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Crescent Nebula
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NGC 6888 (also the Crescent Nebula, Caldwell 27 and Sharpless 105) is a magnitude +7.4 HII emission nebula located 5,000 light-years away in the constellation Cygnus.

The nebula was discovered by German-British astronomer William Herschel using a 47.5 cm (18.7 inch) f/13 speculum reflector at Windsor Road in Slough, Berkshire on the 15th September 1792.

Right Ascension 20h 12m 06.5s, Declination +38° 21' 18"

It is formed by the fast stellar wind from the Wolf-Rayet star WR 136 (HD 192163) colliding with and energizing the slower moving wind ejected by the star when it became a red giant around 250,000 to 400,000 years ago. 
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NGC 6888

 



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Title: Ionisation structure and chemical abundances of the Wolf-Rayet nebula NGC6888 with integral field spectroscopy
Authors: A. Fernández-Martín, D. Martín-Gordón, J.M. Vílchez, E. Pérez Montero, A. Riera, S. F. Sánchez

This work aims to search for the observational footprints of the interactions between the interstellar medium (ISM) and stellar winds in the Wolf-Rayet (WR) nebula NGC6888 in order to understand its ionisation structure, chemical composition, and kinematics. We have collected a set of integral field spectroscopy observations across NGC6888, obtained with PPAK in the optical range performing both 2D and 1D analyses. Attending to the 2D analysis in the northeast part of NGC6888, we have generated maps of the extinction structure and electron density. We produced statistical frequency distributions of the radial velocity and diagnostic diagrams. We have found that the spectra of a localised region to the southwest of this pointing can be represented well by shock models. Furthermore, we performed a thorough study of integrated spectra in nine regions over the whole nebula. We derived electron densities ranging from <100 to 360 cm^(-3). The electron temperature varies from ~7700 K to ~10200 K. A strong variation of up to a factor 10 between different regions in the nitrogen abundance has been found: N/H appears lower than the solar abundance in those positions observed at the edges and very enhanced in the observed inner parts. Oxygen appears slightly underabundant with respect to solar value, whereas the helium abundance is found to be above it. Finally, we provide a scenario for the evolution of NGC6888 to explain the features observed. This scheme consists of a structure of multiple shells.

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Title: Suzaku Observations of the Prototype Wind-Blown Bubble NGC 6888
Authors: Svetozar A. Zhekov, Sangwook Park

We present an analysis of the Suzaku observations of the prototype wind-blown bubble NGC 6888 which is based both on use of standard spectral models and on a direct comparison of theoretical models with observations. The X-ray spectra of NGC 6888 are soft and most of the X-rays are in the (0.3 - 1.5 keV) energy range. But, hard X-rays (1.5 - 4.0 keV) are also detected (~10% of the observed flux). The corresponding spectral fits require a relatively cool plasma with kT < 0.5 keV but much hotter plasma with temperature kT > 2.0 keV is needed to match the observed hard X-ray emission. We find no appreciable temperature variations within the hot bubble in NGC 6888. The derived abundances (N, O, Ne) are consistent with those of the optical nebula. This indicates a common origin of the X-ray emitting gas and the outer cold shell: most of the X-ray plasma (having non-uniform spatial distribution: clumps) has flown into the hot bubble from the optical nebula. If the electron thermal conduction is efficient, this can naturally explain the relatively low plasma temperature of most of the X-ray emitting plasma. Alternatively, the hot bubble in NGC 6888 will be adiabatic and the cold clumps are heated up to X-ray temperatures likely by energy exchange between the heavy particles (hot ions diffusing into the cold clumps).

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Tricrescent_goldman_c50.jpg 
Credit Don Goldman

NGC 6888, also known as the Crescent Nebula, is a cosmic bubble about 25 light-years across, blown by winds from its central, bright, massive star. Near the centre of this widefield view of interstellar gas clouds and rich star fields of the constellation Cygnus, NGC 6888 is about 5,000 light-years away. The three colour composite image was created by stacking exposures through narrow band filters that transmit the light from atoms in the clouds. Hydrogen is shown as green, sulphur as red, and oxygen as blue. NGC 6888's central star is classified as a Wolf-Rayet star (WR 136) and is shedding its outer envelope in a strong stellar wind, ejecting the equivalent of our Sun's mass every 10,000 years. Burning fuel at a prodigious rate and near the end of its stellar life, this star should ultimately go out with a bang in a spectacular supernova explosion.



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