Binocular Double Stars

Binocular Double Stars
an activity for binoculars or a small telescope

The stellar components of some double stars can be separated by using 
only 10 power binoculars. Here are ten binocular doubles to start your 
journey of discovery! 

Map DSA shows the locations of ten double stars in the early evening April sky.

Be sure that the binoculars are steadily supported to minimize vibrations for the 
sharpest view. Brace your arm against a solid surface such as a wall or a car roof, or, 
better yet, use a tripod.

Magnification and Angular Separation. People with average eyesight should be able 
to see two stars that are separated by 4 minutes of arc – with many people being able 
to discern two objects separated by 2 minutes. (Keep in mind that the full moon has a 
true angular diameter of 30 arc minutes.) 

Since the double stars in this activity have a true angular separation of at least 41 arc 
seconds, a pair of 10 power binoculars will increase their apparent separation ten times 
to at least 410 arc seconds, which equals slightly more than 7 arc minutes. Therefore, 
while they will appear close, they should be discernible as two separate points of light. 

Three double stars are challenges. Two because they are very close indeed, and one 
because the primary star is much brighter than the secondary.

An 8th-magnitude star is very dim for 10x50 binoculars. So, go no fainter. Steady 
support is essential. Otherwise, they just won’t be split.

Click here to view/download the PDF of this Observing Challenge.