If stars can only fuse elements as far as iron, where do heavy elements come from?

Typical Star Core fusion happens because of the heat and pressure of all the mass of the star trying to crush it to a smaller diameter- but this amount of pressure and heat is limited to a certain maximum amount due to the majority of the stars' element: Hydrogen/Helium. There is some kind of bar graph arc of this limit- the more mass/bigger star you have, the pressure maximum is still a bit higher, but there can never be enough mass in a star to ever get the pressure/heat high enough to fuse past Iron. Such a massive amount in one location to actually fuse iron to bigger elemtns would become a Black Hole, not a star, as gravity would at that point win the gravity/energy tug of war. So; no stable star can make >Iron fusion reactions in the humdrum universe

BUT, obviously we do have more heavy elements than Iron around here.. A LOT! so like you asked: How did THEY get made, if the above statement is true?

Like the other posters said: In supernovae explosions... but they didn't quite explain WHY/how it happens- so allow me.

To make Uranium, the building block elements (whatever they are) need to be heated and pressured together with enough force/heat that the fusion reaction can occur- but Like I said above, such a large amount is not possible in stellar cores. BUT, at the milliseconds after the instant of a supernova explosion, it actually IMPLODES first, then rebounds outward in its violence. If you had a really well placed high speed camera to view the explosion, you would see that the "balance" of fusion/heat/energy pushing outward VS Gravity pushing inwards suddenly shifts, as the energy within finally runs out of fuel to put up a fight against gravity, and Gravity goes A HA!!!! at last and crushes inwards... THIS is where the additional pressure to make the higher element fusion and heat comes from, and Uranium, and all the others are created in a frenzy of last second fusion reactions happeneing within the now much denser and hotter core--- BUT, since this is a Supernovae, and not an infant black hole, at some point, Gravity- who was like "Ah HA!!" milliseconds before, now hits a brick wall and goes "woa!! What the?!"- that Brick wall most likely being the [not enough mass to become a black hole] new Neutron star cores' resistance, in addition to all the suddenly more energetic fusion reactions goin on in that internal frenzy- and in the end of that particular stars tug of war- energy wins the ultimate battle, and almost all of the mass of the star- the stuff up to iron, and all the recently made heavier stuff- goes flying outward in the big KABOOM we know as the Crab Nebula, or Eta Carrinae.

So that is where the heavier elements come from- the last frenzied reactions occuring at the rebounding hiccup of a giant stars death throes. And even though it was only a few seconds or so- a LOT of matter was fused into heavier elements- due to the scale of such massive stars; enough gold is made to make 6 Earths, for instance, and all of that gets spread out after the explosion, and later consolidates into new planetary systems like ours, where we find all those heavier elements mixed in with all the natural ones.

Fusion does not necessarily stop at Iron. True, Iron marks the boundary between where fusion yields excess energy and where fusion yields less energy than what was required to fuse the elements in the first place, but that does not stop a star from continuing. In a desperate attempt to fight the pull of its own gravity in the last moments of a stars life much heavier elements can be fused within a star. The fusion of heavier elements only lasts for a very brief time, but even so, all the elements heavier than Iron we have in the universe today came from the last moments of a stars life. Often times a super nova is precluded by the formation of these heavier elements and once the star explodes, these elements are scattered throughout the universe.

Nuclear reactions inside stars can only go on if they provide energy to a star. Up to iron, the fusion of a lighter element into a heavier element does provide some energy (less and less as you get closer to iron).

If you want to create elements heavier than iron, you have to ADD energy from outside. This does not happen... unless the star explodes (supernova). Then the shock wave is so violent that it creates heavy elements not only in the core, but also in the upper layers of the star (the ones that get blown away into interstellar space).

In fact, it creates all kinds of heavy elements, including very radioactive ones that otherwise would never exist. These elements then decay over weeks and months and years (depending on which isotopes) and that is the "glow" that can be observed for weeks, months and years after the initial explosion.

Fuse Element

A main cycle star can only fuse elements lighter than iron, but it may contain heavier elements due to accretion; of course this doesn't answer where those elements came from originally, but scientists believe that our heavy elements may have been formed in supernovae; without the main fusion reaction to repel a star's mass, it collapses inward and prompts a much more powerful reaction.

Excellent question...

After Iron, the only way to create heavier elements is to expend energy (rather than to generate it)... and, that comes from stars that go supernova. The sudden collapse of the star's outter regions into the core supply the pressure and energy needed to fuse the heavier elements; the explosion that occurs after that spreads the elements into space.

You need 2 supernovas exploding close to each other to get elements heavier than iron. And the super novas need to already contain elements as heavy as iron.

Supernovae.

When a massive star dies and explodes the energy from the explosion is enough to synthesize most all of the other elements on the periodic table.

While simultaneously disbursing those elements into a stellar nebula.