What is the source of all elements in the universe that are more massive than iron?

Answer 1

The primary sources of these elements are fusion reactions in stars (the plural is there because there are hundreds, if not thousands, of different reactions that take place in stars).

The reason that iron is significant is that two of its isotopes (56Fe and 58Fe) are the around the most stable nuclei of any element (56Fe is often wrongly attributed to be the most stable nuclide, but that distinction actually goes to 62Ni - 56Fe comes in third after 62Ni and 58Fe).

As a result, fusion reactions (nuclear reactions that combine smaller elements to make larger ones) that take place to give progressively heavier elements up to nickel (just beyond iron in the Periodic Table) will give out energy. To form elements larger than iron, energy has to be put in to the reaction. It is the fusion reactions that give elements up to nickel, which give out the energy from stars.

The consequence of this is that any elements heavier than nickel which may be temporarily formed in a star will undergo fission reactions that give smaller elements. Elements heavier than iron are generally formed in supernovae, where a star coming to the end of its life (and therefore containing plenty of heavy elements) produces a massive energy output that fuels the formation of heavy elements and scatters them to interstellar space before significant losses due to fission can take place.

Answer 2

The only thing that can fuse lighter elements into something heavier than iron is the explosion of a super-nova star.

The problem is something called the "packing fraction". If you fuse light elements like hydrogen into heavier elements like helium, it generates energy; this is how a star works. When the star gets old, it collapses a little, heats up a LOT, and begins to fuse helium into carbon. This releases energy too, but not as much. As you fuse heavier and heavier elements, you get less and less energy out - until you get to iron. From the standpoint of nuclear reactions, iron is as "dense" as things get. Fuse elements together into something heavier than iron, and you must put energy IN to drive the reaction.

So in a dying star, getting hotter and hotter and fusing heavier and heavier stuff, at some point it starts fusing iron - and this sucks energy OUT OF the star. Instead of the core of the star feeding energy out to the rest of the star, the core starts sucking energy from the rest of the star into the core, as iron becomes heavier elements like gold, lead, uranium, and even heavier exotic elements. So the middle levels of the star explode in two directions in the supernova - IN, to feed energy to the core to continue the generation of heavy elements, and OUT, to blow the remainder of the star into space. Some of the core elements collapse into a neutron star or a black hole, while the remainder of the core material is blown out into space.

Millions or billions of years later, that gold dust or lead dust or uranium dust will fall into another planetary nebula, and form planets - planets like Earth. Every atom of ANYTHING heavier than helium has been through a star - and every atom heavier than iron has escaped from the core of a supernova explosion!

Answer 3

Nuclear fusion, This process happens in stars where lighter elements fuse to create heavier elements.