The Medusa Nebula, a name chosen due to its apparent meandering, serpentine filaments of gleaming gas, is a planetary nebula in the constellation of Gemini. It is also known as Abell 21 and Sharpless 2-274. George O. Abell originally discovered this in 1955 close to the University of California, Los Angeles. Abell classified it as a planetary nebula however until the early 70’s this nebula was considered a supernova remnant. However yet again the classification was changed, in 1971 according to that of Soviet Astronomers the nebula was truly a planetary nebula as Abell had stated. They supported this conclusion with the computation of the expansion velocities and the thermal character of radio emission. In the late 1800's when stellar spectroscopy was first being used people were surprised to find out that planetary nebulae had pure emission line spectra, rather than a continuum with absorption lines as was found for stars. Thus, doubly ionised oxygen (O III) usually acts as a marker for planetary nebulae as they have one single exciting star which ionises oxygen atoms to O2+ and so planetary nebulae pop up on the spectral line as very bright and noticeable in comparison to other stars also there is an obvious green glow given off. When the green emission of doubly ionised oxygen was first observed, astronomers believed they had even discovered a new element named nebullium. However they soon realised that it was simply an uncommon wavelength of radiation from the doubly ionised oxygen. 
  
The Medusa Nebula is roughly 460 pc or 1500 ly away and is estimated to be over 4 light years across. Similar to its mythological namesake the planetary nebula represents a vast transformation. The planetary nebula phase is the final phase in the evolution of low mass stars such as our sun. They metamorphose from red giants to hot white dwarf stars whilst in this transformation leaving behind their outer layers. The transforming star is a faint star near to the centre of the bright crescent.