(Re)Discover the Ring Nebula
The Ring Nebula is one of the most famous celestial object that can be seen from the northern hemisphere. The film is a journey inside the Nebula, showing its formation and its real shape. The Ring Nebula is not a ring … but an oblate spheroid.
The 3D model used in the film as based on scientific data. It is as close to reality as possible.
The white dwarf formation
90% of the stars in our universe will become white dwarves, including our Sun. The transition will occur in about 100 million years, passing through the state of red giant.
The planetary nebula is a transient object in terms of astronomical time scales. If the life time of the star would be compared to a year, the red giant transformation would occur on the 27th of December. The planetary nebula created in the transformation into a white dwarf would exist in the last 100 milliseconds.
A star life cycle depends on its size. This is balance between the internal pressure due to the gravity and the energy emitted by nuclear reactions. Hydrogen will be converted to heavier elements, like helium, then carbon or oxygen. All heavier atoms, like these in our body, have been created by nuclear reactions in stars.
By creating heavier elements, the star density increases. At one point, the internal balance between the gravitational pressure and the temperature is lost. The nucleus collapses. This could be compared to bones no more able to support a body becoming too heavy.
In the case of the white dwarf, the core collapses. The shockwaves repel the outer stellar matter in space, creating a planetary nebula.
The nebula will finally diffuse in space, taking away these heavier atoms. This gas will later be captured by more dense space regions, and merged to diffuse nebulae. These stellar nurseries will give birth to new stars. The proto-planetary dust ring around the new born stars contains these atoms, and later on, the planets. This completes the cycle.
The Nebula shape
As said in the film, the Ring Nebula is not a ring … but an oblate spheroid. This scientific article describes the process behind in all details (Three-Dimensional Ionization Structure and Evolution of NGC 6720, The Ring Nebula”. Astronomical Journal 134 (4): 1679–1692).
In few words, measuring precisely the Doppler shift of all regions of the corona, the scientists have deduced their speed and direction. Seen from the Earth single point of view, the light coming from several regions are mixed. They have reconstructed the real shape using tomography techniques, with the hypothesis that the corona has been cast of by the central star.
Compared to other planetary nebulae, the shape of Ring Nebula is very “simple”. An excellent summary of complex planetary nebulae shape can be found here.
In a simplified very way, the shape is influenced by three factors:
First, the initial impulse provided by the stellar core collapse is not always homogenous in space. The star plasma fluxes is very complex. Temperature and pressure have a spherical symmetry, but magnetic fields, by essence do not — they conform more to a torus topology. As the star plasma is highly magnetic, and also the source of the magnetic field, the plasma fluxes have highly complex structures, far from being spherical. The arches of matter of some nebulae results from these initial magnetic field lines. Then, once the matter has been repelled into space, it will be influenced by the stellar wind emitted by the central star. Finally, the corona can locally gather into clumps, due to the gravity. This is the case for the Ring or the Dumbell Nebulae.
Other factors, like the presence of planets or of a star companion will also influence the final shape.