Medusa nebula

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Medusa nebula
ESO Very Large Telescope images the Medusa Nebula.jpg
Observational Data
Designation Sharpless 2-274
Abell 21
PK 205+14 1
Right ascension 07h 29m 02.70944s[1]
Declination +13° 14′ 48.5893″[1]
Constellation Gemini
Type of object Planetary nebula
Dimensions 10.25 arcmin[1]
Magnitude Apparent Mag: 15.99[1]
Absolute Mag: 12.68[2]
Distance from Earth 1,500 ly[3]
Radial velocity 28.8 km/s
Proper motion RA: -2.720 mas/yr[1]
Dec.: -8.601 mas/yr[1]
Parallax 1.8597±0.0806 mas[1]

The Medusa nebula (Sharpless 2-274, Abell 21, PK 205+14 1) is a planetary nebula in the constellation of Gemini.[4] The nebula was discovered by Abell in 1955 and designated as number 14 in his list of planetary nebulae.[5] Later in 1966, Abell redesignated it as number 21 in a catalogue of what he considered to be "odd planetary nebulae".[5] It was debated for a while whether the object was in fact a supernova remnant, but measurements of its expansion velocity made in the 1970s determined it to be a planetary nebula instead. Its expansion velocity of 50 km/s is much lower than that of supernova remnants such as the Crab nebula, which is expanding at approximately 1,500 km/s.[3][6]

The nebula's name derives from Medusa, one of the gorgons in Greek mythology, whose "hair" was a mass of writhing snakes. Filamentary structures of gas can be seen to snake their way across the nebula. Though large, the nebula has a low surface brightness making it very faint and tricky to observe.

Properties and structure

The nebula is situated some 1,500 light years in the constellation of Gemini, close to its southern border with Canis Major.[3] It has an apparent size of 10.25 arcseconds, meaning it is roughly 4.5 light years across.[1] The nebula houses a small but hot star, a white dwarf as most planetary nebulae do. The bright star that can be seen in images of the nebula is a foreground star that happens to be located in front of the nebula, and is designated TYC 776-1339-1.[3] The white dwarf can be seen lying just off the crescent shaped region.

The white dwarf emits vast amounts of ultraviolet radiation, ionizing the enveloping gas and causing it to glow.[3] The different colors correspond to different ionization levels of different elements. The green for example is due to doubly ionized oxygen, [O III], that is atoms of oxygen that have lost two of their electrons. This particular emission is extremely rare, so rare that it was originally believed to indicate the presence of an unknown element designated "nebulium".[3] Later it was realised it was only another ionization state of oxygen and not a new element. The transition responsible for the green color is rare due to the effects of quantum mechanics.


  1. 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 PN A66 21 from the SIMBAD Astronomical Database
  2. From definition of absolute magnitude, using apparent magnitude (+10.95) and distance (9.68 ly) given here.
  3. 3.0 3.1 3.2 3.3 3.4 3.5 The Dreadful Beauty of Medusa from
  4. The Medusa Nebula from
  5. 5.0 5.1 Johnson, H. and Rubin, R. (1971). Observation and Classification of the Nebula YM 29. The Astrophysical Journal, 163, p.151. Bibcode: 1971ApJ...163..151J
  6. Bietenholz, M., Kronberg, P., Hogg, D. and Wilson, A. (1991). The expansion of the Crab Nebula. The Astrophysical Journal, 373, p.L59. Bibcode: 1991ApJ...373L..59B