Welcome to the Double Star Section of the AAQ, dedicated to the enjoyment of observing double and multiple stars.

Read on for an introduction to Double Stars. You can then jump to further information on Double Stars for a list of books, journals and web references.

When you are ready to practice what you’ve read and see some beautiful and challenging sights, we’d invite you to take part in the AAQ Resolution Survey. By simply using a telescope, your eyes and some easy to learn techniques you can discover more about your equipment and the sky, and contribute to some useful astronomical research.

What is a double star ?

As the name suggests, it is a pair of stars close together in the sky, in fact so close together that they can only be separated using some kind of optical aid. Some wide, bright doubles can be seen in binoculars, but most need a telescope of some kind. It used to be thought that they were chance alignments of stars in the sky, but it is now known that such alignments are rare and most doubles are actually close to each other in space and bound by gravity, either orbiting each other (when they tend to be referred to as binary stars), or having common proper motion – that is, they are moving through space together. In fact more than half the stars we see in the sky are actually multiple star systems. In that sense, our own sun is in the minority because (as far as we know) it has no companion.

Alpha Centauri is one of the most famous doubles, and only accessible to us blessed southern hemisphere observers. Alp Cen is actually a triple system, with the third member, Proxima Centauri (quite far away from the bright pair and not visible in the same field), being the closest star to our sun. The bright pair are orbiting each other relatively quickly, completing a full orbit in only 79.2 years. So motion along the orbit is measurable in much less than an observer’s lifetime, which is not always true of visual doubles.

Why observe double stars ?

Why observe these fascinating objects ? Well, there are plenty of reasons. The Budget Astronomer lists them as follows:

  1. They are good targets for an urban or suburban location. Many of them are quite bright, and there are binaries visible all year long, whereas planets and the moon are not always up when you want to observe.
  2. They are aesthetically pleasing. A nice binary pair with distinct colour differences can be very pretty.
  3. They provide a challenge – some are faint, or very close, or have a high magnitude difference, and they provide a certain satisfaction.
  4. Binaries give you an excuse to push the magnification on your scope to “crazy high” powers – above 600x.
  5. You will never run out. There are thousands of binaries visible from any given latitude.
  6. Binaries provide amateurs an opportunity to contribute to “real science” – there are more binaries than professional astronomers. Backyard observers can contribute by measuring the separation and position angle, which can then contribute to calculating orbits, which in turn helps us learn more about the mass of the stars, and helps to refine the model of the nature of how stars form and evolve.

The AAQ monthly newsletter includes many interesting doubles to observe month by month, in the Visual Observing section. There are websites which include several lists of doubles worth a look. There are books which deal with double stars or include aspects of doubles and their observation.


Further reading

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Most recent papers by members

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