Messier 17, NGC 6618 - Omega Nebula, Swan Nebula

Messier 17 in Sagittarius is one of the brightest diffuse nebulae in the sky. Commonly known as the Omega, Swan, Horseshoe, or (especially in the southern hemisphere) Lobster nebula, M 17 is just visible to the naked eye under very favorable conditions.

Discovery and Observation

M 17 was first noticed by Philippe Loys de Cheseaux in the spring of 1746, and rediscovered independently by Charles Messier in June of the same year. De Cheseaux's discovery did not become widely known, so Messier cataloged it as M 17 in 1764.

M 17 is located in the rich star fields of the the Milky Way in Sagittarius. It can be found easily, along with its apparent neighbor, M 16, the Eagle Nebula. Starting from Altair (α Aql), follow δ Aql and λ Aql to γ Sct; M 17 is just over 2° SW of this star, and M 16 lies 2° NW. With a visual magnitude of 6.0, M 17 is visible to the naked eye from low latitudes under good observing conditions.

In a small telescope, M 17 rivals the Orion Nebula (M 42) in splendor. The core of the nebula is a checkmark, the bar of which extends 12' ESE-WNW. The check projects SSW from the WNW end of the bar. Fainter nebulosity loops west from the check, enclosing a dark mass popularly called the Fish's Mouth. The involved cluster is poorly concentrated, its stars spread over a 25' area.

The overall color of the Omega Nebula is reddish, due to light emitted from the hot hydrogen gas excited by the stars that have just formed within it. However, the nebula's brightest region is actually white, apparently the result of light emission from the hottest gas, mixed with reflections of bright starlight from dust. The nebula contains a large amount of dark obscuring material, which has been heated by the hidden young stars, and shines brightly in the infra-red.

Properties and Evolution

Distance estimates for M 17 range between 5,000 and 6,000 light years - a little less than its apparent neighbor, M 16. Physically, these two star forming regions are indeed close together, in the Sagittarius-Carina spiral arm of the Milky Way galaxy, and are perhaps part of the same giant complex of interstellar matter.

While the visible nebula is about 15 light-years across, the total gaseous cloud, including low-luminosity material, extends to at least 40 light-years. The total mass of the gas has been estimated at about 800 Suns: enough to form a conspicuous cluster, and a good deal more than the Orion Nebula (M 42).

Some 35 bright but obscured stars, each about six times hotter and 20 to 30 times more massive than the Sun, are embedded in the nebulosity. Unlike in many other emission nebulae, however, these stars are not obvious in optical images, but hidden in the nebula. Star formation is either still active in this nebula, or ceased very recently.

The radiation from these stars erodes the dense cloud of cold gas within which they were formed, exposing dense pockets of gas that may contain other stars in the birthing process. Because these dense pockets are more resistant to the radiation onslaught than is the surrounding cloud, they appear as sculptures in the walls of the cloud, or as isolated islands in a sea of glowing gas. The pressure on the tips of the waves may trigger new star formation within them.