M 27, A PLANETARY NEBULA
1 - The planetary nebula
We call planetary nebula, all nebulas which have at the first sight a circular form. In photography, these objects seem to have globular form, or ring form ( like the Lyre Nebula: M57). But they can include various motifs (form of a 8 …) like the Dumbbell nebula. One around 1700 planetary nebula, 70% haven't a totally circular form. It has a symmetric aspect in comparison to its centre star (example: Dumbbell, Egg Nebula). They are called bipolar. This bipolar aspect may be the consequence of a giant star, which is originally to the nebula died; it may have expelled its gas envelope in two opposite directions.
2 - Origin of this nebula
This nebula is the results of supernovae, that's to say the explosion of giant stars in the end of its life. There is the residue of stars (dwarf, neutron's star …). This residue is generally always observable in the centre of the nebula. After the expulsion of its different gas envelopes, this star warm up until 100 000° K. It emits UV radiations, which by fluorescence permit to produce the gas luminous. With this gas luminescence we can identified the composition of nebula by spectroscopy (green coloration for oxygen, red for hydrogen,…).
3 - Death o f a nebula
With the time, the centre star of a nebula cools progressively. It will fade definitively. After 50 000 years, the gas envelope, which surrounds the nebula, will disappear. So it will stay no more visible traces of the nebula.