Winter's Finest Triple Stars

By: Bob King February 14, 2024 4

Double stars are wonderful, but triples are terrific! Here are 17 trios to enjoy the next clear night.

Beta Mon and Zeta Cnc sketches — Beta (β) Monocerotis and Zeta (ζ) Cancri are two of the season's most scintillating triple stars. An 80-mm scope will reveal the former while a 6-inch the latter. North is up in these sketches made with my 15-inch Dobsonian at magnifications of 142× (left) and 286×(right).
*Bob King*

If you had to name one triple star of the winter sky it might be Beta (β) Monocerotis, a trio of hot B-type stars shaped like a series of knots in a kite's tail. Few threesomes are as bright and exquisite as this gem. William Herschel called it "one of the most beautiful sights in the heavens." Every year it's on my list of must-see seasonal highlights.

Double stars are gorgeous enough, but adding a third star to the mix deepens the visual thrill while also making us wonder exactly who is orbiting whom. While triples are less common than doubles their members continue to orbit the system's center of mass. Often, two of the stars will form a closer, orbiting pair circled at a distance by the third member, a stable arrangement called hierarchical. Beta Monocerotis is likely a hierarchical system with the bright A star in a stable orbit around the tighter B-C pair.

Not all multiple star systems are so neatly nested. In trapezia — multiple star systems named for the Trapezium in the Orion Nebula — there is no hierarchy as stars compete for stable orbits. Such arrangements are inherently unstable until one or more stars are ejected, and the system relaxes into a binary or triple. Chaos in trapezia may give rise to a significant fraction of so-called runaway stars that bolt across the galaxy at hundreds of kilometers per second.

Triple star finder maps — These finder maps were created at different scales to provide context for locating each featured triple. North is up. Click here for a full-size version of the image.
*Stellarium with additions by Bob King*

I've prepared a list of selected wintertime triples that happily will stay put. Stellar components are designated A, B, and C in order of decreasing brightness and separations are in arcseconds. The position angle (PA) gives the offset of the secondary (B) or tertiary (C) component from the primary star (A) measured eastward from the North Celestial Pole — the same as azimuth on a compass. In cases where the B-C components form a tight pair the BC separation and PA are given.

Star	RA.	Dec.	Magnitudes	Separations	PA
Psi Cas	1^h 26^m	+68° 08′	4.7, 9.2, 10.0	AB=20.3″, BC=2.9″	AC=128°, BC=253°
Iota Cas	2^h 29^m	+67° 24′	4.7, 6.9, 9.1	AB=3.0″, AC=6.9″	AB=231°, AC=117°
Pi Ari	2^h 49^m	+17° 28′	5.3, 8.0, 10.7	AB=3.2″, AC=25.5″	AB=118°, AC=113°
STT 64 (Tau)	3^h 50^m	+23° 51′	6.8, 10.2, 10.5	AB=3.3″, AC=10.1″	AB=234°, AC=235°
HD 35162 (Lep)	5^h 22^m	–24° 46′	5.4, 6.6, 9.3	AB=3.5″, AC=59.8″	AB=91°, AC=104°
STF 734 (Ori)	5^h 33^m	–01° 43′	6.7, 8.2, 8.4	AB=1.6″, AC=29.2″	AB=355°, AC=243°
Lambda Ori	5^h 35^m	+09° 56′	3.5, 5.5, 10.7	AB=4.1″, AC=29.3″	AB=43°, AC=185°
Iota Ori	5^h 35^m	–05° 55′	2.8, 7.7, 9.8	AB=11.2″, AC=49.4″	AB=141°, AC=103°
STF 761 (Ori)	5^h 39^m	–02° 33′	7.9, 8.4, 8.6	AB=68.1″, BC=8.5″	AB= 202°, BC=269°
STF 780 (Cam)	5^h 51^m	+65° 45′	7.0, 8.2, 10.2	AB=4.0″, AC=12.5″	AB=105°, AC=148°
Beta Mon	6^h 29^m	–07° 02′	4.6, 5.0, 5.3	AB=7.2″, BC=2.6″	AB=133°, BC=112°
12 Lyn	6^h 46^m	+59° 27′	5.4, 6.0, 7.1	AB=1.9″, AC=8.9″	AB=65°, AC=309°
Bu 324 (CMa)	6^h 50^m	–24° 05′	6.6, 7.9, 8.3	AB=1.8″, AC=30.3″	AB=210°, AC=282°
Alpha Gem	7^h 35^m	+31° 53′	1.9, 3.0, 9.8	AB=5.6″, AC=71.6″	AB=51°, AC=164°
STF 1127 (Cam)	7^h 47^m	+64° 03′	7.0, 8.5, 9.7	AB=5.6″, AC=11.6″	AB=340°, AC=178°
Zeta Cnc	8^h 12^m	+17° 39′	5.3, 6.3, 5.9	AB=1.1″, AC=6.2″	AB=354°, AC=61°
STF 1291 (Cnc)	8^h 54^m	+30° 35′	6.1, 6.4, 9.2	AB=1.5″, AC=54.6″	AB=310°, AC=204°

All data are current and from Stelle Doppie, a searchable double- and multiple-star database sourced from the Washington Double Star Catalog. Note that several of our featured triples — or in some cases a single system member — may or may not be physically related. For additional triple stars check out Dave Cotterell's Best Triple Stars on Cloudy Nights.

Selected triple star impressions

Selected because my best efforts have met with frequent clouds. I look forward to the next clear night to complete the pleasant task. Hopefully, you'll have better luck!

Psi (ψ) Cassiopeiae — What a disparate trio! The bright primary is a succulent yellow-orange sun accompanied at a comfortable distance by a slightly unequal but considerably fainter tight pair. Magnification recommendation: 100× or higher.

Iota (ι) Cassiopeiae — True showpiece and Beta Mon's equal. Three stars arranged in a very compact, obtuse triangle each one two magnitudes fainter than the next. Best magnification: 175× or higher.

STT 64 Tauri — Tucked up within the Pleiades star cluster this looks like an unremarkable pair at first glance, but when I increased the magnification to 286× a third member appeared between the two components forming a nearly straight line. A delightful surprise. Best magnification: 150× or higher.

STF 734 Orionis — Located just below Orion's Belt this triple hosts a very close, unequal pair and a more distant C component. Best magnification: 200× or higher.

Lambda (λ) Orionis a.k.a. Meissa — Another close but easier-to-separate pair accompanied by a fainter, more distant member. The primary is an O8 giant star and emits so much ultraviolet (UV) light it's visually almost four magnitudes fainter than Rigel despite being less than twice as distant. Best magnification: 150× or higher

Iota (ι) Orionis — The easiest triple to find. Park the scope on the Orion Nebula and after you've feasted on nebulosity, slide to its southern edge and spent a few minutes with this scenic arc of three stars. Part of the triple's appeal is the large magnitude difference between the primary and its two companions. Best magnification: 50×-100×.

STF 761 Orionis — You're going to get an eyeful here because this triple (suggested by reader Dave Mitsky) lies just 3′ northwest of the bright quadruple star Sigma (σ) Orionis, located below Alnitak, the easternmost star in Orion's Belt. Sigma is a eye-catching conga line of four stars with separations from 11.4″ to 41.4″. The triple is right next store and consists of a neat, nearly-equal pair and more distant C-component. Both systems are a treat and make for easy observing in any telescope.

Beta (β) Monocerotis— This trio dazzles! They're all so shiny white and jammed in tight. I enjoy doubles and multiples at the lowest magnification possible in order to preserve their sharpness and sparkle. I can make out all three at 64×, but the split between A and B is cleaner at 142×. The stars' masses range from 6 to 9 times solar with luminosities of 3,200, 1,600, and 1,300 times that of the Sun, respectively. Best magnification: 100× and higher.

12 Lyncis— White, compact pair in cahoots with a pale-yellow secondary star. Very attractive trio. Best magnification: 150× and higher.

Alpha (α) Geminorum a.k.a Castor — A knockout system and public-star-party favorite. Even a 60-mm will cleave Castor into a pair of close-set, dazzling white stars. The fainter C component (also known as YY Gem) sits well off to the southeast and appears faintly red in my 15-inch. It's a spectroscopic binary comprising two red dwarfs. They orbit each other every 19.5 hours nearly edge-on from our perspective — when one eclipses the other the system's combined light drops by about half a magnitude.

Castor triple sketch — This sketch nicely captures the bright glowing Castor A-B pair and the fainter red dwarf. The triple star system is visible in telescopes as small as 3 inches. North is up, with east to the left.
*Roberto Mura, CC BY-SA 3.0*

Castor A and B likewise are tightly bound spectroscopic companions, making the whole a sextuplet system. The true separation of the bright pair averages 104 a.u.— roughly the distance to the outer edge of the Kuiper Belt — and they complete an orbit in about 460 years. Best magnification: 100× or higher.

Zeta (ζ) Cancri — Fantastic high-magnification trio. Seen as a double at 86×, the primary splits again into two tremulous dots of pale orange light at 286×. Resembles 12 Lyn but with the close pair yellow instead of white. One of my personal favorites in a dim constellation albeit one rich in double and multiple stars. Best magnification: 200× and higher.

STF 1291 Cancri — A close doublet joined by a more distance companion. Best magnification: 200× or higher.

One of the best things about observing triple stars (doubles, too!) is that they're arresting under less than ideal conditions whether that be broken cloud or bright moonlight. Keep these in your back pocket for those subpar evenings and I guarantee a perfect night.

Read more on current celestial events in the February 2024 issue of Sky & Telescope.

Comments

Dave Mitsky

February 15, 2024 at 10:06 pm

Bob,

I've observed a number of the triple stars in your article. Some of them I was not familiar with, so they'll be added to my list of objects to observe.

One excellent triple star that you didn't mention is Struve 761, which lies just northwest of the multiple star Sigma Orionis.

https://skyandtelescope.org/observing/celestial-objects-to-watch/orions-splendid-double-stars/

Dave

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Bob KingPost Author

February 17, 2024 at 11:49 am

Hi Dave,

Thanks for suggesting that one! I'll put that on my list to observe. (FYI -- I viewed this last night (Feb. 17, 2024). A very nice triple with an pretty close pair and a more distant C-component. Easy for small scopes and in fabulous company with the quadruple Sigma Orionis! I recalled observing both long ago but had forgotten all about them, so it was great to see them again. Of course, I added STF 761 to the list so other observers could enjoy it, too.)

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