Why Do Planetary Nebulae Form?

[Cheman]

Hi,
When a white dwarf is formed, why do the outer layers of gas which made it a red giant drift away to foirm a planetary nebula? Decrease gravity? Outward pressure? What exactly?
Also, when the outer layers of a star move outwards to form a red giant, why do they cool down? (hence becoming red)

 

[quartodeciman]

Hi again!

try this:

link to FUSION planetary nebulae narrative --->
http://fusedweb.pppl.gov/CPEP/Chart_Pages/5.Plasmas/Nebula/Planetary.html

link to a PN fan site --->
http://www.blackskies.com/intro.html#NEBULAE

Researcher Sun Kwok aptly named PNs "Cosmic Butterflies" for a book title. See:

link to Prof. Sun Kwok U. Calgary home page --->
http://www.iras.ucalgary.ca/~kwok/kwok.html

Thank you for starting a terrific subject.

 

[Cheman]

So do the white dwarf and nebula kind of occur as separate events? ie - outer layers pushed out; as the core starts to run out and emitts lots of radiation, the outer layers are pushed away? Correct? By the way, why does nuclear fusion of heavier elements (ie - helium to carbon, etc) produce more energy that hydrogen to helium fusion? (it must do to cause expansion to red giant)

 

[quartodeciman]

So do the white dwarf and nebula kind of occur as separate events?

I reckon it is different activities governed by different states, since the body of a star has been dividing between inner and outer parts since its arrival on the main sequence. There is nuclear activity in the inner part (active energy production) and pure re-activity in the outer part (no active energy production, just energy transfer).

why does nuclear fusion of heavier elements...produce more energy that hydrogen to helium fusion? (it must do to cause expansion to red giant)

I don't think that is quite correct. Red giant expansion is supposed to arise because the inner temperature gets high enough for hydrogen->helium activity to run in a shell around the helium core. That is what expands the outer part and makes the star a giant.

Heavy element "burning" only occurs in stars 3 solar masses and up. The core pressures and temperatures get extremely high. But as long as the extensive outer part behaves like a convective processor of energy, the heated material rises, cools and emits mostly lower frequencies of light--the star becomes red. That finally comes to an end with too much nuclear energy production in shells getting stacked up below. BOOM!