Targets for June 2–9, 2016
Naked eyes: Mizar and Alcor
Small telescope: The Sombrero Galaxy (M104)
Large telescope: Elliptical galaxy M60
This week’s naked-eye object is an optical double star in Ursa Major. At the bend of the Big Dipper’s handle, you’ll find magnitude 2.4 Mizar (Zeta [ζ] Ursae Majoris) and magnitude 4.0 Alcor (80 Ursae Majoris).
The two stars are separated by 12′, an easy split for most observers just using their naked eyes. In fact, 2,000 years ago, spotting two stars at this location was the test for vision in the Roman legion. If a prospective soldier didn’t see two stars, he didn’t get in.
Mizar and Alcor are an optical double star. That means the two stars are not physically related. They just lie in the same direction from our point of view. I like to think of an optical double as a type of optical illusion.
When you’ve enjoyed the two stars without optical aid, point a telescope at them and you’ll see more. Telescopically, Mizar splits into two components separated by 14″. Mizar’s companion shines at magnitude 4.0. This was the first star astronomers telescopically identified as a double. Italian astronomer Giovanni Battista Riccioli made this discovery at Bologna in 1650.
