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Abell 21



Abell 21: The Medusa Nebula in Gemini

RA = 7 hrs 29 min; Dec = +13 deg 14 min

Date & Site:

Imaged on 1/17/2015; Little Blair Valley, Anza Borrego, CA

Conditions: calm; 35 degrees; excellent seeing


Telescope: TEC 140mm Refractor (980mm fl) at f/7
Mount: AstroPhysics 1100
Camera: ST-10XME; CFW10; Hutech OAG5 autoguider

Astrodon Gen II RGB filters


Exposures: 4 hrs Ha (15 min subs 1x1) and 30 min each RGB (5 min subs) for total of 5.5 Hrs.

Commonly known as the Medusa Nebula, the braided serpentine filaments of glowing gas suggests the serpent hair of Medusa of ancient Greek Mythology. Abell 21 is a very ancient planetary nebula, an expanding shell of gas surrounding a dying star, formed from matter ejected from the star's outer layers; the gas is ionized by the remaining hot stellar core, emitting light in the process. Until the early 1970s, the Medusa was thought to be a supernova remnant. With the computation of expansion velocities and the thermal character of its radio emission, Soviet astronomers in 1971 concluded that Abell 21 was most likely a planetary nebula.

1500 light years away and 4 ly across

FOV =52 x 35 arc minutes

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