SKY_September2014.pdf

SkyandTelescope.com September 2014 65

magnitude, spectral type, and distance in light-years of

each. The distances are inexact, more so the farther out

you go, so when the values look similar they may in fact

be essentially the same and the two stars may be a true

binary. Mizar and Alcor are a case in point.

Mizar & AlcorMizar & Alcor, separation: 12′ (ENE)

Mizar: mag. 2.2, A2V, 78 l-y

Alcor: mag. 4.0, A5V, 82 l-y

Easy. R.A. 13h 24 m, Dec. +54.9°

This most famous of naked-eye doubles, at the bend of

the Big Dipper’s handle, was long considered a test of

keen eyesight. But in the modern era of good eyeglasses

it’s no great challenge for most people, with its separation

of^1 / 5 °. A small telescope resolves Mizar itself into a much

closer double star, separation 14 arcseconds.

Alpha Librae (Zubenelgenubi)Alpha Librae (Zubenelgenubi), separation 3.8′ (NW)

Alpha^2 Lib: mag. 2.8, A3III/V, 77 l-y

Alpha^1 Lib: mag. 5.2, F3V, 75 l-y

Very diffi cult. R.A. 14h 52 m, Dec. –16.1°

Famous as a wide and easy pair in binoculars, Alpha

Librae’s toughness to resolve naked-eye may be surprising

— until you imagine Mizar and Alcor three times closer

together, and with a greater magnitude diff erence!

When Greek-lettered stars have superscript numbers,

the numbers are not in order of brightness but right as-

cension from west to east. This was the order they crossed

the north-south hairline in a meridian transit telescope,

the fi rst really accurate tool for measuring star positions.

Omega ScorpiiOmega Scorpii, separation 15′ (SSE)

Omega^1 : mag. 3.9, B1V, 470 l-y

Omega^2 : mag. 4.3, G7III, 290 l-y

Easy. R.A. 16h 07 m, Dec. –20.7°

This lovely naked-eye optical double adds to the overall

fascination of the district around Antares and the head of

Scorpius, as shown at left. Look for a color diff erence.

Mu Scorpii (Little Cat’s Eyes)Mu Scorpii (Little Cat’s Eyes), separation 5.8′ (ENE)

Mu^1 : mag. 3.0, B1.5Vp, 500 l-y

Mu^2 : mag. 3.6, B2IV, 470 l-y

Showcase close naked-eye double! R.A. 16h 53 m, Dec. –38.1°

Beautiful but close — and low. If you’re at 40° north lati-

tude, Mu never rises more than 12° high.

Zeta ScorpiiZeta Scorpii, separation 6.5′ (W)

Zeta^2 : mag. 3.6, K4III, 130 l-y

Zeta^1 : mag. 4.7, B1Ia-Oek, 2,500 l-y

Moderately diffi cult. R.A. 16h 55 m, Dec. –42.4°

Zeta Sco is even lower for northern observers; it’s 4° south

of Mu. For the truly keen-eyed, a third, 5.8-magnitude

star lies 7 arcminutes to the pair’s south. Just north of

Zeta is the bright, sprawling open cluster NGC 6231, add-

ing to the False Comet asterism.

Observing Tips

· Use your arm to block out low-altitude light pollution from

view, or make “air binoculars” with your curled hands.

· Get your vision tested, and make sure your glasses’ prescription

is up to date. Consider acquiring a second pair optimized for

astronomy, perhaps by applying –½ or –¾ diopter to your

regular prescription for each eye. For many people, this off sets

their night myopia, which is generally strongest in youth and

decreases with age. (See “Spectacles for Spectacular Skies,”

S&T: Sept. 2005, p. 30.)

· Get low-refl ection coated eyeglass lenses; they transmit more

light and scatter less.

· Observe often to gain experience. You’ll be surprised at the

diff erence this makes.

Lambda & Upsilon Scorpii (Cat’s Eyes)Lambda & Upsilon Scorpii (Cat’s Eyes), separation 36′ (WSW)

Lambda (Shaula): mag. 1.6, B2IV, 570 l-y

Upsilon (Lesath): mag. 2.7, B2IV, 580 l-y

Easy. R.A. 17h 34.6m, Dec. –37.1°

The Cat’s Eyes in the stinger of Scorpius are unequal and

are usually seen tilted; maybe the cat is wobbling. Both

are hot blue stars in the enormous Scorpius-Centaurus

Association of young stars. It’s hard to believe, but their

separation of 36′ is slightly wider than the full Moon. A

line from Lambda through Upsilon points to Mu.

Epsilon Lyrae (Double Double)Epsilon Lyrae (Double Double), separation 3.5′ (N)

Epsilon^2 : mag. 4.6, A6Vn + A7Vn, 160 l-y

Epsilon^1 : mag. 4.7, A3V + F0V, 160 l-y

Showcase diffi cult pair. R.A. 18h 45 m, Dec. +39.7°

This famous pair is a real challenge because of the stars’

closeness and faintness, but at least they’re nearly equally

bright. Good luck! A telescope at 100× or more resolves

each in turn into a close binary.

Delta LyraeDelta Lyrae, separation 10′ (WNW)

Delta^2 : mag. 4.2, M4II,750 l-y

Delta^1 : mag. 5.6, B2.5V, 1,000 l-y

Moderately diffi cult. R.A. 18h 54 m, Dec. +37.0°

Another famous binocular pair, easier to resolve naked-

eye than Epsilon due to its wide separation.

Nu SagittariiNu Sagittarii, separation 14′ (ENE)

Nu^1 : mag. 4.9 , K1II, 1,100 l-y

Nu^2 mag. 5.0, K1Ib/II, 275 l-y

Easy. R.A. 18h 55m, Dec. –22.7°

Bet you didn’t know this one! Nu Sagittarii is south of the

tip of the Sagittarius Teaspoon, about a third of the way

from there to Nunki, the brightest star in the handle of the

Teapot. The Nu stars are faint but fairly wide. Binoculars

reveal lots of deep-sky wonders around here.

NakedEye_Doubles.indd 65 6/23/14 12:17 PM