|Section Leader:||Richard Jaworski|
|Additional Contacts:||Andrew James|
The Double Star Section disseminates information about double and multiple stars, and is dedicated to promoting observation of these fascinating objects. The Society has a number of members who are active observers and can help you add double and multiple stars to your observing program.
When most amateur astronomers discuss "deep-sky observing" they usually conjure up images of observing nebulae, galaxies, planetary nebulae and distant star clusters. But there is another class of objects, double stars, which can be just as enjoyable to observe.
Two or more stars can sometimes appear to be very close together in the sky. This may occur simply because they happen to lie in nearly the same direction (i.e. a line of sight effect). These are known as optical doubles. More importantly, however, when the stars are connected gravitationally, the system is known as a binary star. Many star systems are even more complicated and may have three, four or more components. For the purposes of this article I will use the term "double star" to encompass star systems with two or more components.
In our galaxy, it has been estimated that more than 50% of the stars are multiple stars. That puts our Sun in the minority. The study of binary stars is a cornerstone of modern astronomy and much of the information about the important properties of stars has been obtained in the last 100 years from the study of these systems.
The first telescopic observation of binary stars was probably made around 1650 by Giovanni Riccioli in Bologna who found that Mizar (z Ursae Majoris) had two components. This was followed by the Trapezium and in 1665 Robert Hooke discovered that Gamma (g) Arietis was double while following comet Hevelius. In 1761 Johann Lambert thought that such pairs could not all occur by chance. John Mitchell in 1767 envisaged the idea of true stellar companions, and this was supported by Christian Meyer in 1779. Meyer, using an eight-foot mural quadrant, published the first real list of double stars in 1781. It was not until 1803 that William Herschel used his observations to show the apparent movement of one star around another. He believed that the movement was related to the mutual interaction of both stars.
This was followed in 1827 by Felix Savary's demonstration that the relative orbit of the binary system z Ursae Majoris was an ellipse with a period of sixty years. This rocked the scientific world because for the first time terrestrial laws of physics were found to apply to objects in the Universe. From these beginnings, binary star observation flourished with improvements in instruments and techniques and the names of the Herschels, the Struves, Dunlop, S.W. Burnham, Dawes, E.E. Barnard and Robert Aitken stand out as great double star observers. Today, techniques such as Speckle Interferometry and Optical Interferometry are providing resolutions that were previously only dreamed about. These techniques allow very accurate observations of close pairs with short periods so that they can be followed in a complete orbit or several orbits during the observer's lifetime.
Binary stars may be detected by various means of observation and can be classified on the basis of the technique by which they were found. Hence we have, visual, astrometric, eclipsing and spectroscopic binaries. What is of most interest to the average amateur is the visual binary. These objects present us with a wide range of colours, brightness, separations and orientations of the component stars.
One of the great things about double star observing is that you don't need a large aperture telescope with a range of expensive eyepieces. In fact a small telescope is ideal for observing these objects. Smaller scopes are not as affected by bad seeing as larger scopes and stopping down your large aperture scope will lead to better views of the brighter doubles.
When describing double stars, astronomers use the parameters "separation" and "position angle" (See image below).
The separation of a pair is the angular distance in seconds of arc between the two. The position angle of a double star is the direction of the fainter member (designated the "B" component) as reckoned from the bright component (designated "A") beginning at 0° at North and going around by East (90°), South (180°) and West (270°) back to North. To determine the orientation of the field in the eyepiece of your telescope let the stars drift across. The stars will move towards the west and you can thus determine the east-west axis. The north south axis lies at right angles to this. Moving the mount along the north south axis will give you the north and south positions. Once you are orientated, you can roughly estimate the position angle of the pair you are observing.
To determine what doubles are in reach of your particular scope, you should calculate the resolving power of your telescope. In the late 1800's, William Dawes in England derived an empirical relationship for resolving double stars. Dawes famous formula (R = 4.56/A) where "R" is the resolution in arc seconds and "A" the telescopes aperture in inches. Many amateurs do not realize that this relationship holds true only for a pair of stars each of about sixth magnitude.
For brighter, fainter and unequal pairs, the actual Dawes' limit departs significantly from this formula and values of 36/A can be obtained in the case of very unequal pairs. Earlier this decade Harold Peterson, using a 75mm refractor and 45x eyepiece, looked at a range of doubles with varying separations and varying magnitudes differences between the two components. He showed that for his telescope, resolution of a pair is independent of the companion star's brightness until about the 9th magnitude, after which greater separations are needed to resolve the pair. What we need is a revision of Dawes' formula that takes into account magnitude differences. This is a project waiting for a keen amateur.
Some amateurs measure the position angles and separations of double stars. Many use a filar micrometer, while others use a diffraction method or an eyepiece fitted with a reticule. Filar micrometers are an expensive piece of hardware, however a simple diffraction grating micrometer, whilst more cumbersome, can be made out of a piece of cardboard. The Webb Society is very active in this area. There is a lot of useful information in the Webb Society Deep-Sky Observer's Handbook.
The study of the colours of double stars is interesting and controversial. Different observers can look at the same pair and see different colours. Also, the descriptions of some doubles vary markedly from one book to another. Perceiving colour is a complex business and involves issues like telescope optics, seeing, individual colour perception, contrast and the preconceived ideas brought to the eyepiece by a particular observer. It is interesting that smaller instruments provide an optimum light level for observing the colours of the brighter pairs. Small telescopes produce a sizeable diffraction disk, making colour easier to discern. You can get an idea of the true colour of a star from its published spectral type.
Double stars show a variety of appearances. Alpha (a) Tauri (Aldebaran) is a giant star with a diameter forty times that of the sun. It has a faint companion, magnitude 13.5 with a separation of 31" at a PA of 112°. Gamma (g) Arietis is an example of a binary system where the components are nearly equal in magnitude (4.6 and 4.7). a Scorpii is a very unequal double. The primary, Antares, is a brilliant orange red star with a companion located to the west whose position has not changed much since the earliest observations. Its colour is pale green, which is a contrast effect as the spectrum shows that the star is bluish white. An example of a rapid visual binary is z Herculis. The maximum separation is 1.6" and the period is 34.5 years. The magnitudes of the components are 2.9 and 5.5. This pair has shown more than six complete revolutions since Sir William Herschel first measured the system in 1782. Castor is a good example of a multiple star. At present Castor is a relatively easy double. Each member is a spectroscopic binary, and there is the 9th magnitude eclipsing binary YY Gem at separation 73". Some binary stars show beautifully contrasting colours. One of the best examples of this is Beta (b) Cygni (Albireo). The main star of this pair is 3rd magnitude and is golden yellow, while its 5th magnitude companion appears blue. Another example is Delta (d) Corvi whose components are described as yellow and lilac.
I enjoy double star observing as I can do most of it from my home - even at times when there is moonlight around and when my neighbours yard lights are on. My strategy is to select a constellation that is well positioned in the sky at the time I want to observe and ark off the doubles in my atlas within the range of my scope. I often look at a close pair to determine how good the seeing is and save the most difficult pairs for the nights with the best seeing. Double star observing is also a terrific method to learn your way around the constellations.
So take the challenge, add some double stars to the list of objects you observe. I think you'll find yourself spending more time observing, and enjoying your hobby.
(* = Triple Star. ** = Quadruple Star)
Our Society has traditionally has held a strong interest among the membership regarding these objects and in 1979 formed this section in response to this. Richard is a keen visual observer; he contributes "This Month's Double Stars" occasionally appearing in Universe Magazine. He has published several articles including a Sky & Telescope (December 2002) article on some of the brighter southern double stars. He has been the ASNSW's Double Star Section Leader since 1991.