From the January 2022 issue

101 Must-See Cosmic Objects: The Omega Nebula

By | Published: May 23, 2022 | Last updated on August 30, 2023
Rodney Pommier

In 1764, Swiss astronomer Philippe Loys de Chéseaux discovered the beautiful Omega Nebula (M17) in Sagittarius. He described it as having “the shape of a ray, or of a comet tail. … Its sides are exactly parallel and quite well defined, much like its ends.”

The object’s nickname, however, didn’t come until John Herschel jotted down his description of it in 1833. Specifically, Herschel wrote that “its form is that of a Greek Omega with the left base-line turned upwards.” Herschel later mentioned that it also reminded him of a horseshoe. Others see it as a swan, a checkmark, and even the number 2 with an extended base.

No matter what you call it, M17 is bright enough that it is easily visible with binoculars. Through a 4-inch telescope, the checkmark or number 2 shape is readily seen. But it will take at least an 8- or 10-inch scope to make out the full bow of the omega or horseshoe. The brighter portions along the nape of the swan’s neck and extended body show a richness in textural detail unparalleled in most nebulae. A narrowband light-pollution or oxygen-III (OIII) filter will greatly improve the view of these textures.

The Omega Nebula is a massive HII region marking the tip of an interstellar iceberg: a giant molecular cloud inside one of our galaxy’s spiral arms. Waves of star formation have produced a packed star cluster hidden within the swan’s neck. Known as NGC 6618, this cluster holds an estimated 10,000 stellar infants, which astronomers believe are no more than about a million years old. Due to opaque cosmic dust, however, the cluster is only readily visible at infrared wavelengths.

Residing along our galaxy’s Sagittarius spiral arm near the Omega Nebula is the Eagle Nebula. At 5,600 light-years away, the Eagle is the closer of the two; Omega is located some 400 light-years farther away. Estimates tell us that the Omega, at 70 light-years across, is roughly twice the size of the Eagle.

Make sure to explore Astronomy’s full list of 101 cosmic objects you must see. New entries will be added each week throughout 2022.

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