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The double star with the greatest colour contrast.

Appreciating the colours of double stars in the eyepiece surely has to be among the most pleasurable of all experiences in visual astronomy. This viewing pleasure becomes even more enhanced if the two components in the double system happen to be of contrasting colours. Since the first telescopes were turned to the sky a few centuries ago, a number of colourful pairs have emerged as clear favourites among double star enthusiasts that top the charts, time and time again. These 'classical doubles', as I would describe them, often consist of reddish primaries accompanied by bluish-green companions, of which Albireo (Beta Cygni), Almaach (Gamma Andromedae), Rasalgethi (Alpha Herculis) and 95 Herculis are prominent examples.

The colour contrast between the two individual components of a double star system becomes greatest when the stars each shine with a light that is at the opposite end of the visible part of the electromagnetic spectrum. That is to say, a red star sitting next to a blue star will produce a greater colour contrast effect in the eyepiece compared to, say, an orange star viewed next to a yellowish companion. This is one of the main reasons why the 'classical doubles' are so highly cherished by amateurs, especially among newcomers to this hobby.

Generally speaking though, descriptions of double star colours are quite subjective and have been known to vary markedly among individual observers. So is there any 'objective' method of determining double star colours? Fortunately there is! In my colour-ranking catalogue of the reddest stars, that I had originally compiled going back to the 1980s, stars are ranked according to their colour indices, B-V. This same technique can be applied to individual components of double stars to gauge their colours. The difference between the B-V values of each star in a pair will then yield a numerical value of the relative contrast between them. Using this method of 'colour differencing', it is thus possible to evaluate the colour contrasts for all visual double stars.

The reddest stars in the sky are of course the carbon stars, whose light is intrinsically reddened by chemical processes within their atmospheres, giving them highly positive B-V colour index values. Consequently, it follows that double star systems in which one component happens to be a deeply red-tinted carbon star and the other a star of non-carbon classification, will have higher than average colour contrasts.

Based on an analysis of 110 carbon stars dotted across the sky brighter than magnitude 8.5, (1) I have determined the double star WZ Cassiopeiae (Otto Struve 254) to be the system with the greatest colour contrast, both visually as well as numerically, out of all double stars in the sky.


The primary in this pair is a deep red variable carbon star of 7th magnitude average brightness, with a colour index, B-V, of +3.08. (2) The secondary is an 8.3 mag A0-class main sequence dwarf of B-V 0.00. Thus the colour index difference between them is a high figure of 3.08.

It should be noted that, in my analysis, there were a couple of other close contenders to WZ Cas in this league, but they proved inferior for one reason or another as follows. U Antliae in the far southern skies is a deep red carbon star, varying around a central magnitude of 6.5. Like WZ Cas, it too has an A0-class colour contrasting companion, albeit much fainter at magnitude 8.7. However, the companion of U Ant (HD 91756) has a B-V of +0.4, with U Ant itself having a B-V of +3.28, giving a net difference of 2.88 between them in the pair as a whole, which is noticeably less than WZ Cas at 3.08.

The red variable star U Cygni in the far northern sky was found to be yet another beautiful object, paired with a (brighter) blue companion of mag 7.8 sitting only 65 arcseconds away. However, this pair did not rank as high as WZ Cas for the fact that the red star is actually a Mira-type variable, pulsating with a much greater amplitude than WZ Cas and fading to as faint as 12th magnitude at minimum. Also, the companion star (HD 193700) happens to be a yellow G0-class main sequence dwarf, having a colour index B-V of+0.8, compared to U Cyg's B-V of +3.08, giving a net difference of 2.28 magnitudes for the pair as a whole. This again is considerably less than the 3.08 of WZ Cas.

I have personally found WZ Cas to be two fine points of twinkling light of rose-red and greenish silver hues, set against the coal blackness of the sky. The colours were best seen through x36 and x100 Plossl eyepieces of my Skywatcher 8-inch Newtonian reflector. I hope readers of this column will take pleasure to observe this pair on the next clear night and see if they concur with my descriptions and ranking of WZ Cas as 'the most colour contrasted' of all double stars in the sky.

Abdul Ahad

100 Selbourne Road, Luton, Bedfordshire, LU4 8LR []

(1) List of carbon stars, http://www.astrosurf. com/buil/us/peculiar2/carbon.htm

(2) SIMBAD database online
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Title Annotation:Letters
Author:Ahad, Abdul
Publication:Journal of the British Astronomical Association
Article Type:Letter to the editor
Date:Dec 1, 2010
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