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Comment by SiVal | Hacker News Reader

Iron is always spoken of as the dividing line, but I'd like to know whether iron is exactly on the line, on one side (which?), or it depends. IOW, does fusion of iron atoms release energy (hydrogen side of the line), absorb energy (uranium side of the line), neither, or either (depending on conditions)?

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My son has a masters in nuclear physics and i've always been curious as to why iron causes stars to explode.

He does an excellent job explaining things and put it to me like this.

Elements to the Left of Iron can undergo fusion and release energy, Elements to the right can undergo fission and release energy.

Iron IS the line because it needs energy to do either of these.

All elements want to find stability, and Iron is that Element because it needs energy for either fission or fusion.

So yes, Iron is the dividing line and this is what makes it so stable.

Edit: forgot to link the chart when referencing left or right..

http://www.splung.com/content/sid/5/page/benergy

Fusion to the left of Fe, fission to the right, here I am stuck in the middle with glu(ons)

(Yes I know it's technically mesons at the scale we're talking about but it doesn't rhyme so there)

Mesons should been called gluons, way stickier for sure.

Look at the binding energy curve: http://hyperphysics.phy-astr.gsu.edu/hbase/NucEne/nucbin.htm...

The periodic table is pretty awesome. But I wonder if there's a layout that takes into account this fission/fusion division? Would it just be a line?

-- Yeah there are a few alternative designs, but nothing that seems to do the division. https://en.wikipedia.org/wiki/Alternative_periodic_tables

That's because the periodic table is essentially about chemistry (i.e. about electron orbitals), not about nuclear physics (i.e. the atom's nucleus). For example it doesn't talk much about isotopes, aside from usually reporting the average atomic mass.

My point is that there are other ways of organizing the same information to perhaps reveal unexpected connections. From the same link, a 'physicists periodic table' - https://en.wikipedia.org/wiki/Alternative_periodic_tables#Th...

And really, i guess everything is physics if you get specific enough.

As I understand it, iron is the first element that absorbs energy under fusion, and therefore won't fuse further. Could be wrong, though.

It will happily fuse further. It just won't support the outside of a star against gravity, while doing it. So the star collapses, fuses lots more stuff even heavier than iron, and then explodes. Most of the iron and heavier stuff fuses into the core of a neutron star, the ultimate in energy-consuming fusion.

None of this is accurate.

Iron will not happily fuse further because this NEEDS energy and where would that energy come from?

"heavier than iron" elements are produced when a star explodes because that collapse produces enormous amounts of energy.

During the collapse, the outer edge of the star is accelerated to something like 20% of the speed of light, that is an ENORMOUS amount of energy slamming down on the core.

Lastly, neutron starts don't produce energy, they are the incompressible remnants of a dead star.

You answer your own question: the energy for further fusion, all the way to neutron degeneracy, is provided by gravitational collapse. The outer layers fusing provide energy for the explosion.

... which, it must be said, is probably more important than gravitation in overcoming nuclear charge repulsion.

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