The Crescent Nebula
Posted by Brian Ventrudo
The constellation Cygnus wheels into view in these early northern-summer months, and stargazers all over the world can inspect dozens of fine sights in this large and famous constellation. An overlooked sight in Cygnus is the splendid Crescent Nebula (NGC 6888), an arc of gas that’s excited by the machinations of a massive, evolved star. Here’s what the Crescent Nebula looks like, and here’s how to find it in a backyard telescope.
All bright nebulae need an energy source to shine. The Crescent Nebula gets its power from a Wolf-Rayet star, an evolved massive star that sprays its outer layers into space at 2,000-3,000 km/s. This fast-moving hot gas collides with cooler gas ejected by the star during its quieter days, and the collision excites the gas to emit light. The star at the centre of the Crescent Nebula sheds mass at a tremendous rate, close to a full solar mass in just 10,000 years.
The nebula is spread out and lies at a distant 4,700 light years. So it’s not the easiest object to see. You’ll need a telescope of 6-8 inches aperture and very dark and clear sky. Look for the nebula just 2.7 degrees southwest of the star Sadr (gamma Cygni), which itself anchors a tremendously rich complex of nebulosity. The Crescent Nebula spans about 1/3 of a degree in its longest dimension. The above image gives you an idea of what you’ll see in an 8-10 inch scope at about 100x. The blazing Wolf-Rayet star that drives the nebula is just right and above the centre of this image.
In a larger scope, you may see visual hints of the Crescent closing back on itself. Longer exposures in large telescopes reveal the full optical extent of this striking object.
Even if you miss the nebula, take time to enjoy the starry fields in this fine region of the heavens. Even a pair of binoculars will show you thousands of stars along the length of Cygnus, along with hints of nebulosity both bright and dark. The large cross-shape of Cygnus rises high in the eastern sky in early summer near midnight, and can be seen well over the northern horizon from the southern hemisphere.