Observing Double Stars by Andrew Wood
Andrew Wood is an amateur astronomer and member of Sydney City Skywatchers. He has spent much of covid outside at night on Dharawal Country observing the stars and planets through his telescopes. He has also been re-examining observations made over the past few years with some interesting results and advice well worth sharing.
Beyond our solar system, in which we can make observations of the Sun, Moon, planets, asteroids and comets; lies the rest of the universe: the realm of star clusters, nebulae and galaxies. We generally refer to these as Deep Sky
Objects. The simplest deep sky objects are Double Stars – also called binary stars. These are systems of two stars (sometimes more – termed multiple stars) rotating around a centre of gravity. They are therefore close together – as close as solar system distances between stars and planets. Planets within such a system would be treated to two or more “suns” in their sky.
From our Viewpoint on Earth
Looking across the light years of space, most double stars appear as single points to the naked eye. Through the telescope, however, their true nature is revealed. You may see two stars fairly equal in brightness, or a bright star with a much fainter companion. In addition, the close proximity of the stars often accentuates any colour difference. Such examples, as well as being visually striking, demonstrate stellar evolution through the eyepiece. Two examples of double stars are shown below:
(left) Albireo in the northern constellation of Cygnus is a fine example of a double star with components of different colours, indicating different spectral classes.
(below)Double and multiple stars come in a wide variety, as shown in this graphic. CPM in the lower right hand example refers to Common Proper Motion, where astronomers have found the stars appear to be traveling through space together. Reference: handprint.com
Observing these often overlooked deep sky objects can be very interesting and does not require premium quality telescopes or perfect sky conditions.
Thousands of binaries are within reach of modest telescopes, and some good examples are available to the suburban observer, even under moonlight.
The telescope you use will place some limitation on your observing. However, observing binary stars can test what your telescope is capable of - its resolution - and this can be an interesting observing project.
Resolution is the ability to separate points of light. The oft-used analogy is watching a vehicle approach from a long distance at night-time. At first its headlights will appear merged: only as the vehicle gets closer will you see the light from each headlight separately.
So the first consideration in observing a binary star is whether your telescope is capable of separating the combined light of the stars – do you see just one star or can your telescope “split it” so you see two? This depends on a few things. The first consideration is the angular separation of the pair. Because the distance to stars is so far, this is a tiny angle, measured in arcseconds. (An angular degree is divided into 60 arcminutes, each of which is divided into 60 arcseconds).
Whether a telescope is theoretically capable of splitting a binary star is given by the following formula, called the Dawe’s Limit.
R = 116/D
In this formula, R is resolution in arcseconds, and D is the telescope aperture in millimetres. This formula, derived by an 18th century English astronomer, William Dawes, using high quality refracting telescopes of the day, actually refers to two stars, each of magnitude 6.0.
This is an important factor, and it brings us to the second consideration in a telescope’s ability to split binary stars: the magnitude difference of the two components. The brighter member in a pair is called the Primary, and its fainter companion the Secondary. The larger the magnitude difference between the stars, the further the Secondary component needs to be from the Primary in order to be seen in the brighter star’s glare.
Resources such as a Norton’s Star Atlas or the Bright Star Atlas will list among the deep sky objects on offer the magnitudes and angular separation of hundreds of binaries within the range of amateur telescopes. There are also publications such as the Cambridge Double Star Atlas, dedicated only to binary stars, which list many thousands of objects. It’s a matter of hunting them down and seeing if your telescope can split them.
Which raises the third concern for the binary star observer – their telescope. As mentioned, the Dawe’s Limit was found using high quality refracting telescopes of more than 100 years ago. An expensive, modern apochromatic refractor may go beyond the theoretical limit, while a cheaper achromat is unlikely to reach it. Large aperture Newtonian “light bucket” reflectors are used in order to see very faint deep sky objects, but their single star images are not as “tight” as a high quality refractor. Whatever telescope you use, there will always be plenty of binary stars within range, but it’s interesting to test your telescope to see what stars can and can’t be split.
I thought it would be interesting, anyway. So I did it...
The telescope I have mainly used for the last 25 years is a 250mm aperture Newtonian Reflector. Before obtaining it I’d used for many years a small achromatic refractor, and some of my favourite objects to observe were brighter binaries. Although the larger telescope was obtained for seeing fainter galaxies and nebulae at dark sky locations, it is still mainly used in my suburban backyard, so my binary observations continued. I’ve been a recorder of what I’ve observed. The graph below, from observations of double stars over a period of ten years, illustrates the telescope’s ability as a “star-splitter”.
In the graph above, I have plotted the magnitude difference of the stars’ components versus their angular separation in arcseconds. [I am certainly not the first to carry out such a study. An American amateur, Harold Peterson, first published a similar set of results for his 75mm refractor in Sky & Telescope more than 50 years ago]. Looking at the overall data, with some exceptions, at a separation of 3 arcseconds stars are either split or not split evenly and above 4 arcseconds the vast majority are split.
Let’s take a look at some individual examples.
One night in October 2014, I observed a double star designated h5014 in the constellation Crater. This is pair a of 5.7 magnitude stars separated by 1.7 arcseconds. Using a magnification of 155 times, I noted that I could see two stars without any space between; they were “touching”. Nearly four years later, in August 2018, I observed the system designated Rmk 20 in Triangulum Australe. The components have magnitudes of 6.2 and 6.4 and are separated by 1.8 arcseconds. I couldn’t split them. I was also unable to separate the system h4728 (Omega Lup), which is a pity as the The Cambridge Double Star Atlas lists it as showpiece pair. It consists of two blue-white stars each of magnitude 4.6 separated by 1.6 arcseconds.
What about some well-known binary systems?
Antares, a variable star of magnitude range 0.9-1.8, has a green companion of magnitude 5.4 separated by 2.5 arcseconds. I once saw the striking green secondary through a 125mm apochromatic refractor owned by another observer. I have on many occasions, when Antares is near zenith and conditions were good, tried with my 250mm reflector; and have never succeeded in splitting it. Similarly with Sirius, which at magnitude -1.5 has a magnitude 8.5 white dwarf secondary 7 arcseconds away. Despite the wide separation, I’ve never seen the secondary in the glare of the primary ten magnitudes brighter.
Rigel has been a happier proposition. I have never failed to see its magnitude 7 secondary 10 arcseconds away from the 0.3 magnitude primary.
It would appear that my personal limit as an observer using this telescope, for similarly bright stars, is 1.7 arcseconds, well above the theoretical 0.5 arcseconds (116/250 = 0.5). At 3 arcseconds stars are either split or not split evenly and above 4 arcseconds the vast majority are split with a few exceptions.
So does the fact that my telescope does not approach the theoretical resolution of its aperture mean that it is of poor quality? No. As mentioned earlier another observer with a smaller high quality refractor was able to split stars better than mine. When looking at faint deep sky objects such as nebulae, however, the amount of detail I could see was far greater than in that smaller, much more expensive telescope, the optics of which did give sharper star images. A good comparison may be a small sports car versus a work-horse ute.
The graph shows that several stars were not resolved when the surrounding data suggest they should have been. This may be due to less than ideal viewing conditions at the time of observation; or possibly because double stars are dynamic systems that orbit each other and their orientation from our Earthly viewpoint can result in the apparent angular distance between them changing over some years.
Another parameter given for double stars in some references is the position angle, measured from north, of the primary to the secondary star. There are many doubles where the position angle and separation have not been measured for decades. These are important measurements as they can be used by professional astronomers to calculate star mass. It is a neglected area of amateur study (and may be the topic of another article).
Whatever telescope you use, there will always be plenty to see, and it’s fun to observe all sorts of astronomical objects, from those that are well within reach of your instrument to those that you know will be more challenging.
Here are some well-known double stars to try:
Any basic astronomy guide such as the Norton’s Star Atlas or the Bright Star Atlas (Tirion and Skiff) will list many double stars among the deep sky objects. The following two books are devoted solely to the observing of Double Stars.
Double Stars and How to Observe Them. James Mullaney. This book is currently free online as a PDF.
The Cambridge Double Star Atlas. James Mullaney and Wil Tirion. Cambridge University Press.
Hartung’s Astronomical Objects for Southern Telescopes, Revised by Malin and Frew, Melbourne University Press, 1995. Hartung included many double stars among the deep sky objects he described.
Southern Double Star Gems. Richard Jaworski. https://skyandtelescope.org/observing/celestial-objects-to-watch/southern-double-star-gems/
Postscript: In researching images for this article I found a lack of good Astro-photos of southern double stars. Anyone inspired to observe double stars who also partakes in digital astrophotography has an opportunity to fill that gap.