The following was originally published on the Stellaris forums by Murmeldjuret.
(Today we will only discuss the visible part of the universe or 4.6% of the total mass. So when I use %mass I mean % of the visible mass.)
Carl Sagan once said we are all made of stardust. There is truth in it. 90% of our mass has been forged in the fusion reactors that power stars. Though it is equally inspiring that the 10% that is hydrogen in us is the same hydrogen big bang created during the recombination 13.8 billion years ago. 10% of your mass is 13.8 billion years old. A lot of (note understatement) hydrogen and helium was created during this recombination, and it is still the same hydrogen that powers the stars.
For most of a star’s life it fuses hydrogen into helium, but as helium content grows and hydrogen content shrinks, it will eventually start fusing helium. This is usually the end of a star’s regular life as the core process that powers it and keeps it in balance falters. The results depend on startype. The new stellar reactor begins fusing helium into heavier elements. See chart below. The heaviest naturally occurring element is plutonium, though it is possible heavier elements have formed earlier in the universe but have decayed to plutonium or lighter by now.
Above is a chart of the different types of elements in the solar system. The same ratio is about correct for the universe in general, except hydrogen andhelium are even more common. Hydrogen is about 74% by mass and helium is about 24% of the mass in the Milky Way. That leaves 2% to be shared by all the other elements. We thus say the Milky Way has a metallicity of 2%. Chemists will probably not consider carbon a metal, but since anything heavier than helium has to have been made after big bang it is a more clear distinction from an astronomical perspective.
Interestingly enough, the third most abundant element is oxygen, with a much higher concentration than any element heavier than helium. In fact, a full 1% of the galaxy is oxygen. This is why ice is not uncommon in the universe, it being a stable bond of hydrogen and oxygen, the most and third most common elements. The fourth most common element is carbon, so score one for carbon lifeforms on watery worlds being the most abundant.
Another interesting thing to note from the curve is that even numbers are much more common. This is due to helium being the main source of heavier elements and helium is two protons. There is no way to get the odd numbered elements without something to alter the regular process. Of the ten most common elements by mass, only nitrogen and hydrogen are odd numbered.
The next thing to note is the peak of iron (Fe) and nickel. Iron is a relatively heavy element, but it is still the sixth most common element by weight. 0.1% of the visible Milky Way is iron by mass. All planetary cores are iron and/or nickel based, and iron can be found everywhere. This is strange, element nr 26 beats 21 preceding elements in abundance. The reason is actually quite straightforward. Iron is the most stable nuclei in the universe. It has the lowest binding energy of all nuclei. All the lower elements give net energy when you fuse them, and all the heavier elements give net energy when you split them. There is no way to change iron without adding energy, both splitting or fusing iron costs energy. When the core of star turns to iron the process that powers a star turns to a process that takes energy from the star.
Fissile elements usually decay to lead, see its peak (Pb). The heaviest stable element lead-208 is still radioactive but with a halflife of 10^19 years. Compare that to the 10^10 years the universe has existed. Everything heavier than iron might be radioactive but with halflives unimaginably long.
The hydrogen created at big bang is limited, and it is being burnt in every star, in every galaxy, in every cluster, every second. For each atom of iron it creates, there is less fuel left to power the universe. If nothing else ends the universe as we know it, the hydrogen and helium will run out, and all that would be left is a universe sized cloud ofiron glittering from the light of long gone stars.