the universe cannot be established yet.
One more kind of nebula, the most numerous of all, remains to be
mentioned, the so-called “white nebulae,” which do not glow green
like many of the brighter planetaries, but shine with a white light
and have more or less star-like spectra, although not even the most
powerful telescopes can resolve the white cloudiness into stars. The
typical nebula of this class is the famous Andromeda Nebula, visible
to the naked eye in northern skies as a large oval spot shining softly
“like a candle shining through horn.” Photography first disclosed
the remarkable fact that it has the form of a great, closely-wound
spiral, and further research has shown that by far the greater number
of “white nebulae” have this form. There is a very fine one in
Aquarius,[12] which has been known since 1824, but visual observations
gave absolutely no idea of its true form. A photograph exposed for four
hours in September 1912 showed it clearly as about two turns of a great
spiral.
[12] N. G. C. 7293.
The distribution of this kind of nebula is quite different from that of
the gaseous nebulae, for, instead of clustering towards the Milky Way,
they avoid it, and especially the brightest region, where we saw that
the others most abound, viz. in Scorpio, Sagittarius, and Ophiuchus. On
the contrary, the largest number of these is found near the north pole
of the Galaxy—that is, as far removed from it as possible, in Virgo.
There is, however, no corresponding group about the south pole of the
Galaxy.
One investigator has found the distance of the Andromeda Nebula to be
twenty light-years, but the distance and the movements of this type
are difficult to discover. They are evidently very different from the
others, and quite as mysterious.
XVI
THE CLOUDS OF MAGELLAN
One of the wonders which most attracted the attention of early
explorers in the southern hemisphere, and roused as much interest as
the Southern Cross, was the pair of faint clouds, looking like detached
pieces of the Milky Way, which are seen in the neighbourhood of the
South Pole. Marco Polo made a sketch of the Greater Cloud, which he
describes wonderingly as “a star as big as a sack.”
Although some star-maps show short branches of Milky Way pointing
towards the two Clouds, this is incorrect, and they are quite separate
from it. Herschel was struck by their isolation, especially in the
case of the Little Cloud, which he described as situated in a “most
oppressively desolate desert,” its only neighbour being the globular
cluster 47 Toucani, which is near, but separated by a perfectly black
sky.
The Greater Cloud is much brighter to the naked eye than the Lesser,
and it is much more complex and interesting in the telescope. It
contains, moreover, the wonderful Looped Nebula, of which we have
already spoken.
Both Clouds consist of gaseous nebulae and star-clusters on a
background of vague nebulosity and crowds of almost indistinguishable
stars. But the white nebulae shun the Clouds, just as they shun the
Milky Way.
An immense number of variable stars have been discovered in the Clouds
of Magellan, of the same type as those in globular clusters. Miss
Leavitt of Harvard Observatory catalogued from photographs no less
than 969 in the Lesser Cloud and 800 in the Greater. In the latter
the greatest number of variables was found in a stream of faint stars
which connects a group of star-clusters with the Looped Nebula, and
others occur locally in certain parts of the Cloud, but few are in
its northern region or in parts where many of the brighter stars
congregate. All the variables are very faint, the usual minimum in both
Clouds being about fourteenth magnitude, and the maximum seldom more
than one magnitude brighter. A few in the Lesser Cloud have been found
with periods unusually long for this “cluster type” of variables,
amounting to 32, 66, and even 127 days. These longer periods seem to
belong to somewhat brighter stars, but they are quite as exact as the
usual period of a few days or a single day.
XVII
THE MILKY WAY
Like a great river returning into itself, the Galaxy encircles the
starry heavens, and those who know only its northern course have no
idea of its brilliance and wonderful complexity in its brightest part.
Its light is soft, milky, and almost uniform, between Cygnus and
Sirius, but when it enters Argo it becomes extremely broad, and spreads
out like a river on a flat marshy plain, in many twisting channels with
spaces between. Where Canopus shines on the bank there is a narrow
winding ford right across its whole breadth, as if a path had been made
by the crossing of a star.
After this it suddenly becomes extremely narrow, but so bright that all
the light which was shining in the broad channel seems to be condensed
in this narrow bed. In the brightest, richest part the Great Nebula of
Argo is easily distinguished by the naked eye. Contrasting with this
and other bright condensations are black gaps, the largest and blackest
of which is the well-known Coal-Sack near the Southern Cross.
[Illustration: THE MILKY WAY IN SCORPIO, LUPUS, AND ARA
Photographed at Hanover, Cape Colony,
by Bailey and Schultz]
The river now divides. One short stream, which goes north from Centaur
towards Antares, is faint and soon lost; but another northern stream
is so bright and so persistent that from Centaur to Cygnus we may say
that the Galaxy flows in a double current. This northern portion forms
first the smoke of the Altar on which the Centaur is about to offer
the Beast, then passes through the Scorpion into the Serpent-Holder,
and here, between η Ophiuchi and Corona Australis, the double stream
has its greatest width. The northern division soon grows dim and seems
to die out, but begins again near β Ophiuchi, and, curving through a
little group of stars, passes through the head of the Eagle and forms
an oval lagoon in the Swan.
The southern stream passes through the Scorpion’s Tail into
Sagittarius, then through the Eagle and the Arrow till it flows close
beside the northern stream in the Swan, and finally rejoins it in a
bright patch round α Cygni. Except just here it is much brighter than
the northern stream, and its structure is even fuller of wonderful
detail than in Argo. In Sagittarius it consists of great rounded
patches with dark spaces between. The brightest of these contains the
star γ Sagittarii; then follows a remarkable region of small patches
and streaks, the portion passing through Sagittarius and Aquila being
thickly studded with nebulae. This is followed by another bright patch,
rivalling that round γ Sagittarii, which involves the stars λ and 6
Aquilae.
This ends the most brilliant and wonderful part of the Milky Way. When
well seen, as we see it in the south, it recalls Herschel’s words,
written at the Cape when it came into view in his telescope:
“The real Milky Way is just come on in great semi-nebulous masses,
running into one another, heaps on heaps.” And again: “The Milky Way is
like sand, not strewed evenly as with a sieve, but as if flung down by
handfuls, and both hands at once.”
What is it? The ancients thought it the pathway of departed spirits,
or fiery exhalations from the earth imprisoned in the skies, or a
former road of the sun through the stars. But Democritus and some
other inquiring Greeks believed it to be the shining of multitudes
of stars too faint and too close together to be seen separately, and
we know this to be the truth. We know also, from simply counting the
stars in different regions of the sky, that their numbers increase
regularly as we go from north or south towards the Milky Way, and stars
of all magnitudes are most abundant within its course. We saw also that
star-clusters and certain kinds of nebulae frequent it, while other
kinds avoid it, and that blue and white stars are the most abundant
near it, and tend to move through space in planes parallel with it,
while the redder stars are scattered and move about in all directions.
Facts like these lead astronomers to believe that the Milky Way has a
definite relation with all the visible universe, that even the most
distant nebula is not an outlying universe apart from ours, but all are
parts of one vast stellar system.
It is possible that the Milky Way, which we see as a great circle,
double in one part, is really an immense spiral, and that we are
nearest one curve of it, the great southern division which looks so
bright. It may be that the spiral nebulae, vast though they are in
terms of earthly measurement, are tiny models of one tremendous spiral
which enfolds the universe with its coils.
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