THE DISTANT SUNS.
Sirius Contrasted with the Sun--Stars can be Weighed, but not in
general Measured--The Companion of Sirius--Determination of the
Weights of Sirius and his Companion--Dark Stars--Variable and
Temporary Stars--Enormous Number of Stars.
The splendid pre-eminence of Sirius has caused it to be observed with
minute care from the earliest times in the history of astronomy. Each
generation of astronomers devoted time and labour to determine the exact
places of the brightest stars in the heavens. A vast mass of
observations as to the place of Sirius among the stars had thus been
accumulated, and it was found that, like many other stars, Sirius had
what astronomers call _proper motion_. Comparing the place of Sirius
with regard to the other stars now with the place which it occupied one
hundred years ago, there is a difference of two minutes (127") in its
situation. This is a small quantity: it is so small that the unaided eye
could not see it. Could we now see the sky as it appeared one century
ago, we should still see this star in its well-known place to the left
of Orion. Careful alignment by the eye would hardly detect that Sirius
was moving in two, or even in three or in four centuries. But the
accuracy of the meridian circle renders these minute quantities evident,
and gives to them their true significance. To the eye of the astronomer,
Sirius, instead of creeping along with a movement which centuries will
not show, is pursuing its majestic course with a velocity appropriate to
its dimensions.
Though the velocity of Sirius is _about_ 1,000 miles a minute, yet it
is sometimes a little more and sometimes a little less than its mean
value. To the astronomer this fact is pregnant with information. Were
Sirius an isolated star, attended only by planets of comparative
insignificance, there could be no irregularity in its motion. If it were
once started with a velocity of 1,000 miles a minute, then it must
preserve that velocity. Neither the lapse of centuries nor the mighty
length of the journey could alter it. The path of Sirius would be
inflexible in its direction; and it would be traversed with unalterable
velocity.
[Illustration: Fig. 92.--The Orbit of Sirius (Professor Burnham).]
The fact that Sirius had not been moving uniformly was of such interest
that it arrested the attention of Bessel when he discovered the
irregularities in 1844. Believing, as Bessel did, that there must be
some adequate cause for these disturbances, it was hardly possible to
doubt what the cause must be. When motion is disturbed there must be
force in action, and the only force that we recognise in such cases is
that known as gravitation. But gravity can only act from one body to
another body; so that when we seek for the derangement of Sirius by
gravitation, we are obliged to suppose that there must be some mighty
and massive body near Sirius. The question was taken up again by Peters
and by Auwers, who were able to discover, from the irregularities of
Sirius, the nature of the path of the disturbing body. They were able to
show that it must revolve around Sirius in a period of about fifty
years, and although they could not tell its distance from Sirius, yet
they were able to point out the direction in which it must lie. Fig. 92
shows the orbit of Sirius as given by Mr. Burnham, of Yerkes
Observatory.
The detection of the attendant of Sirius, and the measures which have
been made thereon, enable us to determine the weight of this famous
star. Let us attempt to illustrate this subject. It must, no doubt, be
admitted that the numerical estimates we employ have to be received with
a certain degree of caution. The companion of Sirius is a difficult
object to observe, and previous to 1896 it had only been followed
through an arc of 90 deg.. We are, therefore, hardly as yet in a position to
speak with absolute accuracy as to the periodic time in which the
companion completes its revolution. We may, however, take this time to
be fifty-two years. We also know the distance from Sirius to his
companion, and we may take it to be about twenty-one times the distance
from the earth to the sun. It is useful, in the first place, to compare
the revolution of the companion around Sirius with the revolution of the
planet Uranus around the sun. Taking the earth's distance as unity, the
radius of the orbit of Uranus is about nineteen, and Uranus takes
eighty-four years to accomplish a complete revolution. We have no planet
in the solar system at a distance of twenty-one; but from Kepler's third
law it may be shown that, if there were such a planet, its periodic time
would be about ninety-nine years. We have now the necessary materials
for making the comparison between the mass of Sirius and the mass of
the sun. A body revolving around Sirius at a certain distance completes
its journey in fifty-two years. To revolve around the sun at the same
distance a body should complete its journey in ninety-nine years. The
quicker the body is moving the greater must be the centrifugal force,
and the greater must be the attractive power of the central body. It can
be shown from the principles of dynamics that the attractive power is
inversely proportional to the square of the periodic time. Hence, then,
the attractive power of Sirius must bear to the attractive power of the
sun the proportion which the square of ninety-nine has to the square of
fifty-two. As the distances are in each case supposed to be equal, the
attractive powers will be proportional to the masses, and hence we
conclude that the mass of Sirius, together with that of his companion,
is to the mass of the sun, together with that of his planet, in the
ratio of three and a half to one. We had already learned that Sirius was
much brighter than the sun; now we have learned that it is also much
more massive.
Before we leave the consideration of Sirius, there is one additional
point of very great interest which it is necessary to consider. There is
a remarkable contrast between the brilliancy of Sirius and his
companion. Sirius is a star far transcending all other stars of the
first magnitude, while his companion is extremely faint. Even if it were
completely withdrawn from the dazzling proximity of Sirius, the
companion would be only a small star of the eighth or ninth magnitude,
far below the limits of visibility to the unaided eye. To put the matter
in numerical language, Sirius is 5,000 times as bright as its companion,
but only about twice as heavy! Here is a very great contrast; and this
point will appear even more forcible if we contrast the companion of
Sirius with our sun. The companion is slightly heavier than our sun; but
in spite of its slightly inferior bulk, our sun is much more powerful as
a light-giver. One hundred of the companions of Sirius would not give as
much light as our sun! This is a result of very considerable
significance. It teaches us that besides the great bodies in the
universe which attract attention by their brilliancy, there are also
other bodies of stupendous mass which have but little
brilliancy--probably some of them possess none at all. This suggests a
greatly enhanced conception of the majestic scale of the universe. It
also invites us to the belief that the universe which we behold bears
but a small ratio to the far larger part which is invisible in the
sombre shades of night. In the wide extent of the material universe we
have here or there a star or a mass of gaseous matter sufficiently
heated to be luminous, and thus to become visible from the earth; but
our observation of these luminous points can tell us little of the
remaining contents of the universe.
The most celebrated of all the variable stars is that known as Algol,
whose position in the constellation of Perseus is shown in Fig. 83. This
star is conveniently placed for observation, being visible every night
in our latitude, and its interesting changes can be observed without any
telescopic aid. Everyone who desires to become acquainted with the great
truths of astronomy should be able to recognise this star, and should
have also followed it during one of its periods of change. Algol is
usually a star of the second magnitude; but in a period between two and
three days, or, more accurately, in an interval of 2 days 20 hours 48
minutes and 55 seconds, its brilliancy goes through a most remarkable
cycle of variations. The series commences with a gradual decline of the
star's brightness, which in the course of four and a half hours falls
from the second magnitude down to the fourth. At this lowest stage of
brightness Algol remains for about twenty minutes, and then begins to
increase, until in three and a half hours it regains the second
magnitude, at which it continues for about 2 days 12 hours, when the
same series commences anew. It seems that the period required by Algol
to go through its changes is itself subject to a slow but certain
variation. We shall see in a following chapter how it has been proved
that the variability of Algol is due to the occasional interposition of
a dark companion which cuts off a part of the lustre of the star. All
the circumstances can thus be accounted for, and even the weight and the
size of Algol and its dark companion be determined.
There are, however, other classes of variable stars, the fluctuation of
whose light can hardly be due to occasional obscuration by dark bodies.
This is particularly the case with those variables which are generally
faint, but now and then flare up for a short time, after which temporary
exaltation they again sink down to their original condition. The periods
of such changes are usually from six months to two years. The best known
example of a star of this class was discovered more than three hundred
years ago. It is situated in the constellation Cetus, a little south of
the equator. This object was the earliest known case of a variable star,
except the so-called temporary stars, to which we shall presently refer.
The variable in Cetus received the name of Mira, or the wonderful. The
period of the fluctuations of Mira Ceti is about eleven months, during
the greater part of which time the star is of the ninth magnitude, and
consequently invisible to the naked eye. When the proper time has
arrived, its brightness begins to increase rather suddenly. It soon
becomes a conspicuous object of the second or third magnitude. In this
condition it remains for eight or ten days, and then declines more
slowly than it rose until it is reduced to its original faintness, about
three hundred days after the rise commenced.
More striking to the general observer than the ordinary variable stars
are the _temporary stars_ which on rare occasions suddenly make their
appearance in the heavens. The most famous object of this kind was that
which blazed out in the beginning of November, 1572, and which when
first seen was as bright as Venus at its maximum brightness. It could,
indeed, be seen in full daylight by sharp-sighted people. As far as
history can tell us, no other temporary star has ever been as bright as
this one. It is specially associated with the name of Tycho Brahe, for
although he was not the discoverer, he made the best observations of the
object, and he proved that it was at a distance comparable with that of
the ordinary fixed stars. Tycho described carefully the gradual decline
of the wonderful star until it disappeared from his view about the end
of March, 1574, for the telescope, by which it could doubtless have been
followed further, had not yet been invented. During the decline the
colour of the object gradually changed; at first it was white, and by
degrees became yellow, and in the spring of 1573 reddish, like
Aldebaran. About May, 1573, we are told somewhat enigmatically that it
"became like lead, or somewhat like Saturn," and so it remained as long
as it was visible. What a fund of information our modern spectroscopes
and other instruments would supply us with if so magnificent a star were
to burst out in these modern days!
But though we have not in our own times been favoured with a view of a
temporary star as splendid as the one seen by Tycho Brahe and his
contemporaries, it has been our privilege to witness several minor
outbursts of this kind. It seems likely that we should possess more
records of temporary stars from former times if a better watch had been
kept for them. That is at any rate the impression we get when we see how
several of the modern stars of this kind have nearly escaped us
altogether, notwithstanding the great number of telescopes which are now
pointed to the sky on every clear night.
In 1866 a star of the second magnitude suddenly appeared in the
constellation of the crown (Corona Borealis). It was first seen on the
12th May, and a few days afterwards it began to fade away. Argelander's
maps of the northern heavens had been published some years previously,
and when the position of the new star had been accurately determined, it
was found that it was identical with an insignificant looking star
marked on one of the maps as of the 9-1/2 magnitude. The star exists in
the same spot to this day, and it is of the same magnitude as it was
prior to its spasmodic outburst in 1866. This was the first new star
which was spectroscopically examined. We shall give in Chapter XXIII. a
short account of the features of its spectrum.
The next of these temporary bright stars, Nova Cygni, was first seen by
Julius Schmidt at Athens on the 24th November, 1876, when it was between
the third and fourth magnitudes, and he maintains that it cannot have
been conspicuous four days earlier, when he was looking at the same
constellation. By some inadvertence the news of the discovery was not
properly circulated, and the star was not observed elsewhere for about
ten days, when it had already become considerably fainter. The decrease
of brightness went on very slowly; in October, 1877, the star was only
of the tenth magnitude, and it continued getting fainter until it
reached the fifteenth magnitude; in other words, it became a minute
telescopic star, and it is so still in the very same spot. As this star
did not reach the first or second magnitude it would probably have
escaped notice altogether if Schmidt had not happened to look at the
Swan on that particular evening.
We are not so likely to miss seeing a new star since astronomers have
pressed the photographic camera into their service. This became evident
in 1892, when the last conspicuous temporary star appeared in Auriga. On
the 24th January, Dr. Anderson, an astronomer in Edinburgh, noticed a
yellowish star of the fifth magnitude in the constellation Auriga, and a
week later, when he had compared a star-map with the heavens and made
sure that the object was really a new star, he made his discovery
public. In the case of this star we are able to fix fairly closely the
moment when it first blazed out. In the course of the regular
photographic survey of the heavens undertaken at the Harvard College
Observatory (Cambridge, Massachusetts) the region of the sky where the
new star appeared had been photographed on thirteen nights from October
21st to December 1st, 1891, and on twelve nights from December 10th to
January 20th, 1892. On the first series of plates there was no trace of
the Nova, while it was visible on the very first plate of the second
series as a star of the fifth magnitude. Fortunately it turned out that
Professor Max Wolf of Heidelberg, a most successful celestial
photographer, had photographed the same region on the 8th December, and
this photograph does not show the star, so that it cannot on that night
have been as bright as the ninth magnitude. Nova Auriga must therefore
have flared up suddenly between the 8th and the 10th of December.
According to the Harvard photographs, the first maximum of brightness
occurred about the 20th of December, when the magnitude was 4-1/2. The
decrease of the brightness was very irregular; the star fluctuated for
the five weeks following the first of February between the fourth and
the sixth magnitude, but after the beginning of March, 1892, the
brightness declined very rapidly, and at the end of April the star was
seen as an exceedingly faint one (sixteenth magnitude) with the great
Lick Refractor. When this mighty instrument was again pointed to the
Nova in the following August, it had risen nearly to the tenth
magnitude, after which it gradually became extremely faint again, and is
so still.
The temporary and the variable stars form but a very small section of
the vast number of stars with which the vault of the heavens is studded.
That the sun is no more than a star, and the stars are no less than
suns, is a cardinal doctrine of astronomy. The imposing magnificence of
this truth is only realised when we attempt to estimate the countless
myriads of stars. This is a problem on which our calculations are
necessarily vain. Let us, therefore, invoke the aid of the poet to
attempt to express the innumerable, and conclude this chapter with the
following lines of Mr. Allingham:--
"But number every grain of sand,
Wherever salt wave touches land;
Number in single drops the sea;
Number the leaves on every tree,
Number earth's living creatures, all
That run, that fly, that swim, that crawl;
Of sands, drops, leaves, and lives, the count
Add up into one vast amount,
And then for every separate one
Of all those, let a flaming SUN
Whirl in the boundless skies, with each
Its massy planets, to outreach
All sight, all thought: for all we see
Encircled with infinity,
Is but an island."