The Crescent Nebula (also known as NGC 6888, Caldwell 27, Sharpless 105) is an

emission nebula An emission nebula is a nebula formed of ionized gases that emit light of various wavelengths. The most common source of ionization is high-energy ultraviolet photons emitted from a nearby hot star. Among the several different types of emissio ...

in the constellation Cygnus, about 5000

light-year A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 101 ...

s away from

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surf ...

. It was discovered by

William Herschel Frederick William Herschel (; german: Friedrich Wilhelm Herschel; 15 November 1738 – 25 August 1822) was a German-born British astronomer and composer. He frequently collaborated with his younger sister and fellow astronomer Carolin ...

in 1792. It is formed by the fast

stellar wind A stellar wind is a flow of gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric. ...

from the Wolf-Rayet star

WR 136 WR may refer to: Arts and media * '' W.R.: Mysteries of the Organism'', a Serbian film, * Wikipedia Review, an Internet forum for the discussion of Wikimedia projects, particularly the English Wikipedia. *Willie Revillame, a Philippine TV host * ...

(HD 192163) colliding with and energizing the slower moving wind ejected by the star when it became a

red giant A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around or ...

around 250,000 to 400,000 years ago. The result of the collision is a shell and two

shock wave In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a me ...

s, one moving outward and one moving inward. The inward moving shock wave heats the stellar wind to

X-ray X-rays (or rarely, ''X-radiation'') are a form of high-energy electromagnetic radiation. In many languages, it is referred to as Röntgen radiation, after the German scientist Wilhelm Conrad Röntgen, who discovered it in 1895 and named it ' ...

-emitting temperatures. It is a rather faint object located about 2 degrees SW of

Sadr The Sahrawi Arab Democratic Republic (; SADR; also romanized with Saharawi; ar, الجمهورية العربية الصحراوية الديمقراطية ' es, República Árabe Saharaui Democrática), also known as Western Sahara, is a ...

. For most telescopes it requires a UHC or OIII filter to see. Under favorable circumstances a telescope as small as 8 cm (with filter) can see its nebulosity. Larger telescopes (20 cm or more) reveal the crescent or a

Euro The euro (symbol: €; code: EUR) is the official currency of 19 out of the member states of the European Union (EU). This group of states is known as the eurozone or, officially, the euro area, and includes about 340 million citizens . ...

sign shape which makes some to call it the "Euro sign nebula". File:NGC 6888 HOO By Don Christopher Deaver.jpg, alt=Crescent Nebula NGC 68888 Imaged by Don Christopher Deaver, NGC 6888 in Hydrogen and Oxygen Light. HOO Pallet. Imaged by Don Christopher Deaver File:Crescent Nebula, Hydrogen alpha and Oxygen III by Stephan Hamel.jpg, Crescent Nebula in H-alpha and OIII. Imaged with a 715mm focal length telescope. The hydrogen gas is red. The oxygen is blue. File:Crescent Nebula in H-alpha.jpg, Image of

NGC 6888 The Crescent Nebula (also known as NGC 6888, Caldwell 27, Sharpless 105) is an emission nebula in the constellation Cygnus, about 5000 light-years away from Earth. It was discovered by William Herschel in 1792. It is formed by the fast stellar ...

using H-alpha filter. File:NGC 6888 (Crescent Nebula) bicolor.jpg, Picture of

captured in narrowband by amateur astronomer Luca Moretti File:NGC 6888, the Crescent Nebula in Cygnus, imaged by amateur astronomer Patrick Hsieh.jpg, NGC 6888 imaged in 3 nm hydrogen-alpha and oxygen-III narrowband at 2800mm focal length by amateur astronomer Patrick Hsieh.

References

External links

* * * * {{Portal bar, Stars, Spaceflight, Outer space, Solar System NGC objects Wolf–Rayet nebulae Cygnus (constellation) Sharpless objects 027b ?

See also

References

External links