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Origin
Planetary nebulae are the final stage of evolution for many stars. Our sun is a star of average size, and only a small number of stars in excess of its mass. Stars with a mass of several times solar in the final stage of becoming supernovae. Stars of intermediate and small mass at the end of the evolutionary path create a planetary nebula.
A typical star with a mass several times less than sunlight for the greater part of his life due to reactions of thermonuclear fusion of helium from hydrogen in its nucleus (often instead of the term fusion term burning, in this case - the burning of hydrogen). The energy released in these reactions keeps the star from collapsing under its own force of gravity, thereby making it stable.
After several billion years, giving a hydrogen supply, and energy becomes insufficient to contain the external layers of the star. The nucleus begins to shrink and heat up. At present, the temperature of the Sun is the nucleus around 15 million K, but after the hydrogen supply is exhausted, the kernel will make the compression temperature rise of up to 100 million K. This outer layers cooled and substantially increased in size due to very high temperatures nucleus. The star becomes a red giant. The nucleus at this stage continues to shrink and heat, with temperatures reaching to 100 million By the beginning of the synthesis of carbon and oxygen from the helium.
Resumption of fusion reactions stop allows further compression of the nucleus. Burn out soon an inert helium nucleus, consisting of carbon and oxygen surrounded by a shell of burning helium. Thermonuclear reactions involving helium are very sensitive to temperature. The speed of the reactions is proportional to T40, ie the temperature increase by only 2% will lead to doubling the speed of the reactions. This makes the star very unstable: a small increase in temperature causes a rapid increase in the speed of progress of the reactions, increasing the allocation of energy, which in turn causes the temperature to rise. The upper layers of the helium burning are beginning to expand rapidly, the temperature decreases, the reaction slows down. All this may be the cause of power fluctuations, sometimes strong enough to discard a significant portion of the atmosphere the stars into space.
Released gas forms the expanding shell around the exposed core stars. As the growing part of the atmosphere separated from the star, seen more and deeper layers with higher temperatures. Upon reaching the exposed surface (the photosphere star) temperature 30 000 K emitted ultraviolet photon energy becomes sufficient for the ionization of atoms in the released substance that makes it glow. Thus, the cloud becomes a planetary nebula.