SKY_September2014.pdf

20

64 September 2014 sky & telescope

Spottable Pairs

Double Stars

From easy to challenging, here are some star pairs every skywatcher should know.

Because I take astrophotos all the time, some people

think I never do any observing. But once I get an exposure

run going, I start looking around. If I’m out with friends

I’ll observe through their scopes. More often I’m by myself,

and then I use either binoculars or just my eyes.

I’ve done a lot of naked-eye observing. It never pales.

Sitting under a clear, dark sky as the rest of the universe

passes overhead in majestic splendor is a profound and

astonishing experience. And there’s almost no end of new

things to discover.

One pastime I’ve come to enjoy is naked-eye double-star

observing. My favorite showcase examples are Theta Tauri,

Alpha Capricorni, Epsilon Lyrae, the Cat’s Eyes pair in

Scorpius, and Mu Scorpii. But you can see many more.

Of course, this pursuit has its limits. The human

eye could theoretically resolve about ½ arcminute if

it were made with machine perfection, but in the real

world, folks with excellent eyesight can resolve about 2

arcminutes. With average eyesight, you can resolve 3′. A

lot also depends on the brightness of the two stars and

especially the brightness diff erence between them.

A note on terminology: Not all double stars are binary

stars. A binary is a true physical pair whose members

orbit each other. “Double stars” include both binaries

and optical doubles, pairs that merely look close together

on the sky but whose stars are at diff erent distances and

physically unrelated. If a double is wide enough to be

resolved with the naked eye, it’s usually an optical pair.

Here’s my list of favorites. It starts with those you can

see on August and September evenings, then works the

rest of the way around the night and the year. Given for

each star are its separation in arcminutes and the direc-

tion of the fainter star from the brighter one, then the

Naked-Eye

JERRY LODRIGUSS

SCORPIUS

Zeta

Mu

M4

Upsilon

Antares

Omega

Lambda

M6

M7

ALL PHOTOS BY JERRY LODRIGUSS

NakedEye_Doubles.indd 64 6/23/14 12:17 PM