Medusa; the frightful woman with snakes in stead of hair and a gaze that would turn any ill-fated man that happened to cross her path into stone. She was said to roam the underworld until Perseus, the valiant half-god, would cut off her head whilst using a mirror to avoid her deadly look. People were so abhorred by her name that the Romans eventually called some very nasty creatures after her: jellyfish.
Now it so happens that there's an exceptionally dark and nasty jellyfish in the winter sky. On the border between the constellations of Gemini and Canis Minor (the small dog), you'll find a nebula so faint that it wasn't discovered until 1955. Scientifically it's called Abell 21, but more popularly it's known as the Medusa Nebula. If you haven't got a big telescope or if your night's sky polluted by the venomous glow of useless street lights, please don't bother. Even with my 2x18" binoscope this nebula wasn't keen on revealing itself at first, although after a couple of minutes some details started to appear to me. The overall jellyfish shape was faint, but fairly obvious and especially the two "tentacles" stood out.
The Medusa Nebula is a planetary nebula, just like the Ring or Saturn nebulae, albeit a very old one. Many tens or possibly even hundreds of thousands of years ago a star died and shed its atmosphere into space. For a while this huge gas bubble would expand and glow under the fierce radiation from the remaining stellar core. But in Medusa's case, the nebula has now expanded to over 4 lightyears in diameter and the gas is slowly dissolving into space. The central star, or what's left of it, is cooling down and is now emitting much less radiation. Therefore the gas bubble's cooling down as well and hardly emits light anymore. Eventually Medusa will disappear forever.
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