Abell 21 (Gemini)
Fig. 1 - Forming a crescent shape in the sky: Abell 21 (Sharpless 2-274) in Gemini, photographed with an 8-inch f/4 Newtonian reflector on a ZWO AM5 mount.
| Object name: | Constellation: | Coordinates: | Apparent size: | Visual brightness: |
|---|---|---|---|---|
| Abell 21 (= Sharpless 2-274) | Gemini | 07h29m / +13°15' | 10.3' x 10.3' | 16 mag |
The planetary nebula Abell 21 (also known as Sharpless 2-274 or the Medusa Nebula) resides in the constellation Gemini. Abell 21 is named after a creature from Greek mythology — the Gorgon Medusa. Medusa bore a crown of living serpents in place of hair. In the image above, those serpents are echoed by the red filaments of luminous hydrogen gas, which sweep into a crescent across the sky.
The Medusa Nebula stretches roughly four light-years across and is located about 1,500 light-years away. Despite its vast size, it remains relatively faint. Abell 21 was discovered by the American astronomer George O. Abell in 1955 (source: Wikipedia).
Exposure time: 7h 42min (154x3min) at gain 100 and -10°C, taken on November 27/28, 2025 and on December 13/14, 2025. Processing with Astro Pixel Processor (APP) and Photoshop. Darks, flats, and bias were used.
Equipment: Cooled ASI 2600MC Pro camera, TeleVue Paracorr Type II coma corrector, 8" f/4 "ONTC" Newtonian telescope riding on a ZWO AM5 Strain Wave Mount, ZWO OAG-L off axis system, ASI 174MM Mini guide camera, ASIAIR Plus.
Abell 21 was also captured with an uncooled DSLR camera through a 16-inch dobsonian on an equatorial platform. The image can be seen here.
Fig. 2 - Search chart for Abell 21 (Sharpless 2-274). Copyright 2025 'The Mag-7 Star Atlas Project', www.siaris.net.