VIIIMIRA,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 whichmakes 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 Starsof Long Period
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.