From what I know, fusion past iron requires more energy than is released so I can't understand how the elements heavier than iron were formed.

Thanks.

__________________
MrObvious

supernovas tend to release alot of energy in a real short time period

What Kenneth said is basically the answer. Supernovae release a whole lot of energy (actually, "a whole lot" still seems on the low side to me). It takes a relatively small amount of energy to fuse iron and heavier elements, IIRC, so a SN could do that easily with all the energy it produces.

Isn't a (very) small quantity of heavy elements also produced in the regular fusion process as well?

They can be formed, it's just that such a reaction isn't sustainable.

__________________
Freedom For Fission A breath of fresh Iodine-131

All of the above are true. However, (1) endothermic fusion processes that occur during supernovae don't take you far from iron on the periodic table; (2) a set of processes known as "neutron capture" is most likely responsible for the remaining elements (and various isotopes thereof) up through uranium. The version known as the "r-process" (r for rapid) occurs during supernova explosions, and is responsible for most or all of the heaviest elements. The slow version or "s-process" is responsible for most of the medium-weight elements beyond zinc (the rest of the 4th row beyond zinc, and perhaps part of the 5th row). Neutron capture is what it sounds like. In an environment containing free neutrons, some of them collide with and stick to elemental nuclei. Since neutrons have no charge, they don't experience the repulsive coulomb barrier when they interact with nuclei. Adding neutrons to nuclei changes their isotope number (thus the origin of various isotopes). However, a nucleus can stably hang onto only so many neutrons relative to the number of protons, and if that number is exceeded, the nucleus undergoes spontaneous beta decay: n --> p + e + anti-neutrino. The added proton changes the element of the nucleus....and we build up the rest of the periodic table.

The s-process occurs deep inside red supergiant stars, where various advanced fusion reactions have released free neutrons deep in its interior. This process is slow, because there are relatively few free neutrons floating around, so the tendancy is to beta decay before adding more than one neutron. The r-process occurs during supernovae (and possibly other, more exotic, high energy events), and is rapid because it occurs in a free-neutron rich environment in which many neutrons can be added onto nuclei before they have a chance to beta decay.

If it's so "easy" (no coulomb barrier) to do, why aren't all elements made this way? This is because free neutron environments are rare. That the abundances of the elements beyond zinc are so rare (e.g., gold) is a reflection of this fact. A free neutron has a mean life time of about 15 minutes before undergoing spontaneous beta decay. Red supergiants generate free neutrons through various fusion reactions and supernovae make zillions of them during the processes that caused the collapse of the stellar core.

They can be formed only in environments in which their formation isn't the source of the energy. Supernova explosions are the main sites for nucleosynthesis beyond iron. Come to think of it, since nova explosions involve hydrogen envelopes, I can't immediately name other places that transferric (is that a word?) elements are produced. (Well, there are those reactors at Berkeley and Darmstadt and Dubna...)

"Ordinary" H-->He fusion (as in the Sun's core) has side reactions that produce small amounts of lithium, which counts as a metal to astronomers but isn't heavy enough to enter the question. In any environment, all nuclear reactions allowed by the temperature, density, and composition will take place; it's just that the specific balance needed for a stellar interior means that the environment won't last long unless there is net release of energy sufficient to counter the star's gravity.

Thanks for the info guy's.
I thought about supernovas before I posted the question but thought I was way off the mark. I just figured there would be more of the heavier elements around than there is after ~15 billion years of production.....

I quess it's a pretty inefficient proccess.

Regards,

__________________
MrObvious

It is not so much inefficiency as it is that the universe is a really big place!

__________________
Any day you wake up on "the right side of the dirt" is a good day.

T. Anderson

Why is uranium the stopping point? Why not Pu or Np?

WAG, but uranium, and all elements after it, are radioactive and decay over time. Most of these isotopes have half-lives of less than a second although some, such as uranium, can have isotopes with half-lives of billions of years. Elements after uranium are formed in supernova explosions but over millions of year they decay into other elements, which is why we don't find natural sources of Plutonium. This is also the reason why there is no Technetium (sp?) naturally found on Earth. Despite being right in the middle of the periodic table (element #43?) it has no stable isotopes so all of the Technetium formed in the stellar fusion that formed the solar system has decayed into other elements.

Why does it have no stable isotopes?

So elements heavier than U are produced in supernovae? How high do they get? And why do all the sources I've ever read not mention a word of this?

In the Naquada/Naquadrium thread in Bad Movies Forum someone posted a link that there there exists the possibility for stable transuranics far up the line from U and Pu. I don't know how much energy it would take to create these elements but if they were possible I would think we might see them resulting from supernovae. Or perhaps they exist but are far too rare to find, maybe having sunken into the interior of the earth.

Edit:
here's the link to the Rare Isotope Accelerator mentioned in the other thread

__________________
"Oh no no no I'm a rocket man Rocket man burning out his fuse up here alone." -- Sir Elton John

J Pax

Sorry, don't know the answer to that one, I haven't done much reading on nuclear chemistry. Certain configurations of protons/neutrons are more stable than others, and once you pass a certain number of protons/neutrons all elements are unstable and will decay, albeit in some cases the decay is very slow.

Side note: I did a quick google search and found that there is natural Technetium on Earth, minute amounts were discovered in some pitchblende in South Africa, produced via a natural decay chain from Uranium.

Yeah, I found that too. Didn't figure out why it was unstable, though.

A very detailed explanation of nuclear stability. LINK
It has to do with something called beta-stability; see pages 51 and 54 of the document.

__________________
At night the stars put on a show for free (Carole King)

There is something new out that suggests the early universe had fewer 100+ solar mass stars than previously expected. Early formation of 30 solar mass stars may have been much more common. The difference in these sizes make a big difference in metal production.

I'd like to know why this is so and will it be viewed as a BB gap filling attempt?

__________________
Lighten up! This is a stellar board! Author: duh.

"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.

No kidding -- since a 100+ solar mass star probably collapses straight into a black hole, it ain't gonna be spewing out much material, heavy or light.

These very large stars supernova first but do not produce the same metal material as the smaller mass supernova or hypernova's. At least, that's my limited understanding. I am curious about the difference in their metal production.

__________________
Lighten up! This is a stellar board! Author: duh.

"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.