Epsilon Equulei (Struve 2737): A Quaint Star System of Yellow and Violet in Equuleus

The charming and often overlooked constellation called Equuleus, which means “little horse” in Latin, is a small constellation, but it has some intriguing celestial objects. In fact, I hate to admit it. I overlooked Equuleus for many years as an observer, often flying past the region on my way to Pegasus. This time around, I took a closer look and landed on the delicate magnitude 5 triple star Epsilon Equulei, which shines down from this region of the sky on late summer and early autumn nights.

The two bright stars that make Epsilon Equulei, A & B, are so close together it’s nearly impossible to separate them. Some observers report seeing the star “elongated,” though. These two stars are both spectral type F, which explains the white-yellow color they combine to make. The orbital period for these two stars is 101 years, which you can see mapped out on Jim Kaler’s awesome site.

The third companion, C, is situated at a distance of 10 arcseconds from A & B. It’s a white-yellow F-type star. However, due to the close proximity of AB and C, some observers (including myself) have reported that star C appears more blue or violet in color. This optical illusion enhances the overall appearance of this triple system, making it visually appealing.

However, there is more to the story. A fourth companion, D, can be found around 80 arcseconds away with a magnitude of 13. Although I perceived its existence using averted vision, I was uncertain if I was truly observing it during my evening of stargazing. I suggest reading John Nanson’s take on Star Splitters for more on star D.

Historically speaking, the astronomer William Herschel first reported this star system on August 2, 1780. Later on, in the 19th century, astronomers Smythe and Chambers described Epsilon Equulei as “a delicate triple star… A white, B lilac…with a bright star following at a distance”.¹

My Observations

Even with some high clouds moving in overhead, I’m able to pick out this new star. The primary is pale yellow in color, while the close companion, star C, appears blue at first, but more violet as I stare longer. I read earlier about star D, and while there are some faint stars in the eyepiece, I’m unable to locate this magnitude 13 star. As I use averted vision, it seems a very faint point blinks in and out of view. I’m not sure if that’s my mind playing tricks or if I’m seeing D. If only I had darker skies, I could confirm my sighting.

Key Stats

Constellation	Equuleus
Best Viewing	Autumn
Visual Magnitude	+5.3, +7
Separation	10.5″
Position Angle (between AB & C)	67°
Spectral Class	AB=F, C=F
Absolute Visual Magnitude	+3.38
Distance from Earth	180 ly
Milky Way Location	Orion Spur
My Viewing Grade	A-
Designations	1 Equ, STF 2737, SAO 126428, ε Equ, 1 Equulei, HD 199766, HIP 103569, HR 8034, BD +03° 4473

Sources and Notes

The banner illustration was created by the European Southern Observatory (ESO) and is under the Creative Commons Attribution 4.0 International License. Per ESO agreement: Here is the original image on the ESO website.

¹ Kaler, J. (2011, September 14). Epsilon Equulei. http://stars.astro.illinois.edu/sow/epsequ.html

Sketch by Wayne McGraw