Origins
Stars more massive than 8 solar masses (M⊙) will likely end their lives in a dramatic supernova explosion. Planetary nebula may result from the death of intermediate and low mass stars down to 0.8 M⊙.
Stars spend most of their lifetime shining as a result of nuclear fusion reactions that convert hydrogen to helium in the star's core. Outward pressure from fusion in the core balances inward collapse due to the star's own gravity. Such stars are said to be in the main sequence.
Intermediate to low mass stars run out of hydrogen in their cores after tens of millions to billions of years in the main sequence. Gravity compresses the core and it heats up. Currently the sun's core has a temperature of approximately 15 million K, but when it runs out of hydrogen, the compression of the core will cause the temperature to rise to about 100 million K.
The outer layers of the star expand enormously and become much cooler in contrast to the very high temperature of the core; and the star becomes a red giant. The core continues to contract and heat up, and when its temperature reaches 100 million K, helium nuclei begin to fuse into carbon and oxygen. The resumption of fusion reactions stops the core's contraction. Helium burning (fusion of helium nuclei) soon forms an inert core of carbon and oxygen, with both a helium-burning shell and a hydrogen-burning shell surrounding it. In this last stage the star will observationally be a red giant again and structurally an asymptotic giant branch star.
Helium fusion reactions are extremely temperature sensitive, with reaction rates being proportional to T40 (under relatively low temperatures). This means that just a 2% rise in temperature more than doubles the reaction rate. These conditions cause the star to become very unstable—a small rise in temperature leads to a rapid rise in reaction rates, which releases a lot of energy, increasing the temperature further. The helium-burning layer rapidly expands and therefore cools, which reduces the reaction rate again. Huge pulsations build up, which eventually become large enough to throw off the whole stellar atmosphere into space.
The ejected gases form a cloud of material around the now-exposed core of the star. As more and more of the atmosphere moves away from the star, deeper and deeper layers at higher and higher temperatures are exposed. When the exposed surface reaches a temperature of about 30,000 K, there are enough ultraviolet photons being emitted to ionize the ejected atmosphere, making it glow. The cloud has then become a planetary nebula.
Read more about this topic: Planetary Nebula
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