TEMPORARY AND VARIABLE STARS
It is a clear night, nearly all the world is asleep, when an astronomer
crosses his lawn on his way to his observatory to spend the dark hours
in making investigations into profound space. His brilliant mind,
following the rays of light which shoot from the furthest star, will
traverse immeasurable distances, while the body is forgotten. Just
before entering the observatory he pauses and looks up; his eye catches
sight of something that arrests him, and he stops involuntarily. Yet any
stranger standing beside him, and gazing where he gazes, would see
nothing unusual. There is no fiery comet with its tail stretching across
from zenith to horizon, no flaming meteor dashing across the darkened
sky. But that there is something unusual to be seen is evident, for the
astronomer breathes quickly, and after another earnest scrutiny of the
object which has attracted him, he rushes into the observatory, searches
for a star-chart, and examines attentively that part of the sky at
which he has been gazing. He runs his finger over the chart: here and
there are the well-known stars that mark that constellation, but here?
In that part there is no star marked, yet he knows, for his own eyes
have told him but a few moments ago, that here there is actually blazing
a star, not large, perhaps, but clear enough to be seen without a
telescope--a star, maybe, which no eye but his has yet observed!
He hurries to his telescope, and adjusts it so as to bring the stranger
into the field of view. A new star! Whence has it come? What does it
mean?
By the next day at the latest the news has flown over the wires, and all
the scientific world is aware that a new star has been detected where no
star ever was seen before. Hundreds of telescopes are turned on to it;
its spectrum is noted, and it stands revealed as being in a state of
conflagration, having blazed up from obscurity to conspicuousness. Night
after night its brilliance grows, until it ranks with the brightest
stars in heaven, and then it dies down and grows dim, gradually
sinking--sinking into the obscurity from whence it emerged so briefly,
and its place in the sky knows it no more. It may be there still, but
so infinitely faint and far away that no power at our command can reveal
it to us. And the amazing part of it is that this huge disaster, this
mighty conflagration, is not actually happening as it is seen, but has
happened many hundreds of years ago, though the message brought by the
light carrier has but reached us now.
There have not been a great many such outbursts recorded, though many
may have taken place unrecorded, for even in these days, when trained
observers are ceaselessly watching the sky, 'new' stars are not always
noticed at once. In 1892 a new star appeared, and shone for two months
before anyone noticed it. This particular one never rose to any very
brilliant size. I twas situated in the constellation of Auriga, and was
noticed on February 1. It remained fairly bright until March 6, when it
began to die down; but it has now sunk so low that it can only be seen
in the very largest telescopes.
Photography has been most useful in recording these stars, for when one
is noticed it has sometimes been found that it has been recorded on a
photographic plate taken some time previously, and this shows us how
long it has been visible. More and more photography becomes the useful
handmaid of astronomers, for the photographic prepared plate is more
sensitive to rays of light than the human eye, and, what is more useful
still, such plates retain the rays that fall upon them, and fix the
impression. Also on a plate these rays are cumulative--that is to say,
if a very faint star shines continuously on a plate, the longer the
plate is exposed, within certain limits, the clearer will the image of
that star become, for the light rays fall one on the top of the other,
and tend to enforce each other, and so emphasize the impression, whereas
with our eyes it is not the same thing at all, for if we do not see an
object clearly because it is too faint, we do not see it any better,
however much we may stare at the place where it ought to be. This is
because each light ray that reaches our eye makes its own impression,
and passes on; they do not become heaped on each other, as they do on a
photographic plate.
One variable star in Perseus, discovered in 1901, rose to such
brilliancy that for one night it was queen of the Northern Hemisphere,
outshining all the other first-class stars.
It rose into prominence with wonderful quickness, and sank equally fast.
At its height it outshone our sun eight thousand times! This star was
so far from us that it was reckoned its light must take about three
hundred years to reach us, consequently the great conflagration, or
whatever caused the outburst, must have taken place in the reign of
James I., though, as it was only seen here in 1901, it was called the
new star of the new century.
When these new stars die down they sometimes continue to shine faintly
for a long time, so that they are visible with a telescope, but in other
cases they may die out altogether. We know very little about them, and
have but small opportunity for observing them, and so it is not safe to
hazard any theories to account for their peculiarities. At first men
supposed that the great flame was made by a violent collision between
two bodies coming together with great velocity so that both flared up,
but this speculation has been shown by the spectroscope to be
improbable, and now it is supposed by some people that two stars
journeying through space may pass through a nebulous region, and thus
may flare up, and such a theory is backed up by the fact that a very
great number of such stars do seem to be mixed up in some strange way
with a nebulous haze.
All these new stars that we have been discussing so far have only
blazed up once and then died down, but there is another class of stars
quite as peculiar, and even more difficult to explain, and these are
called variable stars. They get brighter and brighter up to a certain
point, and then die down, only to become bright once more, and these
changes occur with the utmost regularity, so that they are known and can
be predicted beforehand. This is even more unaccountable than a sudden
and unrepeated outburst, for one can understand a great flare-up, but
that a star should flare and die down with regularity is almost beyond
comprehension. Clearly we must look further than before for an
explanation. Let us first examine the facts we know. Variable stars
differ greatly from each other. Some are generally of a low magnitude,
and only become bright for a short time, while others are bright most of
the time and die down only for a short time. Others become very bright,
then sink a little bit, but not so low as at first; then they become
bright again, and, lastly, go right down to the lowest point, and they
keep on always through this regular cycle of changes. Some go through
the whole of these changes in three days, and others take much longer.
The periods, as the intervals between the complete round of changes are
called, vary, in fact, between three days and six hundred! It may seem
impossible that changes covering so long as six hundred days could be
known and followed, but there is nothing that the patience of
astronomers will not compass.
One very well-known variable star you can see for yourselves, and as an
ounce of observation is worth a pound of hearsay, you might take a
little trouble to find it. Go out on any clear starlight night and look.
Not very far from Cassiopeia (W.), to the left as you face it, are three
bright stars running down in a great curve. These are in the
constellation called Perseus, and a little to the right of the middle
and lowest one is the only variable star we can see in the sky without a
telescope.
This is Algol. For the greater part of three days he is a bright star of
about the second magnitude, then he begins to fade, and for four and a
half hours grows steadily dimmer. At the dimmest he remains for about
twenty minutes, and then rises again to his ordinary brightness in three
and a half hours. How can we explain this? You may possibly be able to
suggest a reason. What do you say to a dark body revolving round Algol,
or, rather, revolving with him round a common centre of gravity? If
such a thing were indeed true, and if such a body happened to pass
between us and Algol at each revolution, the light of Algol would be cut
off or eclipsed in proportion to the size of such a body. If the dark
body were the full size of Algol and passed right between him and us, it
would cut off all the light, but if it were not quite the same size, a
little would still be seen. And this is really the explanation of the
strange changes in the brightness of Algol, for such a dark body as we
are imagining does in reality exist. It is a large dark body, very
nearly as large as Algol himself, and if, as we may conjecture, it is a
mighty planet, we have the extraordinary example of a planet and its sun
being nearly the same size. We have seen that the eclipse happens every
three days, and this means, of course, that the planetary body must go
round its sun in that time, so as to return again to its position
between us and him, but the thing is difficult to believe. Why, the
nearest of all our planets to the sun, the wee Mercury, takes
eighty-seven days to complete its orbit, and here is a mighty body
hastening round its sun in three! To do this in the time the large dark
planet must be very near to Algol; indeed, astronomers have calculated
that the surfaces of the two bodies are not more than about two million
miles apart, and this is a trifle when we consider that we ourselves are
more than forty-six times as far as that from the sun. At this distance
Algol, as observed from the planet, will fill half the sky, and the heat
he gives out must be something stupendous. Also the effects of
gravitation must be queer indeed, acting on two such huge bodies so
close together. If any beings live in such a strange world, the pull
which draws them to their mighty sun must be very nearly equal to the
pull which holds them to their own globe; the two together may
counteract each other, but the effect must be strange!
From irregularities in the movements of Algol it has been judged that
there may be also in the same system another dark body, but of it
nothing has been definitely ascertained.
But all variable stars need not necessarily be due to the light being
intercepted by a dark body. There are cases where two bright stars in
revolving round each other produce the same effect; for when seen side
by side the two stars give twice as much light as when one is hidden
behind the other, and as they are seen alternately side by side and in
line, they seem to alter regularly in lustre.