Sigma Orionis is a star viewable with the naked eye. It lies in the constellation Orion and sits near the left side of Orion’s belt, right under the star Alnitak.
Examining Sigma Orionis reveals a bright quartet of stars, with several fainter stars surrounding it. The brightest star in the pack is actually a pair of stars referred to as σ Ori AB. Notably, astronomers report that Star A is 18 times more massive than the Sun, while Star B is 14 times more massive than the Sun. This makes this binary pair one of the most massive visual binaries known!
While stars A & B orbit each other every 170 years, the other stars surrounding them are not in stable orbits. Instead, it’s believed these remaining stars in the pack will be gravitationally ejected out at some time, leaving Star A & B by themselves until one or both will eventually explode.
Historically, sky watchers have observed Sigma Orionis as a bright single star in Orion since ancient times. Then, using better optics in 1776, the astronomer Christian Mayer described σ Ori as a triple star, setting off future research in the 19th century when other astronomers discovered the additional stars.
The constellation Orion is already a favorite target for amateur astronomers because of the famous Orion Nebula, but observers shouldn’t miss the many wonderful objects in the hunter, including Sigma Orionis and its multiple stars.
My Observations
| Date | December 31, 2021 |
| Time | 8:50 p.m. |
| Location | Seattle, WA |
| Magnification | 169x |
| Scope | Meade 8″ SCT |
| Eyepiece | 12mm |
| Seeing | Average |
| Transparency | Poor |
On this cold, snow-covered night, I’m navigating my way through Orion. As I do so, my mind wanders. I think of 2021 and all that has happened. When 2021 started, we were hopeful the end of the pandemic was in sight. However, unfortunately, a new version of the virus (called Omicron) appears to be spreading rapidly everywhere.
Sigma Orionis is a nice target, especially for those interested in binary and multiple star systems, and I agree with the assessment made by my star app that there are few multiple stars more attractive than σ Orionis.
Key Stats
| Constellation | Orion |
| Best Viewing | Winter |
| Visual Magnitude | +3.8, +8.4 |
| Absolute Visual Magnitude | +0.8 |
| Distance from Earth | 1100 ly |
| Separation | 0.3″ | 85 AU |
| Milky Way Location | Orion Spur |
| My Viewing Grade | A |
| Designations | SAO 132406 |
Sources and Notes
The banner photo was created by the European Southern Observatory (ESO) and is licensed under a Creative Commons Attribution 4.0 International License. ESO and Digitized Sky Survey 2. Acknowledgment: Davide De Martin. Link to photo.
Sketch by Wayne McGraw
The Garden Astronomer 