One: Observing Double Stars
John A. Barra
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.
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.
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.
main interest in amateurs who view double stars is to learn to see doubles
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.
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.
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.
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:
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"
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
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.
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
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.
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
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.
ISSUE: Lesson Two: Observing Globular Clusters