What to Record
Okay, so now you’ve found your double star, what exactly are you looking at? And what should you record?
Perhaps the first thing to note is the color of the two stars. It is not uncommon for observers to disagree on color. This can be due to a number of factors, such as your color vision, the quality of your binoculars, the contrast between the two stars, their magnitudes, how high they are above the horizon, how clear the sky is and even the time of night or the time of year. Do not be surprised to find that when you return to look at a double at a later date then one or both stars appear to have changed color. If you want to know what color they should appear as then visit the Alcyone website and enter the name of the star. When the results come up click into “The Washington Double Star Catalog 1996". If you do this for, say, Iota Bootis then it will tell you that the stars’ spectral groups are A7IV and K0V. The A7 means that one of the stars should appear yellowish, while the K0 means the other star should be yellowish-orange. The color sequence is:
B: Blue to bluish-white
A: Bluish-white to pale yellow
F: Pale yellow to yellow
G: Yellow to yellowish-orange
K: Yellowish-orange to reddish-orange
M: Reddish-orange to deep red
Next you should note the Position Angle or PA. The PA is centered on the brightest star. The angle of the faintest star is measured counter-clockwise from due north. So a star that has a PA of 90 degrees is the fainter star that is directly to the left of the brighter star. If it had a PA of 180 degrees then it would be due south, and 270 degrees would put it directly to the right. The Alcyone website will give you some idea of the PA and how it has changed over the years.
With practice you will become familiar with the angles. A little more tricky is the separation of the two stars. Again, Alcyone will help, as will a good star atlas such as Norton’s.
Now, how about testing your binoculars to see how good they really are? As we previously said, with a 10 x 50 you should be able to separate stars that are 40 arc seconds apart. So why not see if you can separate something a little closer, such as Lambda Arietis at just 37 arc seconds?