NGC6888 - The Crescent Nebula (Peter Hannah)

NGC6888 - The Crescent Nebula

NGC6888 is an emission nebula about 7000 light years away in Cygnus, near the star Sadr which marks the centre of the Northern Cross asterism.The nebula is produced by the star at its centre, WR136. This is a Wolf-Rayet star, which are among the most massive and hottest in the universe. They are characterised by an absence of hydrogen in their spectra, this being the result of their outer envelopes having been stripped of hydrogen by the intense stellar winds these stars generate.WR136 is about 21 times as massive as our Sun and 600,000 times as luminous, with a surface temperature of about 70,000K. Such stars live fast and die young, because the temperature and pressure in their cores burns through their nuclear fuel very quickly.It is believed that WR136 ejected a shell of material about 250,000 years ago when it became a red supergiant, and this shell is expanding at about 80 km/s. At the time, the star was still burning hydrogen but since then it has started helium fusion and again, because of its extreme mass, this is proceeding at a prodigious rate.The intense stellar wind now streaming away from the star is travelling at some 1,700 km/s and is catching up with the older shell and shaping it into the structure seen in the image. The collision of the two shells of material is generating shockwaves moving both outward and inward, and these shockwaves – along with the ultraviolet radiation now being emitted from the star’s surface – are energizing the nebula and causing it to glow.This is a bi-colour image using Hydrogen-alpha and Oxygen-III narrowband filters. The H-α is mapped to the red channel and the OIII to the blue and green.Outside of the nebula other clouds are emitting H-α and OIII light as the nebula lies in a very rich part of the milky way.

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Distribution of astrophotographers

This table shows the number of distinct astrophotographers who have been assigned and IOTD/TP award compared to the total number of images awarded.
	Distinct awarded users	Total awarded images
Image of the day
Top picks
Top pick nominations

Distribution of subject types

This table shows the distribution of awards per subject type, compared to the popularity of images of that subject type.
	Image of the day	Top pick	Top pick nominations	Total submitted
Deep sky
Solar system
Extremely wide field
Star trails
Northern lights
Noctilucent clouds
Landscape

Distribution of data sources

This table shows the distribution of awards per data source, compared to the popularity of images acquired from that data source.
	Image of the day	Top pick	Top pick nominations	Total submitted
Backyard
Traveller
Own remote observatory
Amateur hosting facility
Public amaeteur data
Professional, scientific grade data
Mix of multiple sources
Other
Unknown

The “AstroBin Image of the Day and Top Picks”, or IOTD/TP, is a long-running system to promote beautiful, interesting, peculiar, or otherwise amazing astrophotographs, with a focus on technical excellence. Learn more.

Submitted The date and time when you submitted this image for IOTD/TP consideration.	...
Views by Submitters (available since September 19th, 2023) Every image is assigned to 50% of available Submitters. In the event that at least 80% of them don't view the image before its time in the IOTD/TP process expires, it's assigned to the other 50% of Submitters and the process begins anew.
Promotions by Submitters When 3 distinct Submitters promote the image, it moves on to the next stage of the process: evaluation for Top Pick status. This requirement, in addition to anonymization of images and distribution to only a subset of them, prevents biases and ensures that the best images are selected.
Promotions by Reviewers When 3 distinct Reviewers promote the image, it moves on to the next stage of the process: evaluation for IOTD status.
Early dismissal Staff members have a lot of images to inspect on a daily basis, and they can dismiss images if they believe they don't meet the requirements for IOTD/TP selection. If an image is dismissed 5 times, it's removed from the process. This streamlines the process and ensures that any bias present in promotions could be overruled by other staff members.

Status	Basic failed
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Astrometry.net job	7855043
PixInsight job	n/a
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