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Brightness of Stars.

All the variations we have hitherto considered take place with such rapidity that they can be observed by comparisons embracing but a short interval Possible Seof time-a few days or months at the out- cular Variaside. A somewhat different question of tions in the great importance is still left open. May not individual stars be subject to a slow variation either in their colour or their brightness, which are sensible in the course of only one generation of men, but admit of being brought out by a comparison of the brightness of the stars at widely distant epochs? Is it certain that, in the case of stars which we do not recognise as variable, no change has taken place since the time of Hipparchus and Ptolemy? This question has been investigated by C. S. Pierce and others. The conclusion reached is that no real evidence of any change can be gathered. The discrepancies are no greater than might arise from errors of estimates.

There is, however, an aspect of the question which is of great interest and has been much discussed in recent times. In several ancient writings the colour of Sirius is described as red. This fact would, at first sight, appear to afford very strong evidence that, within historic times, the colour of the brightest star in the heavens has actually changed from red to bluish white.

Two recent writers have examined the evidence on this subject most exhaustively and reached opposite conclusions. The first of these was Prof. T. J. J. See, who collated a great number of cases in which Sirius was mentioned by ancient writers as red or fiery,

and thus concluded that the evidence was in favour of a red colour in former times. Shortly afterwards, Schiaparelli examined the evidence with equal care and thoroughness and reached an opposite conclusion, showing that the terms used by the ancient authors which might have indicated redness of colour were susceptible of other interpretations; they might mean fiery, blazing, etc., as well as red in colour, and were therefore probably suggested by the extraordinary brightness of Sirius and the strangeness with which it twinkled when near the horizon. In this position a star not only twinkles, but changes its colour rapidly. This change is not sensible in the case of a faint star, but if one watches Sirius when on the horizon, it will be seen that it not only changes in appearance, but seems to blaze forth in different colours.

It seems to the writer that this conclusion of Schiaparelli is the more likely of the two. From what we know of the constitution of the stars, a change in the colour of one of these bodies in so short a period of time as that embraced by history is so improbable as to require much stronger proofs than any that can be adduced from ancient writers. In addition to the possible vagueness or errors of the original writers, we have to bear in mind the possible mistakes or misinterpretations of the copyists who reproduced the manuscripts.

It

CHAPTER VIII

NEW STARS

may be glorious to write

Thoughts that shall glad the two or three

High souls, like those far stars that come in sight

Once in a century.-LOWELL

HE stars considered in the preceding chapter go

THE

through their changes of light in a limited and generally more or less regular period, so that a prediction of their brightness at future epochs is in most cases possible. They are distinguished by the remarkable fact, pointed out at the beginning of the chapter, that the period seems to be limited, none so long as two years being yet known.

New stars, or “Nova" as they are frequently called, are distinguished from the irregularly variable stars already described by their blazing forth, so far as is yet known, only once in the period of their his tory.

The limitation of the period seems to form a wellmarked distinction between periodic stars and the irregularly variable ones now to be considered, and to indicate some radical difference in the cause of variability.

The most remarkable among these stars is undoubtedly Eta Argus, which, though now invisible to the naked eye, was, at various times between 1830 and 1850, of the first magnitude. It falls so closely on a line between the new or temporary stars and those which are irregularly variable that it may form a distinct class. Being in 58° of south declination it is not visible except in latitudes south of 32°. For this reason it could not be made a subject of observation in northern European countries. Of the greatest interest is the question whether it was visible in early historic times. On this question no decisive evidence can be gathered. The catalogues of Ptolemy and Ulugh Beigh are among the earlier authorities which we consult on the subject. Much confusion, however, is found in the data to be consulted. In Halma's edition of Ptolemy's catalogue, two stars in the constellation Argo are marked as having the Bayer letter Eta. But neither of these is near the position of the star under consideration. In fact, Ptolemy's constellation Argo seems scarcely to extend as far east as the point in question. The same remark applies to the mediæval catalogue of Ulugh Beigh. The only conclusion we can draw on the subject is that the star was probably not so conspicuous in early historic times as to excite the attention of observers.

On Bayer's charts, published about 1600, there is a star marked Eta, but this is nowhere near the place of the modern Eta, nor is there any star shown in the position of the latter. The fact appears to be that

Bayer's maps of this constellation are so erroneous that little correspondence can be found between his figures and the heavens, and the certain identification of any particular star scarcely seems possible, except in the case of Canopus and possibly a few other bright ones. Near the position of the modern Eta are several small stars marked d, but from what has been said we have no reason to identify these with the star in question.

The first authentic observation of the star is found in Halley's catalogue, made at St. Helena in 1677, where it appears as of the fourth magnitude. The next observation is by Lacaille, who observed it at the Cape of Good Hope about 1750. In the catalogue at the end of his Calum Australe Stelliferum, the star is given as of the second magnitude; but in the original observations it is marked of magnitude 2.3. It may be added that Lacaille was the first one to assign the symbol Eta. From a remark at the end of the catalogue, it seems that he assigned these symbols in accordance with Bayer only when the Bayer stars could be identified, but it would seem that there could have been few such identifications in Argo. In catalogues made between the years 1822 and 1832 it still appears as of the second magnitude; whether this magnitude was an independent one or merely taken from Lacaille may be an open question, but we cannot suppose that the variation from Lacaille's estimate was at all striking. A traveller named Birchell noted it as of the first magnitude in 1827, but this seems doubtful in view of the records of other observers.

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