Much has been written on this subject and it will continue to exercise fascination amongst observers. It is perhaps the most compelling reason why people observe double stars. Although watching the stars swing around their huge orbits over the years can also be interesting, it does not strike with the same immediacy.

Here some optical aid makes all the difference. With the naked eye, few colours can be ascertained. The contrast between the reddish-orange Betelgeuse and the white Rigel in Orion can be seen and the deep red of Antares certainly stands out, but none of the more subtle colours visible in telescopes appear. Colours tend to be much easier to see when some optical aid is used, for a number of reasons. Firstly, there is more light incident on the eye and the cones, which are small receptors in the eye which detect colour can be more easily stimulated. Next, if the telescope is then deliberately defocused, the star colours become more prominent. The reason for this appears to be pychological in origin. Thirdly, star colours become more intense when contrasted with other stars of different hues. In some double stars such as iota Cancri the companion (distant 30'') appears blue alongside the orange-yellow of the primary star. Yet the spectral types of G7 and A3 indicate that the secondary star should be white and it is simply the contrast with the primary which gives the star its blue colour. In alpha Herculis, the companion which is less than 5'' away is distinctly green although no single stars of this colour are known to exist. (Some observers have reported that Beta Librae is green or pale green but Robert Burnham who mentions this in his Handbook, states that the star is white.) It might be interesting to see how the contrast effect varies as the distance between the two stars in a double star system, for stars of similarly different spectral types and brightnesses.

Whilst a telescope enhances the colours in double stars, if too large an aperture is used as Dennis di Cicco9 pointed out some years ago, colour perception is made more difficult. This can be partly explained by the fact that a smaller telescope produces a larger diffraction disk and the eye is more susceptible to colour in extended images than in point sources.

Colours can be determined in a more systematic manner than by eye estimates which are affected by

Figure 2.1. A CCD

image of Albireo (P Cygni) taken from Australia by Steve Crouch. The separation is 34'.'7 North is at the bottom, east to the right.

personal equation. One method is to take colour slides of double stars and project the resulting images against a commercially available colour chart (such as the Macbeth Color Checker) to determine the colour of each component. Such a project was carried out some years ago by a group led by Joseph Kaznica and others10 at the Mount Cuba Observatory in Delaware.

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