MAGNITUDE AND COLOR DIFFERENCES
Double stars of different magnitudes and small separa-tions can be both a joy and a challenge to observe. Tau Eridani's 5.7 arcsecond separation would not usually be a problem for, say, a 6 inch (150 mm) telescope to split. But in this case, the primary's magnitude of 4.0 can overwhelm the magnitude 10.0 secondary, particularly in pool-seeing. Still, it is not impossible, and seeing a bright star next to a faint one with just a hairbreadth between them can be breathtaking.
Many doubles display dramatically different colors, which add to the overall splendor of the system. One ot the finest doubles in the sky, Eta Cassiopeiae, has yellow and red stars 12.7 arcseconds apart.