Lesson One: Observing Double Stars

by John A. Barra




Having completed a series of five articles in the Primer for the Beginner series, I am beginning a new series aimed at those persons who are now ready to use some of those astronomical techniques learned in the former series. The intermediate amateur astronomer will learn how to observe the various deep sky objects and gain a little understanding of the science involved. The emphasis, however, will remain on having fun while observing. The first subject will be observing double stars.

Some double stars are actually optical doubles. They happen to almost be in the same line of sight, so they appear close together when they may be far apart in actual distance. Delta Herculis and Alpha Ceti are examples of such optical pairs. However, the vast majority of double or multiple star systems are stars that are gravitationally related.

They are called binary stars. I will deal exclusively with visual binaries, pairs that can be separated with the use of optical aids. Other binaries can only be detected by spectroscopic means. Some of those pass in front of each causing a change in brightness and are said to be variable binaries. They are not to be confused with the single variable stars that will be the subject of a future expert series.


The main interest in amateurs who view double stars is to learn to see doubles that
are real close together. The separation is measured in terms of angular distances, being segments of a circle or arc called arcminutes and arcseconds. An arcminute ( ' ) is one-sixtieth of a degree while an arcsecond ( " ) is one-sixtieth of an arcminute. Some doubles such as Alcor and Mizar in Ursa Major are as wide as 12'. On clear nights, they can be separated by the naked eye. However most are separated by arcseconds. Mizar is a double itself with a separation of only 14.4". In fact, each of those two are spectroscopic doubles that cannot be separated by visual means.
The diameter of a telescope's main mirror or lens determines how close of a double star that can be separated by that telescope. The theoretical limit of a telescope in resolving a pair is called the Dawes Limit named after the astronomer who discovered it. The formula used to determine this limit is:

S(arcseconds) = 4.6/D(inches) or S(arcseconds) = 11.7/D(cm.)

S is the theoretical angular limit of a telescope in separating two stars each with a apparent visual magnitude of 6. D is the diameter of the main optic in inches or centimeters. Thus an eight inch telescope should be able to split a double that is .575" apart. Be warned, this limit assumes perfect conditions. Seeing conditions, disparity of magnitude of the stars, and many other factors can influence this limit. It is at best an approximation test.


Another fascinating aspect of viewing double stars is observing the colors or, more significantly, the color contrast between the two stars. Astronomers know that the color of the surface of a star is a good estimation of the surface temperature of a star. Temperatures range from 3000 to 3500 degrees Kelvin for red stars to 11,000 to 70,000 degrees for blue/white stars. To get a star's true color one needs to use spectroscopic equipment.

Estimating temperatures by visual colors seen through a telescope, however, is inexact at best. Many factors influence what we perceive the color of a star to be. The most important factor is that each persons eyes are different and see shades of colors differently. Also the color of one star may affect what we see as the color of the double.

Many people can disagree on the color of any given star. In fact Peterson Field Guide: Stars and Planets (a good source I use) describes the famous double Albireo in different parts of the book as orange and blue, orange and green, blue and green, and yellow and green. So be satisfied in knowing that a star that appears to you to be closer to the red end of the spectrum is cooler that one that appears closer to the blue end.

The important thing is to enjoy the colorful spectacle. It is easy to find lists of good doubles stars to observe. Just about any good astronomy book will have such a list.
Here are few favorites--some difficult-- to get your started:

Name Magnitudes Colors Separation

1.  Gamma Arietis 4.6, 4.7 orange, green 7.8"

2.  Gamma Andromadae 2.2, 5.1 orange, blue 9.8"
3.  Iota Trianguli 5.3, 6.9 yellow, blue 3.9"
4.  Beta Orionis (Rigel) 0.2, 6.7 blue, white 9.2"
5.  Alpha Canis Maj. (Sirius) -1.5, 8.5 blue, white 4.5"
6.  Alpha Gem. (Castor) 1.9, 2.9 blue, white 3.0"
7.  Iota Cancri 4.0, 6.6 yellow, blue 30.4"
8.  Gamma Virginis 3.5, 3.5 yellow, yellow 3.0"
9.  Epsilon 1 Lyrae 5.0, 6.1 white, white 2.6"
10. Epsilon 2 Lyrae 5.2, 5.5 white, white 2.4"
11. Beta Cygni (Albireo) 3.2, 5.4 yellow, green 34.4"
12. 61 Cygni 5.2, 6.0 orange, orange 29.7"

Differing Magnitudes

Separating stars of similar magnitudes is not difficult. It is much more challenging to try to separate close stars of widely differing magnitudes. The observation of double stars is one area in which high magnification is used. But highest power is not always best. The higher the magnification, the more unstable the seeing. You can enjoy the beauty of the doubles better if use high power to split the pair and then back off the power a little if you can still keep them separated. A general rule of thumb is:

X = 240 / S ( " )

where X is the power needed and S is the expected separation in arcseconds of the double sought to be split. You would therefore need approximately 60x to split a double separated by four arcseconds.

Trying to resolve those doubles with greatly differing magnitudes does requires as high of magnification as seeing allows. Even then you may have difficulty splitting some pairs. You may be able to split Rigel this way. But the brightest star in the sky Sirius may be too bright to allow you to split its close, but dim companion. One final way to try to separate Sirius, without resorting to more complex means, would be to use very high power and put Sirius just outside your field. You would need to know the position angle of the pair in order to know in which direction to put Sirius. Good luck.

Position Angle

The last item normally mentioned with double stars is the position angle of the pair. The number ranges from 0 for north to 90 for east, and so forth. The angle represents the direction or orientation of the fainter star with respect to the brighter star.
A position angle of 270 would mean that the fainter star is due west of the brighter one.
Remember, when looking at the sky, east and west are reversed from their orientation on the ground.

Astronomers chart the position angles of doubles over time to compute the orbit and other calculations necessary to determine their masses. You can use them in splitting doubles like Sirius mentioned above. Another use would be to verify that you have split both pairs of a double, double. For example, when splitting both pairs of Epsilon Lyrae, you might not be sure whether you actually separated both doubles or are seeing double vision. The position angles of the two sets are 353 and 80 which means their orientations are about 90 degrees a part. If you are seeing two close dots arranged at a right angle to another pair of close dots, you have successfully split both pairs.

Now that you have learned how to view double stars, you might want to obtain an observing certificate by observing the 100 pairs in the Astronomical League's Double Star Club.


NEXT ISSUE: Lesson Two: Observing Globular Clusters

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