Image Credit & Copyright: Ken Crawford, Rancho Del Sol Observatory (http://www.imagingdeepsky.com)
The Medusa Nebula (also known as Abell 21, Sh2-274 and PK205+14.1) is an old, large planetary nebula of more than 4 light-years across, located some 1,500 light-years away from Earth in the constellation of Gemini (the Twins), on the Canis Minor border. The nebula is estimated to be about 8,800 years old.
When a star with a mass up to eight times that of the Sun runs out of fuel at the end of its life, it blows off its outer shells and begins to lose mass. This allows the hot, inner core of the star (collapsing from a red giant to a white dwarf) to radiate strongly, causing this outward-moving cocoon of gas to glow brightly. These glowing clouds can show complex structures, as the ejection of mass from the star is uneven in both time and direction. They are called “planetary” nebulae because early observers thought they looked like planets; but they don’t have anything to do with planets at all.
Planetary nebulae last for only about 10,000 years, a fleeting episode in the 10-billion-year lifespan of Sun-like stars. So, over the next several thousand years, the nebula will gradually disperse into space, and then the white dwarf will cool and fade away for billions of years. Our own Sun is expected to undergo a similar fate, but this will not occur until some 5 billion years from now.
The Medusa Nebula got its name because of the braided serpentine filaments of glowing gas that suggests the serpent hair of Medusa found in ancient Greek mythology. Besides the clearly visible filaments, the nebula also shows fainter filaments that extend below and to the left of the bright crescent region. Medusa’s central star is the bluish colored star lying just left of the nebula’s center.
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 it is often recommended to use at least an 8-inch (200 mm) telescope with an [O III] filter to find this object, although it is probably possible to image with smaller apertures.