| Abell 21, aka It is also known as Sharpless 274. It was originally discovered in 1955 by UCLA astronomer George O. Abell, who classified it as an old planetary nebula. The braided serpentine filaments of glowing gas suggests the serpent hair of Medusa found in ancient Greek mythology. Until the early 1970s, the Medusa was thought to be a supernova remnant. With the computation of expansion velocities and the thermal character of the radio emission, Soviet astronomers in 1971 concluded that it was most likely a planetary nebula. As the nebula is so big, its surface brightness is very low, with surface magnitudes of between +15.99 and +25 reported. Because of this most websites recommend at least an 8-inch (200 mm) telescope with an [O III] filter to find this object although probably possible to image with smaller apertures |
Abell 21, également catalogué PK205+14.1 et SH2-274 Nom commun : Medusa nebula Abell21 est une large nébuleuse planétaire localisée dans les Gémeaux, découverte par G. Abell en 1955. La taille importante de la nébuleuse lui confère une luminosité surfacique assez faible, la rendant plus difficile à photographier ou à observer. Filtres à bande étroite préférables. Magnitude: 11.30 Luminosité de surface: 16.40 Dimension: 600.0 x360.0 " |
 Composite color image R=H-alpha (3h00) G=synthetic B=OIII (3h15) Takahashi FSQ106ED refractor CCD camera Atik 4000 (Kodak KAI-4021M) @-20°C Astrodon 6nm bandwidth filters |
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 Ha-OIII-OIII visualization |
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 Faint details. 
Relative response for the two considered emission lines.
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 Full field of the camera - 1.6x1.6°. [click on the image for a larger view] |
Other images: http://www.starimager.com/Image%20Gallery%20Pages/Planetary%20Nebulae/medusa%20nebula.htm http://panther-observatory.com/gallery/deepsky/doc/Abell21_cass.htm http://www.capella-observatory.com/ImageHTMLs/PNs/Abell21.htm |
