AAQ Special Interest Sections

Double Stars

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.

Enjoy them on many levels

Colour

Like so many astronomical objects, double and multiple stars can be enjoyed on many levels. Their aesthetic beauty is more than enough for some - the contrast between stars of different magnitude or between different colours, and the sheer astonishment of seeing those two pinpricks of light and knowing that if we were alien observers on a planet in their solar system there would be two suns in our sky. One of the most beautiful doubles is the famous northern hemisphere star Albireo (Beta Cygni). The colour contrast is absolutely spectacular, with the brighter (primary) star yellow and the secondary bright blue. And then you can experience the subjective nature of assessing colour in stars ! The eye is not good at perceiving colour in low light conditions so there is sometimes disagreement amongst visual observers. In the books listed below, Hartung calls them “deep yellow and pale bluish”, Burnham “golden yellow or topaz, and sapphire”, Norton’s “yellow and greenish (by contrast)”, Menzel and Pasachoff “yellow and green”, and the recent book by James Mullaney “orange and blue” in a photograph of the pair. So take your pick. What do you see ? It’s said to be resolvable in steadily held binoculars, with a separation of 35 seconds of arc.

And there are a surprising number of doubles with this particular colour combination visible in the southern hemisphere, including Beta Capricorni, h3945 Canis Majoris, and Iota Cancri.

A test of Telescope and Observer

For the amateur astronomer, close or uneven doubles (with very different magnitudes) are a great test for telescope and observer, and of course the seeing conditions. Hartung’s classic book includes a myriad of doubles and multiples with statements like “10.5cm (aperture) will split them”, and to my embarrassment I find myself struggling with 20cm or even 35cm. Hartung was of course a superb observer with acute eyesight, superb optics, and a dark sky site. Most ordinary mortals like us are lucky to claim one of these advantages let alone all three, so Hartung’s casual feats are usually wildly optimistic for the rest of us. However if you harbour a competitive spirit, then you can get a lot of fun out of picking some tough doubles and seeing whether you can separate them with your telescope. And splitting doubles is an excellent way of testing resolution (the capacity to distinguish detail).

The AAQ Resolution Survey

There are many guidelines and methods for determining whether a telescope of particular aperture can split a particular double star. Obviously very close pairs, and those with large magnitude differences, are more difficult to split than others. However there are many other factors which contribute, most importantly the seeing, magnification and the acuity of the observer. The AAQ has embarked on a programme to observe a wide range of doubles (listed in the monthly newsletter) with many observers and different telescopes, to develop a guide to what a particular ‘scope can do. It’s easy to participate, and your observations can add important information to the study, as well as introducing you to the fun and challenge of observing doubles. Instructions and observing report forms can be downloaded.

The Science of Doubles

And finally, binary stars have a vital role in astrophysics.

If their separation and position relative to each other can be measured over a period of time, then their orbit can be calculated. If their distance from us is known (using parallax or some other method) then their masses can be calculated from Kepler’s and Newton’s laws, the only direct way of doing so. It was this kind of data which led to Sir Arthur Eddington’s discovery of the mass-luminosity relationship, one of the most important in astrophysics. Years of patient observation of binary star orbits has yielded the masses of many stars, providing important insights into stellar physics.

The measurement of position and separation is something which amateurs can do very effectively, with a variety of methods. AAQ members are undertaking such observations.

You can continue on to further information on Double Stars for a list of books, journals and web references.

Happy hunting!

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Astronomical Association of Queensland 2006. www.aaq.org.au