University Lowbrow Astronomers

Color Contrast in Double Stars.

by Mark Deprest
Printed in Reflections:  September, 1998.
Revised: September 2001.

Most often, colors are associated with binary stars.  Binaries are stars that have formed in pairs, a common occurrence in nature, because human eyes are sensitive to contrast effects, subtle differences in the tints of these stars stand out when observed in a telescope.  The colors in these stars arise from the fact that the outer layers of gas that form a star can be much different in temperature.  Much like heating a piece of steel from a dull red to hotter orange and then white hot, stars display different colors depending upon their temperature.  Therefore, cooler stars appear reddish or orange and the hotter stars seem white or blue-white.

Binary stars form a large part of our known universe.  One of the best things about observing double stars is that they can be easily seen from even light polluted areas.  My backyard in the northeast corner of Ann Arbor is not by any stretch of the imagination a dark site.  My only clear view is from ESE to west, and even on the best nights I can only just barely see a hint of the Milky Way, but I can see and “split” all of the double stars I will present here.

Before I give you my list of Color Contrasting Doubles, we need to talk about two more things; Separation and Position Angle (PA).  Separation is the apparent distance in arc seconds one star is from its companion.  So it follows that the closer the pair the harder it is the “split” them into two separate stars.  Position Angle (PA) is the angle measured from the relative position of the secondary star and how that relates to a line drawn to north from the primary star.

Now, if you just understood what I just wrote “you’re a better man than I Gunga Din.”  The first question is which is the primary and which is the secondary?  Well, the way I always explain it is...  the brighter of the two is the primary.  Well, that works most of the time, until you get two stars of equal brightness.  Then do what I do... guess, the worst thing that can happen is your PA will be 180 degrees off.  Well, then you know you guessed wrong.  Now come the really fun part...  figuring out which way in your telescope is north, south, east and west.  Well, east and west are fairly simple, if you have a drive, turn it off.  Watch the stars enter from the east and exit the FOV to the west.  Now, for north move your scope toward Polaris... the stars are now entering from the north.  With a little practice you can determine a reasonable estimate of the PA of your binary.  You could also make a PA ring for your scope using a paper plate and a protractor (see me for further details).

Alright, here comes my list of Color Contrasting Doubles.  Check them out and compare my colors to what you see.  I think you will find that ‘color’ in stars is very subjective.

  Beta Cygni  

3/3.5 mag   34.3” sep   54 deg   gold/sapphire

  Omicron Draconis  

4.5/7.5 mag   34.2” sep   326 deg   orange/blue

  h3945 (CMa)  

4.8/6.8 mag   26.6” sep   55 deg   gold/blue

  Alpha CVn  

2.7/5.4 mag   19.4” sep   229 deg   yellow/blue

  Str 872 (Aur)  

6.0/7.0 mag   11.3” sep   2l7 deg   gold/blue

  Gamma And  

2.3/5.0 mag   9.8” sep   64 deg   gold/blue

  xi Bootis  

4.8/6.7 mag   6.6” sep   3l8 deg   yellow/red

  95 Herc  

5.0/5.0 mag   6.3” sep   258 deg   green/red

  Alpha Herc  

3.0/5.5 mag   4.6” sep   l04 deg   red/green

  6 Trianguli  

5.0/6.6 mag   3.9” sep   71 deg   yellow/blue

  70 Ophiuchi  

4.3/6.3 mag   3.8” sep   l47 deg   yellow/red

PA and Sep are from StarList 2000 by Richard Dibon-Smith and are plotted for 2000.0 epoch.

I hope you will enjoy hunting these few down and tell me what you think and if you find some others that fit this bill.  I hope you will share them with me.

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Copyright © 2013, the University Lowbrow Astronomers. (The University Lowbrow Astronomers are an amateur astronomy club based in Ann Arbor, Michigan).
This page originally appeared in Reflections of the University Lowbrow Astronomers (the club newsletter).
This page revised Sunday, March 9, 2014 4:30 PM.
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