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  Planetary Nebula.jpg -  A Planetary Nebula is a type of emission nebula consisting of an expanding glowing shell of ionized gas ejected during the final stage of a star's path to becoming a white dwarf.  The name is due to their disk like appearance, similar to that of planets, when viewed through small telescopes.  Stars spend most of their lifetime on the Main Sequence fusing hydrogen into helium in their central core - approximately  the innermost 10%. When a star, having a mass between 0.8 to 8 times that of the Sun, converts all the hydrogen in its core to helium nuclear reactions cease. The inward pressure of gravity can now overcome the extinguished outward pressure of those reactions causing the core to collapse and the shell around the core to heat up due to increased gravitational pressure starting hydrogen fusion in the shell. Because there is more area in the shell than the core, the energy and temperature produced will increase. The temperature increases enough to start helium fusion in the compressed core. Because the outer layers of the star are being heated more, the star will greatly expand in size while the star's surface temperature will slightly decrease because of its increased distance from the temperature source. At this point the star leaves the Main Sequence becoming a Red Giant. When all the core's helium is converted to carbon and oxygen, nuclear reactions in the core again stop. Once more, the inward pressure of gravity can overcome the extinguished outward pressure of fusion in the core causing the core to further collapse and the shell around the core to heat up even more this time starting helium fusion in the shell immediately around the core. This again causes the temperature to increase, but in stars with a mass less than 8x that of the Sun, the temperature does not increase enough to start fusion of the carbon/oxgen core. At this point the inert core contains carbon and oxygen in a shell undergoing helium fusion inside another shell undergoing hydrogen fusion surrounded by an inactive hydrogen/helium mixture. Helium fusion is extremely temperature sensitive. A very small change in temperature can drastically change reaction rates causing the star to alternately expand as the temperature rises then contract as the expansion cools the star. This pulsating action throws off large amounts of gas into space at a speed of 10 to 20 miles per second. During this pre-planatary nebula phase, the discarded gas blocks light from the dying star. This stage lasts a few thousand years. Eventually all the nuclear fuel is exhausted through fusion and mass loss. This leaves a very hot exposed core which now heats the ejected gas causing it to glow. This is the material we see in a Planetary Nebula. As the ejected material spreads and the remaining star cools becoming a white dwarf, the glow will fade out. This takes just a few tens of thousands of years making it a relatively short lived event. The remaining white dwarf is comprised almost entirely of carbon and oxgen.  It is believed that 95% to 99% of the stellar population will go through the Planetary Nebula stage.When the Sun becomes a Red Giant in about 4.5 billion years it will increase its size by 100x becoming 1000x brighter. It will take about 1 billion years to produce a Planetary Nebula which will last perhaps just 50,000 years. The resulting white dwarf will retain about half its original mass and be about the size of the Earth.   
M27 Dumbbell
M27 LRBG+Hubble palette
M57 Ring
M76 Little Dumbbell
M97 Owl

A Planetary Nebula is a type of emission nebula consisting of an expanding glowing shell of ionized gas ejected during the final stage of a star's path to becoming a white dwarf. The name is due to their disk like appearance, similar to that of planets, when viewed through small telescopes.

Stars spend most of their lifetime on the Main Sequence fusing hydrogen into helium in their central core - approximately the innermost 10%. When a star, having a mass between 0.8 to 8 times that of the Sun, converts all the hydrogen in its core to helium nuclear reactions cease. The inward pressure of gravity can now overcome the extinguished outward pressure of those reactions causing the core to collapse and the shell around the core to heat up due to increased gravitational pressure starting hydrogen fusion in the shell. Because there is more area in the shell than the core, the energy and temperature produced will increase. The temperature increases enough to start helium fusion in the compressed core. Because the outer layers of the star are being heated more, the star will greatly expand in size while the star's surface temperature will slightly decrease because of its increased distance from the temperature source. At this point the star leaves the Main Sequence becoming a Red Giant.

When all the core's helium is converted to carbon and oxygen, nuclear reactions in the core again stop. Once more, the inward pressure of gravity can overcome the extinguished outward pressure of fusion in the core causing the core to further collapse and the shell around the core to heat up even more this time starting helium fusion in the shell immediately around the core. This again causes the temperature to increase, but in stars with a mass less than 8x that of the Sun, the temperature does not increase enough to start fusion of the carbon/oxgen core. At this point the inert core contains carbon and oxygen in a shell undergoing helium fusion inside another shell undergoing hydrogen fusion surrounded by an inactive hydrogen/helium mixture. Helium fusion is extremely temperature sensitive. A very small change in temperature can drastically change reaction rates causing the star to alternately expand as the temperature rises then contract as the expansion cools the star. This pulsating action throws off large amounts of gas into space at a speed of 10 to 20 miles per second. During this pre-planatary nebula phase, the discarded gas blocks light from the dying star. This stage lasts a few thousand years.

Eventually all the nuclear fuel is exhausted through fusion and mass loss. This leaves a very hot exposed core which now heats the ejected gas causing it to glow. This is the material we see in a Planetary Nebula. As the ejected material spreads and the remaining star cools becoming a white dwarf, the glow will fade out. This takes just a few tens of thousands of years making it a relatively short lived event. The remaining white dwarf is comprised almost entirely of carbon and oxgen.

It is believed that 95% to 99% of the stellar population will go through the Planetary Nebula stage. When the Sun becomes a Red Giant in about 4.5 billion years it will increase its size by 100x becoming 1000x brighter. It will take about 1 billion years to produce a Planetary Nebula which will last perhaps just 50,000 years. The resulting white dwarf will retain about half its original mass and be about the size of the Earth.

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