Abell 21 (Gemini)

The planetary nebula Abell 21 (Sharpless 2-274, Medusa Nebula) in the constellation 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

Abell 21, better known as the Medusa Nebula, is a faint planetary nebula in the constellation Gemini. Named after the Gorgon Medusa of Greek mythology, it recalls her crown of living serpents through the intricate red filaments of glowing hydrogen gas that sweep across the sky in a giant crescent.

For many years, astronomers debated the true nature of the Medusa Nebula. Its filamentary appearance and large size initially suggested that it might be a supernova remnant. However, measurements of its expansion velocity revealed speeds of only about 64 to 90 km/s, far lower than those typically observed in supernova remnants. Together with its thermal radio emission and characteristic spectrum, these observations confirmed that Abell 21 is an evolved planetary nebula.

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).

Technical Details:
Exposure time: 7h 42min (154x3min)
Gain: 100
Temperature: -10°C
Date: November 27/28, 2025 and December 13/14, 2025
Processing: Astro Pixel Processor (APP) and Photoshop
Calibration: Darks, flats, and bias

Equipment:
Cooled ASI 2600MC Pro camera, TeleVue Paracorr Type II coma corrector, 8" f/4 "ONTC" Newtonian telescope, 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.

M106 M1