When an intermediate mass star, such as the Sun (or any star with between about 0.7 and 8 solar masses)
nears the end of its life it becomes a helium shell-burning supergiant AGB star. These stars swell to around 100
times their original size, and the tenuous outer layers cool and redden (indeed they become so cool that dust
may form, obscuring and apparently dimming the central star). These stars are burning helium, which means
that they are hotter than hydrogen-burning Main Sequence stars and helium burning emits massive emits of
radiation in a shell just beneath the outer tenuous envelope and above the core. This intense radiation
pressure generates a superwind, which is much more intense than the normal stellar wind of a main sequence
star like the Sun. This wind meets the tenuous outer layers which are so far from the core as to be only loosely
bound by the star's gravity and as a result massive amounts of material are blown outwards from the star and
the outer envelope is shed in one or more expanding shells. These shells typically expand outwards at great
speeds, typically 30-60 kilometres per second. Over thousands of years these shells may be hundreds or
thousands of times the diameter of the Solar System. Relieved of the weight (pressure) of these overlying
layers, the core expands slightly, causing what is left of the envelope around the core to contract and heat up
considerably. When this core plus envelope remnant reach about 30 000 degrees K, they emit enough
ultraviolet radiation to cause the massively expanded outer shells to fluoresce, and a planetary nebula is born!
The planetary nebula appears as a ring when seen through a telescope (light passing through the edges has
passed through more gas creating the illusion that the spherical nebula is thickest here). This ring-like
appearance gives rise to the 'planetary' part of the name, but planetary nebulae actually have nothing to do
with planets (although at first they were mistaken for planets, debris and dust around the central star).
Sometimes, the central star is hard to see (as in the one above) in visible light, especially in a young nebula
which may contain a bubble of hot gas around the core. The core will eventually free itself of the expanding
nebula and cool to become a C-O white dwarf, after about 20 000 years. The Sun will (if left to its own devices)
become a planetary nebula in about 6 billion years time.
Above: the hot central core is visible in the centre of this nebula. Often a planetary nebulae have the appearance
of 'cosmic eyes' such as the Cat's Eye Nebula. Sometimes (largely for unexplained reasons) the core of planetary
nebulae appears off-centre.
Below: some (probably most) planetary nebulae are bilobed as the expanding material is preferentially shed
along a particular axis. Examples include the Hourglass Nebula (which also has a central 'eye' with a stellar core
that is off-centre) and the Egg Nebula. Others more complicated structures may also occur, including radiating
spokes of cooler gas, knots of gas at shock fronts and bipolar jets. Sometimes spherical shells and jets or a pair
of lobes occur together. Some of these more complex patterns probably arise when the star is part of a binary
star and so orbits a nearby companion star that may perturb the nebula with its gravity. It maybe that a companion
star will start to accrete (draw onto itself) some of the material from the nebula, some of which may be jetted off
from the companion star's poles. In short planetary nebulae occur in an extraordinary array of forms and are one
of the most beautiful classes of object in space.