Abell 21 (Gary Imm)

Celestial hemisphere: Northern · Constellation: Gemini (Gem)

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Abell 21

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Abell 21

This object, also known as Sh2-274, is a large planetary nebula located 1500 light years away in the constellation of Gemini. It was discovered in 1955 by Dr. George Abell who classified it as the 21st item in his catalog. The nebula spans a distance of 4 light years. It is nickcamed Medusa because the arcing gas filaments remind some of the living venomous snakes that take the place of hair for the female monster of ancient Greek mythology.

In some images of this object, the arcing gas fronts look like this could be a supernova remnant instead of a planetary nebula. In fact, this object was thought to be a SNR until the early 1970s. But in this image the gas fronts do not look as crisp and detailed as those of a typical SNR, so it looks more like a planetary nebula to me, albeit a very unsymmetrical one.

Strong interstellar winds are likely responsible for the unusual shape. Faint arcs are visible on the lower right of the image, mirroring the brighter arcs at upper left. Because of its large size (10 arc-minutes in apparent diameter), its surface brightness is very low and this is a tough object to see visually.

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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
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Extremely wide field
Star trails
Northern lights
Noctilucent clouds
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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
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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	Advanced failed
Started	...
Astrometry.net job	2606510
PixInsight job	HI93N70AE7O9TMCGO7RFK4WYWVLVD3XA
PixInsight queue size	n/a
PixInsight stage	PLATE_SOLVING_IMAGE

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Abell 21

Abell 21

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Abell 21

Abell 21

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