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20 fun naked-eye 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 1/2 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 difference 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 different 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 direction of the fainter star from the brighter one, then the 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 & Alcor, separation: 12' (ENE)

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

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

Easy. R.A. 13h 24m, Dec. +54.9[degrees]

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 [degrees]. A small telescope resolves Mizar itself into a much closer double star, separation 14 arcseconds.

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

[Alpha.sup.2] Lib: mag. 2.8, A3III/V, 77 l-y

[Alpha.sup.1] Lib: mag. 5.2, F3V, 75 l-y

Very difficult. R.A. 14h 52m, Dec. -16.1[degrees]

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 difference!

When Greek-lettered stars have superscript numbers, the numbers are not in order of brightness but right ascension from west to east. This was the order they crossed the north-south hairline in a meridian transit telescope, the first really accurate tool for measuring star positions.

+Omega Scorpii, separation 15' (SSE)

[Omega.sup.1]: mag. 3.9, BIV, 470 l-y

[Omega.sup.2]: mag. 4.3, G7III, 290 l-y

Easy. R.A. 16h 07m, Dec. -20.7[degrees]

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 difference.

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

[Mu.sup.1]: mag. 3.0, B1.5Vp, 500 l-y

[Mu.sup.2]: mag. 3.6, B2IV, 470 l-y

Showcase close naked-eye double! R.A. 16h 53m, Dec. -38.1[degrees]

Beautiful but close--and low. If you're at 40[degrees] north latitude, Mu never rises more than 12[degrees] high.

Zeta Scorpii, separation 6.5' (W)

[Zeta.sup.2]: mag. 3.6, K4III, 130 l-y

[Zeta.sup.1]: mag. 4.7, B1la-Oek, 2,500 l-y

Moderately difficult. R.A. 16h 55m, Dec. -42.4[degrees]

Zeta Sco is even lower for northern observers; it's 4[degrees] 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, adding to the False Comet asterism.

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[degrees]

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), separation 3.5' (N)

[Epsilon.sup.2]: mag. 4.6, A6Vn + A7Vn, 160 l-y

[Epsilon.sup.1]: mag. 4.7, A3V + F0V, 160 l-y

Showcase difficult pair. R.A. 18h 45m, Dec. +39.7[degrees]

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 100x or more resolves each in turn into a close binary.

Delta Lyrae, separation 10' (WNW)

[Delta.sup.2]: mag. 4.2, M4II, 750 l-y

[Delta.sup.1]: mag. 5.6, B2.5V, 1,000 l-y

Moderately difficult. R.A. 18h 54m, Dec. +37.0[degrees]

Another famous binocular pair, easier to resolve naked-eye than Epsilon due to its wide separation.

Nu Sagittarii, separation 14' (ENE)

[Nu.sup.1]: mag. 4.9 , K1II, 1,100 l-y

[Nu.sup.2] mag. 5.0, K1lb/II, 275 l-y

Easy. R.A. 18h 55m, Dec.-22.7[degrees]

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.

[Omicron.sup.1] & [Omicron.sup.2] Cygni, separation 61' (NNE)

[Omicron.sup.1]: mag. 3.8, K211, 900 l-y

[Omicron.sup.2]: mag. 4.0, K3lb, 1,050 l-y

Easy. R.A. 20h 14m, Dec. +46.8[degrees]

[Omicron.sup.1] & 30 Cygni, separation 5.6' (NW)

[Omicron.sup.1]: mag. 3.8, K2II, 900 l-y

30 Cyg : mag. 4.8, A5IIIn, 600 l-y

Moderately difficult. R.A. 20h 14m, Dec. +46.8[degrees]

The two Omicron stars in Cygnus, 5[degrees] from Deneb, are so far apart that they're a double only under the loosest definition. The challenge here is Omicron1 Cygni and its much closer neighbor, 30 Cygni. Binoculars and telescopes split Omicron1 into an orange-red and bluish pair, which make a lovely triple with white 30 Cygni.

Alpha Capricorni (Algedi), separation 6.3' (WNW)

[Alpha.sup.2]: mag. 3.6, G8.5III-IV, 105 l-y

[Alpha.sup.1]: mag. 4.3, G3lb, 570 l-y

Showcase naked-eye double! R.A. 20h 19m, Dec. -12.5[degrees]

The western tip of Capricornus is one of the loveliest naked-eye doubles in the sky. Although they're just an optical pair, both stars are yellow giants, and both have extremely faint telescopic companions.

Sterope, separation 2.5' (SE). In the Pleiades.

21 Tauri: mag. 5.8, B8V, 370 l-y

22 Tauri: mag. 6.4, A0Vn, 380 l-y

Extremely difficult. R.A. 3h 47m, Dec. +24.6[degrees]

Altas & Pleione, separation 5.0' (N). In the Pleiades.

27 Tauri (Atlas): mag. 3.6, B8III, 380 l-y

28 Tauri (Pleione): mag. 5.0, B8Vne, 380 l-y

Moderately difficult. R.A. 3h 50m, Dec. +24.1[degrees]

The Pleiades contain two naked-eye challenge pairs. The famous Atlas and Pleione are the legendary parents of the Seven Sisters. I rank them as only moderately difficult, a beautiful pairing amidst the other jewels of the Pleiades if you can manage it. The Sterope (or Asterope) pair, on the other hand, is very tight and very faint as well.

Theta Tauri, separation 5.6' (NNW). In the Hyades.

[Theta.sup.2]: mag. 3.4, A7III, 150 l-y

[Theta.sup.1]: mag. 3.8, K0lllb, 155 l-y

Showcase naked-eye double! R.A. 4h 29m, Dec. +16.0[degrees]

Delta Tauri, separation 18' (ESE). In the Hyades.

[Delta.sup.1]: mag. 3.8, G9.5III, 155 l-y,

[Delta.sup.2]: mag, 4.8, A2Vs, 160 l-y

Fairly easy. R.A. 4h 24m, Dec. +17.6[degrees]

The Hyades contain two much easier targets. Theta Tauri, near Aldebaran, is right up there with Alpha Capricorni as one of the finest near-equal naked-eye pairs in the sky. Binoculars show a color difference: orange and bluish.

In the opposite arm of the Hyades' V pattern, the [Delta.sup.1]-[Delta.sup.2] pair is fainter but much wider. [Delta.sup.3] Tauri, twice as far to their northeast, outshines faint [Delta.sup.2] by a half magnitude and forms an elongated triangle with the closer Deltas.

42 & 45 Orionis, sep. 4.2' (ESE). In Orion's Sword.

42 Ori: mag. 4.6, 61V, 900 l-y

45 Ori: mag. 5.2, F0III, 360 l-y

Very difficult. R.A. 5h 36m, Dec. -4.8[degrees]

Theta Orionis, sep. 2.2' (NW). In Orion's Sword.

[Theta.sup.2]: mag. 5.0, 61,1,400 l-y

[Theta.sup.1]: mag. 5.1, 60.5V, 1,400 l-y

Extremely difficult. R.A. 5h 36m, Dec. -5.4[degrees]

Iota Orionis, sep. 8' (SW). In Orion's Sword,

Iota: mag. 2.8, O9111, 2,300 l-y

[SIGMA]747: mag. 4.7, 60.5V, 1,600 l-y

Moderately difficult. R.A. 5h 36m, Dec. -5.9[degrees]

The familiar Sword of Orion is a complex and fascinating area in binoculars and telescopes. But did you know that each of its three brightest stars is a challenging naked-eye pair?

The northern pair consists of 42 and 45 Orionis. They're embedded in a very faint nebula for telescopes under dark skies.

The toughest is [Theta.sup.1] and [Theta.sup.2] in the brightest part of the Great Orion Nebula; they're the closest pair in this entire list, very faint, and they contend with the nebulosity and other stars in the immediate area. I've never been able to split them, but it might just be possible for someone with exceptional eyesight and observing skills.

At the south end of Orion's Sword is the least difficult pair: bright Iota Orionis and its neighbor Struve 747. The latter is a lovely binocular double, magnitudes 4.8 and 5.7, separation 0.6'.

Zeta Corvi, separation 6' (WNW)

Zeta: mag. 5.2, 68V, 420 l-y

HD 107295: mag. 5.9, K0III, 400 l-y

Moderately difficult. R.A. 12h 21m, Dec. -22.3[degrees]

Jumping forward into the spring sky, Zeta Corvi and its companion lie within the four-star "sail" pattern of Corvus. This constellation is never very high, and the Zeta double is a challenge for its faintness.

I often imagine myself sharing the sky with our pre-technological ancestors, who only had their unaided eyes with which to explore the stars and find these pairings. I imagine that they must have found this pursuit fascinating too.

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--1/2 or--34 diopter to your regular prescription for each eye. For many people, this offsets 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-reflection coated eyeglass lenses; they transmit more light and scatter less.

* Observe often to gain experience. You'll be surprised at the difference this makes.

Jerry Lodriguss is an author, professional photographer, and educator who writes about and teaches astrophotography. For more of his images of the double stars in this article, see
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Title Annotation:Spottable Pairs
Author:Lodriguss, Jerry
Publication:Sky & Telescope
Geographic Code:1USA
Date:Sep 1, 2014
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