Observing project: Double and Multiple Stars


There are over 100,000 known and catalogued double stars. Some of these are “visual doubles” which simply appear to be paired from our point of view. Many more are “binary stars” where the individual stars are gravitationally bound to each other. These will orbit each other, although the period of orbit may be many tens of thousands of years.

There are a number of factors that affect the way we see double stars: Colour – often the individual component stars have differing colours, blue and yellow pairs being particularly common; Separation – the angular distance between component stars may be so large as to make the double visible to the naked eye or so small that the double can only be detected by sophisticated analysis of observation data.

A small telescope can resolve doubles down to a separation of around 3 arcseconds, depending on the telescope size; Magnitude – or specifically the difference in magnitude between the component stars. It is easier to resolve two close stars of equal brightness than two stars of widely different brightness. At first sight it is not obvious that Polaris is a double star as it has a separation of 18 arcseconds but a magnitude difference of 7 (so Polaris A is 600 times as bright as Polaris B).

Although “double star” is a common term, many “doubles” have three or four visible component stars. Star systems are known with six or more components, although these are beyond the reach of small telescopes.

Some doubles can be seen with the naked eye or with binoculars, others are a challenge even using a telescope at high magnification. The coordinates for each are included to help you if you are using a goto telescope. The easiest way to find them without goto is to use a good star atlas.

When observing doubles it might be helpful to record the equipment used, the magnification, how easy or difficult it was to resolve the individual stars and what you think the colours looked like (as everyone’s perception of colour is different).

It is possible to image double stars. They can look particularly attractive when imaged using a Newtonian telescope as the diffraction spikes make it easier to identify the individual stars. Keep the exposure time and ISO as low as possible to preserve the differences in colour.

There are many good lists of doubles to observe with a small scope. These can be a good starting point if you are new to doubles. Details of some are given below.

So get out and have a look at some double stars. Observe them, photograph them or draw them. Please share your findings or questions on our projects forum http://basprojects.forumotion.co.uk and share your images on our Flickr group http://www.flickr.com/groups/basmembers .

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