Abell 21 - Medusa Nebula
Abell 21 is an old planetary nebula in the constellation Gemini, discovered in 1955 by George Abell. The reason for this is probably the low surface
brightness of 16mag/'². The apparent diameter in the sky is 12 arcminutes.
The distance to Abell 21 could be determined by the GAIA- mission to be 1.890 light-years - before that the distance was mostly given with 1.500
light-years. From this a true size of 6.6 light-years can be determined, which is quite a lot for a planetary nebula. The PN got its name "Medusa nebula"
because of the snake-like filaments which resemble the head of Medusa. The age of the nebula is estimated to be close to 9,000 years, so it is almost
fading and will probably become invisible in a few thousand years.
The central star (ZS) belongs to the class of "PG1159 stars". These are stars which are between the red giant stage and a white dwarf. This is because
about 10% of all white dwarfs manage to ignite a helium flash for one last time - so in the end they contain almost no hydrogen anymore, but mainly helium,
carbon and oxygen. A characteristic of these stars is their extremely high surface temperature. And here the central star of Abell 21 with a temperature
of 140,000 Kelvin makes no exception, whose mass by the way is given with 0.53 solar masses.
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Abell 21 is a very interesting object in my 20" telescope, but because of its low surface brightness you will need a dark sky and a low magnification to see the
PN in its true beauty.
At a magnification of 120x in combination with a nebula filter, a pale, irregularly shaped disk can be seen at first, which is also quite large. After some time
the first details peel out in the form of some brighter filaments - the brightest can be found in the northern part. Towards the northwest (top-right) the nebula
gently fades out into space, towards the southeast (bottom-left) the PN has a gap in the periphery. There is also a darker, crescent-shaped area within the nebula.
The central star is not visible, but the surrounding area is quite starry due to the Milky Way.
