THE STARRY HEAVENS
We now leave the bounds of our own system, and pass outwards towards
the almost infinite spaces and multitudes of the fixed stars. In doing
so we are at once confronted with a wealth and profusion of beauty and
a vastness of scale which are almost overwhelming. Hitherto we have
been dealing almost exclusively with bodies which, though sometimes
considerably larger than our world, were yet, with the exception of
the sun, of the same class and comparable with it; and with distances
which, though very great indeed, were still not absolutely beyond the
power of apprehension. But now all former scales and standards have to
be left behind, for even the vast orbit of Neptune, 5,600,000,000
of miles in diameter, shrinks into a point when compared with the
smallest of the stellar distances. Even our unit of measurement has
to be changed, for miles, though counted in hundreds of millions, are
inadequate; and, accordingly, the unit in which our distance from the
stars is expressed is the 'light year,' or the distance travelled by a
ray of light in a year.
Light travels at the rate of about 186,000 miles a second, and
therefore leaps the great gulf between our earth and the sun in
about eight minutes. But even the nearest of the fixed stars--Alpha
Centauri, a star of the first magnitude in the Southern Hemisphere--is
so incredibly distant that light takes four years and four months to
travel to us from it; while the next nearest, a small star in Ursa
Major, is about seven light-years distant, and the star 61 of the
constellation Cygnus, the first northern star whose distance was
measured, is separated from us by two years more still.
At present the distances of about 100 stars are known approximately;
but it must be remembered that the approximation is a somewhat
rough one. The late Mr. Cowper Ranyard once remarked of measures of
star-distances that they would be considered rough by a cook who was
in the habit of measuring her salt by the cupful and her pepper by the
pinch. And the remark has some truth--not because of any carelessness
in the measurements, for they are the results of the most minute and
scrupulous work with the most refined instrumental means that modern
skill can devise and construct--but because the quantities to be
measured are almost infinitely small.
It is at present considered that the average distance from the earth
of stars of the first magnitude is thirty-three light years, that of
stars of the second fifty-two, and of the third eighty-two. In other
words, when we look at such stars on any particular evening, we are
seeing them, not as they are at the moment of observation, but as they
were thirty-three, fifty-two, or eighty-two years ago, when the rays
of light which render them visible to us started on their almost
inconceivable journey. The fact of the average distance of
first-magnitude stars being less than that of second, and that of
second in turn less than that of third, is not to be held as implying
that there are not comparatively small stars nearer to us than some
very bright ones. Several insignificant stars are considerably nearer
to us than some of the most brilliant objects in the heavens--_e.g._,
61 Cygni, which is of magnitude 4·8, is almost infinitely nearer to us
than the very brilliant first magnitude star Rigel in Orion. The rule
holds only on the average.
The number of the stars is not less amazing than their distance. It is
true that the number visible to the unaided eye is not by any means
so great as might be imagined on a casual survey. On a clear night the
eye receives the impression that the multitude of stars is so great
as to be utterly beyond counting; but this is not the case. The
naked-eye, or 'lucid,' stars have frequently been counted, and it
has been found that the number visible to a good average eye in
both hemispheres together is about 6,000. This would give for each
hemisphere 3,000, and making allowance for those lost to sight in
the denser air near the horizon, or invisible by reason of restricted
horizon, it is probable that the number of stars visible at any one
time to any single observer in either hemisphere does not exceed
2,500. In fact Pickering estimates the total number visible, down to
and including the sixth magnitude, to be only 2,509 for the Northern
Hemisphere, and on that basis it may safely be assumed that 2,000
would be the extreme limit for the average eye.
[Illustration:
PLATE XXVII.
Region of the Milky Way in Sagittarius. Photographed by Professor E.
E. Barnard.]
But this somewhat disappointing result is more than atoned for when
the telescope is called in and the true richness of the heavenly host
begins to appear. Let us take for illustration a familiar group of
stars--the Pleiades. The number of stars visible to an ordinary eye
in this little group is six; keen-sighted people see eleven, or even
fourteen. A small telescope converts the Pleiades into a brilliant
array of luminous points to be counted not by units but by scores,
while the plates taken with a modern photographic telescope of 13
inches aperture show 2,326 stars. The Pleiades, of course, are a
somewhat notable group; but those who have seen any of the beautiful
photographs of the heavens, now so common, will know that in many
parts of the sky even this great increase in number is considerably
exceeded; and that for every star the eye sees in such regions a
moderate telescope will show 1,000, and a great instrument perhaps
10,000. It is extremely probable that the number of stars visible with
the largest telescopes at present in use would not be overstated at
100,000,000 (Plate XXVII.).
It is evident, on the most casual glance at the sky, that in the words
of Scripture, 'One star differeth from another star in glory.' There
are stars of every degree of brilliancy, from the sparkling white
lustre of Sirius or Vega, down to the dim glimmer of those stars
which are just on the edge of visibility, and are blotted out by the
faintest wisp of haze. Accordingly, the stars have been divided into
'magnitudes' in terms of scales which, though arbitrary, are yet found
to be of general convenience. Stars of the first six magnitudes come
under the title of 'lucid' stars; below the sixth we come to the
telescopic stars, none of which are visible to the naked eye, and
which range down to the very last degree of faintness. Of stars of the
first magnitude there are recognised about twenty, more or less.
By far the brightest star visible to us in the Northern Hemisphere,
though it is really below the Equator, is Sirius, whose brightness
exceeds by no fewer than fourteen and a half times that of Regulus,
the twentieth star on the list. The next brightest stars, Canopus and
Alpha Centauri, are also Southern stars, and are not visible to us
in middle latitudes. The three brightest of our truly Northern stars,
Vega, Capella, and Arcturus, come immediately after Alpha Centauri,
and opinions are much divided as to their relative brightness, their
diversity in colour and in situation rendering a comparison somewhat
difficult. The other conspicuous stars of the first magnitude visible
in our latitudes are, in order of brightness, Rigel, Procyon, Altair,
Betelgeux, Aldebaran, Pollux, Spica Virginis, Antares, Fomalhaut,
Arided (Alpha Cygni), and Regulus, the well-known double star Castor
following not far behind Regulus. The second magnitude embraces,
according to Argelander, 65 stars; the third, 190; fourth, 425; fifth,
1,100; sixth, 3,200; while for the ninth magnitude the number leaps
up to 142,000. It is thus seen that the number of stars increases with
enormous rapidity as the smaller magnitudes come into question, and,
according to Newcomb, there is no evidence of any falling off in
the ratio of increase up to the tenth magnitude. In the smaller
magnitudes, however, the ratio of increase does not maintain itself.
The number of the stars, though very great, is not infinite.
A further fact which quickly becomes apparent to the naked eye is that
the stars are not all of the same colour. Sirius, for example, is of a
brilliant white, with a steely glitter; Betelgeux, comparatively near
to it in the sky, is of a beautiful topaz tint, perhaps on the whole
the most exquisite single star in the sky, so far as regards colour;
Aldebaran is orange-yellow, while Vega is white with a bluish cast,
as is also Rigel. These diversities become much more apparent when
the telescope is employed. At the same time the observer may be warned
against expecting too much in the way of colour, for, as a matter of
fact, the colours of the stars, while perfectly manifest, are yet of
great delicacy, and it is difficult to describe them in ordinary terms
without some suspicion of exaggeration. Stars of a reddish tone, which
ranges from the merest shade of orange-yellow up to a fairly deep
orange, are not uncommon; several first-magnitude stars, as already
noted, have distinct orange tones. For anything approaching to real
blues and greens, we must go to the smaller stars, and the finest
examples of blue or green stars are found in the smaller members of
some of the double systems. Thus in the case of the double Beta Cygni
(Albireo), one of the most beautiful and easy telescopic objects in
the northern sky, the larger star is orange-yellow, and the smaller
blue; in that of Gamma Andromedæ the larger is yellow, and the smaller
bluish-green; while Gamma Leonis has a large yellow star, and a small
greenish-yellow one in connection. The student who desires to pursue
the subject of star colours should possess himself of the catalogue
published in the Memoirs of the British Astronomical Association,
which gives the colours of the lucid stars determined from the mean of
a very large number of observations made by different observers.
In this connection it may be noticed that there is some suspicion that
the colours of certain stars have changed within historic times, or
at least that they have not the same colour now which they are said
to have had in former days. The evidence is not in any instance strong
enough to warrant the assertion that actual change has taken place;
but it is perfectly natural to suppose that it does, and indeed must
gradually progress. As the stars are intensely hot bodies, there must
have been periods when their heat was gradually rising to its maximum,
and there must be periods when they will gradually cool off to
extinction, and these stages must be represented by changes in the
colour of the particular star in question. In all probability, then,
the colour of a star gives some indication of the stage to which it
has advanced in its life-history; and as a matter of fact, this proves
to be so, the colour of a star being found to be generally a fair
indication of what its constitution, as revealed by the spectroscope,
will be.
Another feature of the stars which cannot fail to be noticed is the
fact that they are not evenly distributed over the heavens, but are
grouped into a variety of configurations or constellations. In the
very dawn of human history these configurations woke the imaginations
of the earliest star-gazers, and fanciful shapes and titles were
attached to the star-groups, which have been handed down to the
present time, and are still in use. It must be confessed that in
some cases it takes a very lively imagination to find any resemblance
between the constellation and the figure which has been associated
with it. The anatomy of Pegasus, for example, would scarcely commend
itself to a horse-breeder, while the student will look in vain for any
resemblance to a human figure, heroic or unheroic, in the straggling
group of stars which bears the name of Hercules. At the same time a
few of the constellations do more or less resemble the objects from
which their titles are derived. Thus the figure of a man may without
any great difficulty be traced among the brilliant stars which form
the beautiful constellation Orion; while Delphinus presents at least
an approximation to a fish-like form, and Corona Borealis gives the
half of a diadem of sparkling jewels.
A knowledge of the constellations, and, if possible, of the curious
old myths and legends attaching to them, should form part of the
equipment of every educated person; yet very few people can tell one
group from another, much less say what constellations are visible at
a given hour at any particular season of the year. People who are
content merely to gape at the heavens in 'a wonderful clear night
of stars' little know how much interest they are losing. When the
constellations and the chief stars are learned and kept in memory, the
face of the sky becomes instinct with interest, and each successive
season brings with it the return of some familiar group which is
hailed as one hails an old friend. Nor is the task of becoming
familiar with the constellations one of any difficulty. Indeed, there
are few pleasanter tasks than to trace out the figures of the old
heroes and heroines of mythology by the help of a simple star-map,
and once learned, they need never be forgotten. In this branch of the
subject there are many easily accessible helps. For a simple guide,
Peck's 'Constellations and how to Find Them' is both cheap and useful,
while Newcomb's 'Astronomy for Everybody' and Maunder's 'Astronomy
without a Telescope' also give careful and simple directions.
Maunder's volume is particularly useful for a beginner, combining,
as it does, most careful instructions as to the tracing of the
constellations with a set of clear and simple star-charts, and a most
interesting discussion of the origin of these ancient star-groups.
A list of the northern constellations with a few of the most notable
objects of interest in each will be found in Appendix II.
Winding among the constellations, and forming a gigantic belt round
the whole star-sphere, lies that most wonderful feature of the heavens
familiar to all under the name of the Milky Way. This great luminous
girdle of the sky may be seen in some portion of its extent, and at
some hour of the night, at all seasons of the year, though in May it
is somewhat inconveniently placed for observation. Roughly speaking,
it presents the appearance of a broad arch or pathway of misty light,
'whose groundwork is of stars'; but the slightest attention will
reveal the fact that in reality its structure is of great complexity.
It throws out streamers on either side and at all angles, condenses at
various points into cloudy masses of much greater brilliancy than the
average, strangely pierced sometimes by dark gaps through which we
seem to look into infinite and almost tenantless space (Plate XXVII.),
while in other quarters it spreads away in considerable width, and
to such a degree of faintness that the eye can scarcely tell where it
ends. At a point in the constellation Cygnus, well seen during autumn
and the early months of winter, it splits up into two great branches
which run separate to the Southern horizon with a well-marked dark gap
dividing them.
When examined with any telescopic power, the Milky Way reveals itself
as a wonderful collection of stars and star-clusters; and it will also
be found that there is a very remarkable tendency among the stars to
gather in the neighbourhood of this great starry belt. So much is this
case that, in the words of Professor Newcomb, 'Were the cloud-forms
which make up the Milky Way invisible to us, we should still be able
to mark out its course by the crowding of the lucid stars towards it.'
Not less remarkable is the fact that the distribution of the nebulæ
with regard to the Galaxy is precisely the opposite of that of the
stars. There are, of course, many nebulæ in the Galaxy; but, at the
same time, they are comparatively less numerous along its course, and
grow more and more numerous in proportion as we depart from it. It
seems impossible to avoid the conclusion that these twin facts are
intimately related to one another, though the explanation of them is
not yet forthcoming.
In the year 1665 the famous astronomer Hooke wrote concerning the
small star Gamma Arietis: 'I took notice that it consisted of two
small stars very near together; a like instance of which I have not
else met with in all the heavens.' This is the first English record
of the observation of a double star, though Riccioli detected the
duplicity of Zeta Ursæ Majoris (Mizar), in 1650, and Huygens saw three
stars in Theta Orionis in 1656. These were the earliest beginnings
of double-star observation, which has since grown to such proportions
that double stars are now numbered in the heavens by thousands. Of
course, certain stars appear to be double even when viewed with the
unaided eye. Thus Mizar, a bright star in the handle of the Plough,
referred to above, has not far from it a fainter companion known as
Alcor, which the Arabs used to consider a test of vision. Either it
has brightened in modern times, or else the Arabs have received too
much credit for keenness of sight, for Mizar and Alcor now make a
pair that is quite easy to very ordinary sight even in our turbid
atmosphere. Alpha Capricorni, and Zeta Ceti, with Iota Orionis are
also instances of naked-eye doubles, while exceptionally keen sight
will detect that the star Epsilon Lyræ, which forms a little triangle
with the brilliant Vega and Zeta Lyræ, is double, or at least that it
is not single, but slightly elongated in form. Astronomers, however,
would not call such objects as these 'double stars' at all; they
reserve that title for stars which are very much closer together than
the components of a naked-eye double can ever be. The last-mentioned
star, Epsilon Lyræ, affords a very good example of the distinction. To
the naked eye it is, generally speaking, not to be distinguished
from a single star. Keen sight elongates it; exceptionally keen sight
divides it into two stars extremely close to one another. But on using
even a very moderate telescope, say a 2-1/2-inch with a power of
100 or upwards, the two stars which the keenest sight could barely
separate are seen widely apart in the field, while each of them has
in its turn split up into two little dots of light. Thus, to the
telescope, Epsilon Lyræ is really a quadruple star, while in addition
there is a faint star forming a triangle with the two pairs, and a
large instrument will reveal two very faint stars, the 'debilissima,'
one on either side of the line joining the larger stars. These I have
seen with 3-7/8-inch.
What the telescope does with Epsilon Lyræ, it does with a great
multitude of other stars. There are thousands of doubles of all
degrees of easiness and difficulty--doubles wide apart, and doubles so
close that only the finest telescopes in the world can separate
them; doubles of every degree of likeness or of disparity in their
components, from Alpha Geminorum (Castor), with its two beautiful
stars of almost equal lustre, to Sirius, where the chief star is the
brightest in all the heavens, and the companion so small, or rather so
faint, that it takes a very fine glass to pick it out in the glare
of its great primary. The student will find in these double stars an
extremely good series of tests for the quality of his telescope.
They are, further, generally objects of great beauty, being often
characterized, as already mentioned, by diversity of colour in the
two components. Thus, in addition to the examples given above,
Eta Cassiopeiæ presents the beautiful picture of a yellow star in
conjunction with a red one, while Epsilon Boötis has been described as
'most beautiful yellow and superb blue,' and Alpha Herculis consists
of an orange star close to one which is emerald green. It has been
suggested that the colours in such instances are merely complementary,
the impression of orange or yellow in the one star producing a purely
subjective impression of blue or green when the other is viewed; but
it has been conclusively proved that the colours of very many of the
smaller stars in such cases are actual and inherent.
Not only are there thousands of double stars in the heavens, but there
are also many multiple stars, where the telescope splits an apparently
single star up into three, four, or sometimes six or seven separate
stars. Of these multiples, one of the best known is Theta Orionis. It
is the middle star of the sword which hangs from the belt of Orion,
and is, of course, notable from its connection with the Great Nebula;
but it is also a very beautiful multiple star. A 2-1/2-inch telescope
will show that it consists of four stars in the form of a trapezium;
large instruments show two excessively faint stars in addition. Again,
in the same constellation lies Sigma Orionis, immediately below the
lowermost star of the giant's belt. In a 3-inch telescope this
star splits up into a beautiful multiple of six components, their
differences in size and tint making the little group a charming
object.
Looking at the multitude of double and multiple stars, the question
can scarcely fail to suggest itself: Is there any real connection
between the stars which thus appear so close to one another? It can
be readily understood that the mere fact of their appearing close
together in the field of the telescope does not necessarily imply
real closeness. Two gas-lamps, for instance, may appear quite close
together to an observer who is at some distance from them, when in
reality they may be widely separated one from the other--the apparent
closeness being due to the fact that they are almost in the same
line of sight. No doubt many of the stars which appear double in the
telescope are of this class--'optical doubles,' as they are called,
and are in reality separated by vast distances from one another.
But the great majority have not only an apparent, but also a real
closeness; and in a number of cases this is proved by the fact that
observation shows the stars in question to be physically connected,
and to revolve around a common centre of gravity. Double stars which
are thus physically connected are known as 'binaries.' The discovery
of the existence of this real connection between some double stars is
due, like so many of the most interesting astronomical discoveries,
to Sir William Herschel. At present the number of stars known to be
binary is somewhat under one thousand; but in the case of most
of these, the revolution round a common centre which proves their
physical connection is extremely slow, and consequently the majority
of binary stars have as yet been followed only through a small portion
of their orbits, and the change of position sufficient to enable
a satisfactory orbit to be computed has occurred in only a small
proportion of the total number. The first binary star to have its
orbit computed was Xi Ursæ Majoris, whose revolution of about sixty
years has been twice completed since, in 1780, Sir William Herschel
discovered it to be double.
The star which has the shortest period at present known is the fourth
magnitude Delta Equulei, which has a fifth magnitude companion. The
pair complete their revolution, according to Hussey, in 5·7 years.
Kappa Pegasi comes next in speed of revolution, with a period of
eleven and a half years, while the star 85 of the same constellation
takes rather more than twice as long to complete its orbit. From such
swiftly circling pairs as these, the periods range up to hundreds of
years. Thus, for example, the well-known double star Castor, probably
the most beautiful double in the northern heavens, and certainly the
best object of its class for a small telescope, is held to have a
period of 347 years, which, though long enough, is a considerable
reduction upon the 1,000 once attributed to it.
But the number of binary stars known is not confined to those which
have been discovered and measured by means of the telescope and
micrometer. One of the most wonderful results of modern astronomical
research has been the discovery of the fact that many stars have
revolving round them invisible companions, which are either dark
bodies, or else are so close to their primaries as for ever to defy
the separating powers of our telescopes. The discovery of these
dark, or at least invisible, companions is one of the most remarkable
triumphs of the spectroscope. It was in 1888 that Vogel first applied
the spectroscopic method to the well-known variable star, Beta
Persei--known as Algol, 'the Demon,' from its 'slowly-winking eye.'
The variation in the light of Algol is very large, from second to
fourth magnitude; Vogel therefore reasoned that if this variation were
caused by a dark companion partially eclipsing the bright star, the
companion must be sufficiently large to cause motion in Algol--that
is, to cause both stars to revolve round a common centre of gravity.
Should this be the case, then at one point of its orbit Algol must be
approaching, and at the opposite point receding from the earth; and
therefore the shift of the lines of its spectrum towards the violet
in the one instance and towards the red in the other would settle
the question of whether it had or had not an invisible companion.
The spectroscopic evidence proved quite conclusive. It was found that
before its eclipses, Algol was receding from the sun at the rate
of 26-1/3 miles per second, while after eclipse there was a similar
motion of approach; and therefore the hypothesis of an invisible
companion was proved to be fact. Vogel carried his researches further,
his inquiry into the questions of the size and distance apart of
the two bodies leading him to the conclusion that the bright star is
rather more, and its companion rather less than 1,000,000 miles in
diameter; while the distance which divides them is somewhat more than
3,000,000 miles. Though larger, both bodies prove to be less massive
than our sun, Algol being estimated at four-ninths and its companion
at two-ninths of the solar mass.
The class of double star disclosed in this manner is known as the
'spectroscopic binary,' and has various other types differing from
the Algol type. Thus the type of which Xi Ursæ Majoris was the first
detected instance has two component bodies not differing greatly in
brightness from one another. In such a case the fact of the star being
binary is revealed through the consideration that in any binary system
the two components must necessarily always be moving in opposite
directions. Hence the shift of the lines of their spectrum will be
in opposite directions also, and when one of the stars (A) is moving
towards us, and the other (B) away from us, all the lines of the
spectrum which are common to the two will appear double, those of A
being displaced towards the violet and those of B towards the red.
After a quarter of a revolution, when the stars are momentarily in
a straight line with us, the lines will all appear single; but after
half a revolution they will again be displaced, those of A this time
towards the red and those of B towards the violet.
There has thus been opened up an entirely new field of research, and
the idea, long cherished, that the stars might prove to have dark, or,
at all events, invisible, companions attendant on them, somewhat as
our own sun has its planets, has been proved to be perfectly sound.
So far, in the case of dark companions, only bodies of such vast size
have been detected as to render any comparison with the planets of
our system difficult; but the principle is established, and the
probability of great numbers of the stars having real planetary
systems attendant on them is so great as to become practically a
certainty. 'We naturally infer,' says Professor Newcomb, 'that ...
innumerable stars may have satellites, planets, or companion stars so
close or so faint as to elude our powers of observation.'
From the consideration of spectroscopic binaries we naturally turn
to that of variable stars, the two classes being, to some extent at
least, coincident, as is evidenced by the case of Algol. While the
discovery of spectroscopic binaries is one of the latest results of
research, that of variability among stars dates from comparatively
far back in the history of astronomy. As early as the year 1596 David
Fabricius noted the star now known as Omicron Ceti, or Mira, 'the
Wonderful,' as being of the third magnitude, while in the following
year he found that it had vanished. A succession of appearances and
disappearances was witnessed in the middle of the next century by
Holwarda, and from that time the star has been kept under careful
observation, and its variations have been determined with some
exactness, though there are anomalies as yet unexplained. 'Once in
eleven months,' writes Miss Clerke, 'the star mounts up in about 125
days from below the ninth to near the third, or even to the second
magnitude; then, after a pause of two or three weeks, drops again
to its former low level in once and a half times, on an average, the
duration of its rise.' This most extraordinary fluctuation means that
at a bright maximum Mira emits 1,500 times as much light as at a low
minimum. The star thus subject to such remarkable outbursts is, like
most variables, of a reddish colour, and at maximum its spectrum shows
the presence of glowing hydrogen. Its average period is about 331
days; but this period is subject to various irregularities, and
the maximum has sometimes been as much as two months away from the
predicted time. Mira Ceti may be taken as the type of the numerous
class of stars known as 'long-period variables.'
Not less interesting are those stars whose variations cover only short
periods, extending from less than thirty days down to a few hours. Of
these, perhaps the most easily observed, as it is also one of the most
remarkable, is Beta Lyræ. This star is one of the two bright stars of
nearly equal magnitude which form an obtuse-angled triangle with the
brilliant first-magnitude star Vega. The other star of the pair
is Gamma Lyræ, and between them lies the famous Ring Nebula, to be
referred to later. Ordinarily Beta Lyræ is of magnitude 3·4, but from
this it passes, in a period of rather less than thirteen days, through
two minima, in one of which it descends to magnitude 3·9 and in
the other to 4·5. This fluctuation seems trifling. It really means,
however, that at maximum the star is two and three-quarter times
brighter than when it sinks to magnitude 4·5; and the variation can be
easily recognised by the naked eye, owing to the fact of the nearness
of so convenient a comparison star as Gamma Lyræ. Beta Lyræ is a
member of the class of spectroscopic binaries, and belongs to that
type of the class in which the mutually eclipsing bodies are both
bright. In such cases the variation in brilliancy is caused by the
fact that when the two bodies are, so to speak, side by side, light
is received from both of them, and a maximum is observed; while, when
they are end on, both in line with ourselves, one cuts off more or
less of the other's light from us, thus causing a minimum.
A third class, distinct from either of the preceding, is that of the
Algol Variables, so-called from the bright star Beta Persei, which has
already been mentioned as a spectroscopic binary. Than this star
there is no more notable variable in the heavens, and its situation
fortunately renders it peculiarly easy of observation to northern
students. Algol shines for about fifty-nine hours as a star of small
second magnitude, then suddenly begins to lose light, and in four and
a half hours has fallen to magnitude three and a half, losing in so
short a space two-thirds of its normal brilliancy. It remains in
this degraded condition for only fifteen minutes, and then begins to
recover, reaching its normal lustre in about five hours more. These
remarkable changes, due, as before mentioned, to the presence of
an invisible eclipsing companion, are gone through with the utmost
regularity, so much so that, as Gore says, the minima of Algol 'can
be predicted with as much certainty as an eclipse of the sun.' The
features of the type-star are more or less closely reproduced in the
other Algol Variables--a comparatively long period of steady light
emission, followed by a rapid fall to one or more minima, and a rapid
recovery of light. The class as yet is a small one, but new members
are gradually being added to it, the majority of them white, like the
type-star.
The study of variable stars is one which should seem to be specially
reserved for the amateur observer. In general, it requires but little
instrumental equipment. Many variables can be seen at maximum, some
even at minimum, with the unaided eye; in other cases a good opera or
field glass is all that is required, and a 2-1/2 or 3-inch telescope
will enable the observer to command quite an extensive field of work.
Here, again, the beginner may be referred to the _Memoirs_ of the
British Astronomical Association for help and guidance, and may be
advised to connect himself with the Variable Star Section.
With the exception of such variations in the lustre of certain stars
as have been described, the aspect of the heavens is, in general,
fixed and unchanging. There are, as we shall see, real changes of the
vastest importance continually going on; but the distances separating
us from the fixed stars are so enormous that these changes shrink into
nothingness, and the astronomers of forty centuries before our era
would find comparatively little change today in the aspect of the
constellations with which they were familiar. But occasionally a
very remarkable change does take place, in the apparition of a new or
temporary star. The accounts of the appearance of such objects are not
very numerous, but are of great interest. We pass over those recorded,
in more or less casual fashion, by the ancients, for the reason
that the descriptions given are in general more picturesque than
illuminative. It does not add much to one's knowledge, though it may
excite wonder, to find the Chinese annals recording the appearance, in
A.D. 173, of a new star 'resembling a large bamboo mat!'
The first Nova, of which we have a really scientific record, was the
star which suddenly blazed out, in November, 1572, in the familiar
W of Cassiopeia. It was carefully observed by the great astronomer,
Tycho Brahé, and, according to him, was brighter than Sirius, Alpha
Lyræ, or Jupiter. Tycho followed it till March, 1574, by which time
it had sunk to the limit of unaided vision, and further observation
became impossible. There is at present a star of the eleventh
magnitude close to the place fixed for the Nova from Tycho's
observations. In 1604 and 1670, new stars were observed, the first by
Kepler and his assistants, the second by the monk Anthelme; but from
1670 there was a long break in the list of discoveries, which was
ended by Hind's observation of a new star in Ophiuchus (April, 1848).
This was never a very conspicuous object, rising only to somewhat less
than fourth magnitude, and soon fading to tenth or eleventh. We
can only mention the 'Blaze Star' of Corona Borealis, discovered by
Birmingham in 1866, the Nova discovered in 1876 by Schmidt of Athens,
near Rho Cygni--an object which seems to have faded out into a
planetary nebula, a fate apparently characteristic of this class of
star--and the star which appeared in 1885, close to the nucleus of the
Great Nebula in Andromeda.
In 1892, Dr. Anderson of Edinburgh discovered in the constellation
Auriga a star which he estimated as of fifth magnitude. The discovery
was made on January 31, and the new star was found to have been
photographed at Harvard on plates taken from December 16, 1891, to
January 31, 1892. Apparently this Nova differed from other temporary
stars in the fact that it attained its full brightness only gradually.
By February 3 it rose to magnitude 3·5, then faded by April 1 to
fifteenth, but in August brightened up again to about ninth
magnitude. It is now visible as a small star. The great development
of spectroscopic resources brought this object, otherwise not a very
conspicuous one, under the closest scrutiny. Its spectrum showed many
bright lines, which were accompanied by dark ones on the side next the
blue. The idea was thus suggested that the outburst of brilliancy was
due to a collision between two bodies, one of which, that causing the
dark lines, was approaching the earth, while the other was receding
from it. Lockyer considered the conflagration to be due to a collision
between two swarms of meteorites, Huggins that it was caused by the
near approach to one another of two gaseous bodies, while others
suggested that the rush of a star or of a swarm of meteorites through
a nebula would explain the facts observed. Subsequent observations of
the spectrum of Nova Aurigæ have revealed the fact that it has obeyed
the destiny which seems to wait on temporary stars, having become a
planetary nebula.
Dr. Anderson followed up his first achievement by the discovery of a
brilliant Nova in the constellation Perseus. The discovery was made
on the night of February 21-22, 1901, the star being then of magnitude
2·7. Within two days it became about the third brightest star in the
sky, being a little more brilliant than Capella; but before the middle
of April it had sunk to fifth magnitude. The rapidity of its rise must
have been phenomenal! A plate exposed at Harvard on February 19, and
showing stars to the eleventh magnitude, bore no trace of the Nova.
'It must therefore,' says Newcomb, 'have risen from some magnitude
below the eleventh to the first within about three days. This
difference corresponds to an increase of the light ten thousandfold!'
Such a statement leaves the mind simply appalled before the spectacle
of a cataclysm so infinitely transcending the very wildest dreams of
fancy. Subsequent observations have shown the usual tendency towards
development into a nebula, and in August, 1901, photographs were
actually obtained of a nebulosity round the star, showing remarkable
condensations. These photographs, taken at Yerkes Observatory, when
compared with others taken at Mount Hamilton in November, revealed the
startling fact that the condensations of the nebula were apparently
in extraordinarily rapid motion. Now the Nova shows no appreciable
parallax, or in other words is so distant that its distance cannot be
measured; on what scale, therefore, must these motions have been to be
recorded plainly across a gulf measurable perhaps in hundreds of light
years!
Nova Geminorum, discovered by Professor Turner, at Oxford, in March,
1903, had not the striking features which lent so much interest to
Nova Persei. It showed a crimson colour, and its spectrum indicated
the presence in its blaze of hydrogen and helium; but it faded so
rapidly as to show that the disturbance affected a comparatively small
body, and it has exhibited the familiar new star change into a nebula.
One point with regard to the Novæ in Auriga and Perseus deserves
notice. These discoveries, so remarkable in themselves, and so
fruitful in the extension of our knowledge, were made by an amateur
observer with no greater equipment than a small pocket telescope and a
Klein's Star-Atlas. The thorough knowledge of the face of the heavens
which enabled Dr. Anderson to pick out the faint glimmer of Nova
Aurigæ and to be certain that the star was a new one is not in the
least unattainable by anyone who cares to give time and patience to
its acquisition; and even should the study never be rewarded by a
capture so dramatic as that of Nova Persei, the familiarity gained
in its course with the beauty and wonder of the star-sphere will in
itself be an ample reward.