Colored Stars

( Originally Published Early 1900's )

MOST persons would say on a casual glance that the stars are specks or points of white light, and so no doubt the majority of them are ; but more attentive examination will disclose the fact that a very considerable number of them exhibit definite colours, though those in which any colour is very pronounced are in a great minority. The student who is familiar with the intense colours of the solar spectrum will be disappointed if he expects to find amongst the stars many colours as pronounced as those which he sees in the solar spectrum. Nevertheless, it is possible in a general way to find here and there stars which if they were all brought together in a row would constitute some similitude of the solar spectrum.

There are many difficulties in the way both of observing and of recording the colours of stars, and this explains the discrepancies in the accounts put forth by different observers. In the first place, people's eyes are differently constituted some eyes are more capable than others of accurately appreciating and describing a colour. Some eyes, indeed, as is well known, are totally incapable of appreciating certain colours at all. Possessors of such eyes are said to be " colour-blind." But, disregarding extreme cases of this sort, it is quite certain that ordinary eyes will differ not a little in appreciating a given colour. It suffices to visit a picture gallery and take note of the differences in the copies of one and the same original picture which are being made by different copyists, to realise the fact that particular hues in the original are reproduced in a very different way by the different persons.

Then, again, the quality of the glass of the telescope employed influences much the apparent colours of the objects looked at; and still greater is the effect of the good or bad grinding of the lenses. In other words, lenses made of very pure glass and very accurately ground and polished will yield images and indications of colour which will be much more true to nature than the indications afforded by inferior glass inaccurately figured. It is a very noteworthy fact that metallic mirrors always give to objects seen through them a reddish tinge. This is strikingly brought out in connection with Sir John Herschel's observations of red stars. To many of these objects he has attached such qualifying words as " carmine," " ruby," "intense crimson," where ordinary observers employing ordinary telescopes would see only ordinary red hues. Nor is magnifying power entirely an unimportant matter; with a low power white will dominate, and other tints will in a measure be lost, because no star is absolutely mono-chromatic ; on the other hand, a high magnifying power diminishes the total light, and, exaggerating the dimensions of the spurious discs, renders the colours more easily distinguishable. Again, the state of the atmosphere and the proximity of a star to the horizon greatly affect its appearance. It is only when a star is well up in the heavens above the horizon that its true colour, whatever it may be, can be noted, because near the horizon all celestial objects apparently acquire red or orange hues, which do not really belong to them.

Perhaps the greatest of all the difficulties which beset the observer who wishes to make an accurate record of star colours, is the difficulty of providing and using a standard of colour for comparisons. Such a standard is furnished naturally by the solar spectrum ; but astronomers have hitherto been altogether baffled in their at-tempts to reproduce the prismatic colours in such a way that they can be rendered practically available in the darkness of night, side by side with the image of a star produced at the eye-end of a telescope. There is herein, in point of fact, a double difficulty : that which may be called the manual or mechanical difficulty just alluded to, and that which arises from the fact that the artificial light employed by night being yellow, injures the neutrality of the eye and falsifies all artificial colours. It was with the idea of getting over these difficulties that Secchi proposed to make use of an electric spark, which, if derived from different substances, would give for each of them a different hue, but I am not aware that any attempt has ever been made to put this idea into practice.

Single stars of a red or orange hue are not uncommon, but isolated blue or green stars are very rare. Indeed. ß Librae appears to be the only conspicuous star which is green. In the ease, however, of double stars it is much more easy to define their colours, for in many Instances they exhibit very well-marked colours, and frequently the colours of the 2 stars are what are called " complementary." The reader may be reminded that this is a term applied by physicists to the colours which, when united, make white light. In order to obtain a strictly exact idea of what these colours are recourse must he had to a special instrument of which several kinds have been contrived. But for the elementary purposes of this work it will suffice to state that the principal pairs of colours which are mutually complementary are red and green, orange and blue, and yellow and violet. The intermediate tints are too innumerable to be described in words, and they can only be realised by instrumental means.

When we speak of double stars as exhibiting different colours it is not permissible in all cases to regard the colours as an optical illusion or an effect of contrast, for in some instances certainly the colours are an actual physical reality. We may draw this conclusion in some cases from the circumstance that the colours seen are not always complementary ; and in other cases from the fact that, by concealing the principal star by means of a bar in the eye-piece, formed of watch-spring or something of that sort, we shall notice that the companion star, when thus cleared of the effects of its primary, preserves its colour unchanged.

Secchi compiled the following list of conspicuous stars of the colours stated :— White, Procyon, Altair ; Blue, Sirius, Vega, Castor, Regulus ; Yellow, Capella, Pollux, a Ceti ; Orange, Aldebaran, Arcturus, Betelguese ; Ruddy, Antares, a Herculis.

Krüger, an experienced German observer, has given the following list, which, it will be seen, is not wholly in accord with Secchi's :— White, Sirius, Altair, Regulus ; Yellow, Capella, Pollux, Arcturus ; Orange or Red, a Herculis, Betelguese.

All the really red stars that is, stars of pronounced depth of colour are comparatively small in size scarcely, if at all, visible to the naked eye. There are a few perhaps half a dozen to which the designation " carmine " may be applied, but the bulk of the so-called red stars are more orange than red. I shall have something more to say about some of these in the chapter on " Variable Stars."

The question of whether the stars vary in colour has attracted some attention, but the evidence is on the whole, meagre and inconclusive. From a passage in Seneca, an ancient Roman writer, it has been inferred that he wished it to be understood that in his day Sirius, the Dog Star, was red, whereas now it is white, or bluish-white. Ptolemy seems also to have regarded Sirius as a red star, and to have used a word to describe it which he also applied to Pollux. Now Pollux is certainly a reddish-yellow star in the present day, and if it and Sirius could ever have been appropriately designated by the same adjective of colour, then the conclusion follows as a matter of course that Sirius no longer exhibits the colour it once did. Capella is perhaps another star which has changed from red, or reddish, to blue but one could have wished for a larger number of instances. At present we can only say that whilst change of brilliancy in the case of stars is a common occurrence, change of colour is not a well-established fact.