Sh2-091, Supernova Remnant
Besides the well known Cirrus Nebula there is another Supernova Remnant (SNR) in the constellation "Swan". It is called "Sh 2-091" or
"G65.3+3.7" and is located near the beautiful double star "Albireo".
The SNR was first discovered in 1977 during a photographic survey (Emission Line Survey) and showed filamentary structures with a
diameter of 4.0°x3.3° on the images. The distance was determined in a first study to be 2,900 light-years, but more recent data assume
2,600 light-years. With the help of the apparent size the true diameter of "Sh 2-091" can be determined to be 180 lightyears. Since the
filaments move outward with a velocity of about 160 km/s, a value of 340,000 years is obtained for the age. This makes "Sh 2-091" the
oldest known SNR in our Milky Way. Because of the high age the nebula is already in the cooling phase for a long time, nevertheless
the gas masses have a temperature of 38,000 Kelvin.
Currently no neutron star or pulsar is known to be responsible for this SNR.
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Observing Sharpless 2-091 is quite difficult. Even with 20" it needs a very dark sky to see anything at all, because when you arrive at the
right location, you first stare into dark nothingness.
But eventually, after a while, the pale glow of a filament peels out of the black sky background - so faint and fine that you lose it the very
next moment. An OIII filter is highly recommended for this object.
