Beta Monocerotis: A Delightful Triple Star

Beta Monocerotis is a unique-looking multiple star system in the constellation Monoceros. It’s best viewed in winter.

To naked eye observers, Beta Monocerotis will appear as the brightest star in the Monoceros constellation. Upon viewing the object through the telescope, one will make out three pale blue stars in a unique, attractive pattern that features two close stars with a third star orbiting nearby. The stars (referred to as Beta Mon A, B, and C) make for an undoubtedly attractive trio in the eyepiece. Of interest, it’s believed that the two closest stars that look like a pair (B&C) orbit each other, while Beta Mon A orbits those two.

In 1781, famed astronomer William Herschel described this trio as “one of the most beautiful sights in the heavens.” I believe part of what makes it an attractive object is all three stars are nearly identical in mass and all are hot, blue-white stars. See Jim Kaler’s description to learn more about this.

My Observations

I’m struggling at the moment to draw these three stars as the dew has made my iPad screen all wet. And the seeing conditions are getting poorer as a light haze is noticeable around the streetlight in the distance. All this aside, this is a great multiple-star target, and I can’t wait to visit it again. The colors are perhaps a pale blue, and it looks like a small double star pair with a bigger, brighter friend hanging on.

Key Stats

Constellation	Monoceros
Best Viewing	Winter / Early Spring
Visual Magnitude	+4.6, +5.4, +5.6
Absolute Visual Magnitude	-1.24
Distance from Earth	680 ly
Separation	2.6″ | 539 AU
Orbital Period	Unknown
Milky Way Location	Orion Spur
My Viewing Grade	A

Sources and Notes

The banner art is in the public domain in the United States because it was solely created by NASA. It is not a photo of Beta Monocerotis, but an artist rendition of new stars by NASA. This image or video was cataloged by Jet Propulsion Laboratory of the United States National Aeronautics and Space Administration (NASA) under Photo ID: PIA03520.

Sketch by Wayne McGraw

AKA: SAO 133317, 11 Mon