The Crescent Nebula

A bi-color image of NGC 6888, the Crescent Nebula, by Terry Hancock. The total integration time for this image was 18 hours.

A bi-color image of NGC 6888, the Crescent Nebula, by Terry Hancock. The total integration time for this image was 18 hours.

The Crescent Nebula, located near the middle star that marks the heart of the constellation Cygnus, is a complex arc of gas that’s powered by the machinations of a massive dying star. Called WR 136, this star is just 4-5 million years old, but it’s big enough to have quickly burned through its store of fuel in its core and has now entered a stage where it sheds mass from its outer layer at a prodigious rate, nearly one full solar mass every 10,000 years. This fast-moving hot gas, which moves at a speed of 2,000-3,000 km/s, collides with cooler gas ejected by the star during its quieter days, and the collision excites the gas to emit light. Massive and mass-losing stars like WR 136 are called Wolf-Rayet stars. There are only about 150 such stars known in the Milky Way.

WR 136 will eventually detonate as a supernova when it finally runs of fuel and collapses. The explosion will obliterate the nebula and most other material within several dozen light years of the star. In its place will be a new type of nebula called a “supernova remnant” that consists of a rapidly expanding shock wave that excite the scant atoms lingering in the interstellar medium.

The location of the Crescent Nebula near the star Sadr in the constellation Cygnus.

The location of the Crescent Nebula near the star Sadr in the constellation Cygnus.

The Crescent Nebula is spread out over about 25 light years and lies at a distant 4,700 light years. It’s sometimes called the “Euro” nebula because of its resemblance to the symbol for the currency.

Because of its size and great distance, it’s not the easiest object to see. You’ll need a telescope of 4″ to 6″ aperture and very dark and clear sky. Look for the nebula just 2.7º southwest of the star Sadr (γ Cygni), which itself anchors a tremendously rich complex of nebulosity. The Crescent Nebula spans about 1/3 of a degree in its longest dimension.

(Image above courtesy of Terry Hancock at the Downunder Observatory).

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