In Medieval times alchemists strove to turn lead into gold. They were Epochs late in their quest and limited in their goals. Cosmic alchemy had been occurring in the stars for billions and billions of years. The stars we wonder at every night have been creating the elements that make up the universe, even the carbon that forms the building blocks of life as we know it, since the first star flamed into existence 13.5 billion years ago (give or take a million).
The alchemy starts when the ball of swirling hydrogen that has coalesced into a protostar reaches a sufficiently high temperature for the hydrogen to begin fusion. The fusion process turns hydrogen into helium. This holds true for stars of any mass above 0.08 solar mass, even giants many times larger than the Sun.
Hydrogen burning stars larger than that burn for billions of years. Our Sun is expected to continue to burn its hydrogen for 10 billion years.
But eventually, all the hydrogen in the core is used up, transformed into helium. Then the star transforms.
When the hydrogen in the core is exhausted, the radiation pressure cannot balance gravity, so the star contracts'”but only slightly. This is because there is still hydrogen in the outer layers of the star, and it begins fusion. But fusion in these layers causes them to expand. The star becomes a Red Giant.
As the outer layers burn their hydrogen, the helium produced migrates to the core. Eventually, the helium builds up to a point that gravitational pressures and temperatures in the core cause the helium to begin fusion. The new energy produced by the core makes it expand. This reduces the fusion of the outer layers, so the total energy output of the star is reduced. This causes the star to contract.
An important point about the helium cycle is the elements it produces. In stars up to about four solar mass, helium fusion first produces neon, then that fuses to oxygen and carbon. The planetary nebula surrounding the remnants of these former stars is filled with these vital elements.
Creation of Heavy Elements
Stars more massive than 40 solar masses have very strong stellar winds, and lose mass so rapidly they can’t expand into red giants. If the remaining core is no more than 1.4 solar masses, the fusion process continues as in less massive stars, except that the temperature is sufficient to fuse some of the neon into magnesium. The star eventually becomes a white dwarf.
If the core is more than about 2.5 solar masses, the oxygen begins to fuse into sulfur and silicon. As the core continues to collapse, the temperature becomes high enough to break down any nucleus. This forms alpha particles that fuse with other nuclei to form aluminum and sodium. If the star is too massive to form a white dwarf but not sufficiently massive to maintain the conversion of neon to oxygen and magnesium, it will collapse completely and explode as a supernova.
In even more massive stars, the fusion process continues until it produces iron. This is a very important phase of the cosmic alchemy process. It lays the groundwork for the creation of the heavier elements'”even gold. But more energy is consumed producing iron than is generated, and the core undergoes a sudden, catastrophic collapse. The force of this collapse overcomes the forces keeping nuclei apart and forces the electrons of the atoms into the protons to form neutrons. The star becomes a very dense sphere of neutrons with a thin outer layer of iron.
In the final act of cosmic alchemy, in the most massive stars, the collapse is even more catastrophic. The remaining protons and electrons in the collapsing outer layers also are compressed into neutrons. The star releases the energy created by this transmutation as a burst of neutrinos. The neutrinos bombard the iron core and create elements heavier than iron, up to and possibly beyond uranium.
Of course, transuranic elements have very short half lives, so they do not survive.