The Propeller Nebula (Simeis 57) is part of a vast and rich HII-region in Cygnus, known as Cygnus X Complex. The Propeller Nebula is located close to Gamma Cygni in the direction of north-east. The propeller shape is actually a small portion of a much larger emission nebula, which can be seen as faint red hydrogen-alpha nebulosity throughout the image frame.
The Propeller Nebula is probably about 5,500 light-years away from Earth.Like many emission nebulae in Cygnus, the Propeller is part of the giant molecular cloud, the Cygnus X, which is about 4600 light-years apart from the Solar System.
There is another, much more closer dark molecular cloud between it and us, in a distance of about 300 light-years, called the Cygnus Rift, which makes its observation much more difficult. Cygnus Rift can be observed by the naked eye under dark skies: it is the dark strip dividing the Milky Way into two parts, spanning from Cygnus to Sagittarius. The enormous quantity of dust and plasma contained by Cygnus Rift absorbs the most of the light coming from stars and other objects behind it.
Unfortunately we know quite little about the Propeller Nebula, even its distance is undetermined yet. It mostly emits at the spectra line of hydrogen-alpha, while it is much dimmer on other wavelengths. The stars or other sources responsible for the ionisation of the gas is also still to be identified. The mass of the cloud is less than 50 Solar masses, according to estimations.
