Equuleus, the Constellation Nobody Knows

Often passed over in favor of showier sights, the constellation of the Little Horse has charms of its own. Let's saddle up and go for a ride.

Horses head to head

Equuleus and Pegasus look like they're in a horse race in this mythological depiction. Equuleus is the "Little Horse" and though faint, it's home to several attractive double stars and other sights worth seeking.
Sidney Hall / Urania's Mirror

When was the last time you dropped by Equuleus? Been a while? I can't blame you — at first glance there's nothing here. A few faint stars to pass over en route to the globular cluster M15 or the riches of the Aquila Milky Way.

Equuleus (ee-KWOO-lee-us) doesn't have a bright star to its name. Alpha (α) Equulei (Kitalpha) glimmers at magnitude 3.9, while the other three suns that make up the skewed parallelogram hover around 5th magnitude. The group represents a little horse and is the second smallest of the modern constellations after the Southern Cross; it's also one of the original 48 cataloged by the ancient Greek astronomer Ptolemy. In this regard at least it has more cachet than Sculptor, Scutum, Lynx, and Lacerta, all 17th-century latecomers.

But Equuleus is no one-trick pony. Within the constellation's 10° × 7° confines you'll find a full evening's worth of observing from double stars to galaxies to asterisms for telescopes of all sizes. Earlier this month, I paid a visit to the Little Horse while attending the annual Northern Night Star Fest put on by the Minnesota Astronomical Society. The evening was cool, breezy and damp with temperatures dipping into the upper 40s. After a sticky summer, many of us welcomed the fall-like weather.

Just horsing around

Equuleus is tucked between Pegasus, the Flying Horse, and Delphinus about a quarter of the way from Enif, the horse's nose, to bright Altair in Aquila. The constellation stands due south around 11 p.m. local time in mid-September.
Stellarium

Equuleus is easy to find despite its lack of bright stars, courtesy of its brighter neighbors. The 5°-tall figure is located just 7° (three fingers) due west of Enif, the 2nd-magnitude star that marks the nose of Pegasus, the Flying Horse, and two fists to the left or east of Altair, the bright star at the bottom of the Summer Triangle. I dropped anchor at the bright "pair" of stars, Delta (δ) and Gamma (γ) Equulei, spaced just 1° apart. Both happen to be double stars.

Doubling Down

Delta is a wide pair and probably not a true double but rather two stars viewed along the same line of sight, called an optical double. Delta's "companion" shines at a feeble magnitude 10.2, 77″ due north of the primary star. Delta also has another much closer magnitude-5.5 companion with an orbital period of just 5.7 years. When farthest separated, they're only 0.3″ apart, making it a test for telescopes 16-inches and up. The Washington Double Star Catalog (WDS) gives a separation of 0.3″ in 2017, the most recent listing.

Separation and position angle diagram

The two measures of a double star are separation and position angle (PA). The directions shown here are for an inverting scope, such as a Newtonian reflector.

An easy 1° hop to the west of Delta takes you to 5th-magnitude Gamma Equulei, which also has a close companion (magnitude 8.7) at a distance that varies from about 2″ to 0.6″. Currently (2014) at 0.6" in PA 248°, the squeaky tight separation and magnitude difference make it a great challenge. I could detect no trace of elongation in my 10-inch Dob at 317×. I had better luck with Σ2742, a delicate, equal pair (magsnitues 7.4, 7.6) separated by 2.9″ in PA 215°. 64× almost split them, but 142× proved a far better ax for this pretty pair of pearl earrings.

S781, 2° north of Alpha (α) Equulei, is another nice, equal double, but it's wide enough to drive a truck through with a separation of 186″ in PA 172°. Both magnitude-7.2 stars appeared white through my 15-inch, but to be honest all I needed was binoculars on this one! The "S" in the star's name refers to 19th-century astronomer James South who discovered the pair in 1824.

From here we slide 5° to the southwest to visit the constellation's finest sight, Epsilon (ε) Equulei, a colorful double star ideal for low magnification and modest telescopes.

Equine excellence

Epsilon Equulei is one of the constellation's most eye-catching double stars.
Bob King

The orange-red primary (magnitude 5.3) is accompanied by an off-white (some see blue) secondary 10.5″ in PA 67°. The WDS also lists a 13th-magnitude "C" companion 69″ to the west of the closer pair. I easily spotted this faint point of light using 142× on the 10-inch. The primary also has an extremely close magnitude-6 companion just 0.18″ distant. Come 2090, when its separation widens to 1″, Epsilon will become a stunning triple star. If you ignore everything else in Equuleus, don't pass up this double.

By any indication, I should have split β163, a magnitude-7.3 star with a magnitude-8.8 companion 0.9″ away in PA 258°, but it wouldn't yield on the first try. So I returned several nights later armed with 317× and finally pried it apart. So-so seeing and the relatively large magnitude difference between primary and secondary made this the most challenging of our featured doubles.

The "β" in the star's name refers to the massive catalog of double stars plucked from the sky by the late-19th–early-20th-century falcon-eyed superstar S.W. Burnham. It should be resolvable in a 6-inch scope, so I'd love to hear back if you crack it. Σ2786 was another sweet, snug pair of suns of unequal magnitudes (7.5, 8.2) separated by 3″ in PA 189°, near the limit using 64× but perfect at 142x.

Galactiphilia

Stunning spiral

NGC 7015 is a "bright" spiral galaxy located 203 million light-years from Earth in Equuleus. 
Göran Nilsson & The Liverpool Telescope / CC BY-SA 4.0

If you like faint fuzzies, you'll love Equuleus. Several dozen lie singly or in loose groups across the constellation. All are fainter than 13th magnitude and make great challenge objects for medium to larger instruments. I dug around with the 15-inch and observed four of the brighter ones, including NGC 7015,the  brightest of the bunch at magnitude 13.2. It was faintly visible in my 15-inch reflector at 64×.

I discerned a fuzzy, round patch with a brighter, nonstellar core. About 15′ to its northwest lies IC 5083, a small but compact and relatively bright blot with a faint stellar nucleus.

Near Equuleus' southern border you'll find a galactic trio — NGC 7046, IC 1364, and IC 1365. The last remained elusive, but I discerned IC 1364's elongated shape and stellar nucleus. NGC 7046 was very dim and diffuse and only visible with averted vision. I included these and several additional galaxies on the finder map (below), but if you'd like to track down more, the free Stellarium program plots a dozen and programs like MegaStar many more.

Asterisms

Club member Jerry Jones was kind enough to loan me his Interstellarum Deep Sky Atlas which plots more than 500 telescopic asterisms — striking patterns of unrelated stars spotted by amateur astronomers during routine sweeps of the sky. Equuleus has three, which help to bulk up a constellation that lacks even a single open cluster.

Let's get there!

Use this map to help you find your way to Equuleus's deep-sky wonders. Stars are shown to magnitude 9 (fainter near galaxies) and north is up. Double stars have a line through them. Click for a large version you can print for use outdoors.
Stellarium with additions by the author

When I first saw the alphabet soup designation LeWa J2108.8+0621 on the map, I thought I'd need a radio telescope to pull it out. Instead I was pleasantly surprised. Here was a delightful faux-cluster 22′ × 8′ across in the shape of a spoon. It was discovered by Davy Levy and his wife Wendee Wallach Levy. Next up was Alessi J2108.8+0621 (Brazilian amateur Bruno Alessi), a loose spread of bright field stars half-a-degree wide arranged in two parallel lines. I wrapped up with O'Neal 1 (California amateur Mike O'Neal), an attractive geometric arrangement of six 7th-magnitude stars spanning 45′.

If it's possible to get lost in such a small constellation, that's what it felt like after spending an 1-1/2 hours of pure exploration within its tidy boundaries. Sure, the bigger horse, Pegasus, gets most of the attention every fall, but neighboring Equuleus will forever be a nose ahead, the reason it's sometimes called Equus Primus, or First Horse. Giddy-up!

8 thoughts on “Equuleus, the Constellation Nobody Knows

      1. Tom-Reiland

        I make mistakes like that one on occasion. I guess it happens when you’re typing and your fingers don’t do what your mind wants them to do. I’m looking forward to some clear weather, one of these months, so that I can check out as many objects on your list as possible. Very nice article.

  1. Fabrice MoratFabrice Morat

    Bob,
    Your fine report about Equuleus is certainly a rarity. For many years, i collect images, drawings with observational notes on many deep sky objects and i store them by constellation in plastic sleeves. But my Equ holder is definitely empty, the only one among constellations delta <-52° with Antlia, the air pump. Even Columba, microscopium or Pyxis have more notes !
    My only own record is a drawing of the little horse with naked eye and there was M15 not too far. In your quadrilateral (the chart above), i had seen S781 like an unique star, 9 Equ and that'all !
    So, congratulations for your telescopic descriptions…
    Fabrice M.

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