Formation of Planets

[Chirios]

All stars produce heavier elements as a result of nuclear fusion. From my 2nd year of college (equivalent to last year of high school) understanding of this process, this + supernovae is the source of all elements heavier than helium in the universe. My question is this, how do the elements produced as a result of fusion leave the stars, and how does this result in the formation of planets?

If any of my assumptions in this question are wrong, please inform me.

 

[Nelson]

The material leaves the stars through supernovae. Which, as you mentioned, is also how elements heavier than iron are produced. When stars supernova, they fling all of their material out which enriches the interstellar medium within the galaxy. If a region of this material becomes dense enough, a cloud will form and eventually, due to perturbations, stars will begin to form. Planets are formed when stars form as a result of density perturbations within the circumstellar disk. The cool thing here is that these new stars will have larger ratios of heavy to light elements than previous generations. We can actually look at distant galaxies (which is really showing us what they were like billions of years ago) and see that their ratio of heavy to light elements is much lower. Overall, generation after generation of stars will have higher and higher ratios of these heavy elements.

 

[Josan]

Yes.

Supernovae are really the all-star here (get it? ), they both create and distribute the higher elements. And often it is actually the mass flinged out by the supernova that will begin the procsess of creating new stars.

 

[Pulsar]

It doesn't have to be as violent as a supernova. When a star runs out of hydrogen in its center, hydrogen fusion stops. The core contracts, increasing the temperature. As a result, the outer layers expand and the star becomes a red giant. If the central temperature gets high enough, helium fusion starts in the center, as well as hydrogen fusion in a surrounding layer. Unlike hydrogen fusion, helium fusion is a very violent, unstable reaction, with sudden increases and drops in temperature. This causes pulsations, which propagate through the star and push the outer layers away from the star: a planetary nebula is formed. This planetary nebula drifts away, expands, and mixes with the interstellar material.

Depending on the mass of the star, successive fusions continue, H->He->C->N->O and so forth. If the stellar mass isn't high the fusion processes stop at some point and the star becomes a white dwarf. If not, the processes continue until iron is formed, which doesn't fuse, and the star explodes as a supernova. Naturally, a supernova casts out even more material.

 

[JRChadwick]

Iron does not fuse in the core of stars. It is the most stable element. It is impossible for any star to become large enough to produce the temperatures and pressure necessary to fuse iron because at some point the amount of energy radiating off of the largest stars overcomes the gravitational force attempting to pull in additional matter. When a star supernovas, the pressure and temperature is great enough in that brief time to fuse all of the other elements.

 

[Nelson]

Actually Iron is produced in the cores of massive stars (more than roughly 8 solar masses).

at some point the amount of energy radiating off of the largest stars overcomes the gravitational force attempting to pull in additional matter

This idea is basically right, and prevents stars from forming anything heavier than iron, but here are the basics of how Iron is formed. Stars start out producing Helium and Deuterium from Hydrogen through the PP I chain (PP=proton-proton, these terms can be googled for a better description than what I am giving). Following this are the PP II and PP III chains which produce amounts of Lithium, Beryllium, and Boron. When the abundances are significant, Carbon can then be formed from Beryllium and Helium through the triple alpha process. The Oxygen, Nitrogen, and Fluorine can then be produced through the CNO cycle. I'm sure you get the gist of it by now, but these reaction cycles continue up to Silicon. Silicon-burning in the core of a massive enough star will produce a large number of nuclei with energy right around the binding energy per nucleon of Iron-56. This will produce small amounts of Iron-56 and Iron-54. However, as discussed before, any reactions that produce more massive elements than Iron-56 are endothermic and will not contribute to the luminosity of the star.

I apologize if that was a pain to follow. I was basically trying to summarize a 15 page section of a text book (An Introduction to Modern Astrophysics by Carroll and Ostlie) in one paragraph.

 

[JRChadwick]

Thanks for the added details. I knew the fusion process of stars was more complicated, I just did not want to look up all the information at the time.