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.