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The only exceptions are nuclides that decay by the process of electron capture, such as beryllium-7, strontium-85, and zirconium-89, whose decay rate may be affected by local electron density.For all other nuclides, the proportion of the original nuclide to its decay products changes in a predictable way as the original nuclide decays over time.Notice that there’s a huge jump between, say, hydrogen (H stops the process, but at the same time: it doesn’t help. Without fuel, the rest of the star is free to collapse the core without opposition, and generally it does.When there’s a lot of iron being produced in the core, a star probably only has a few hours or energy to be created (throwing liquid nitrogen on a fire, maybe? That extra energy (which is a lot) isn’t generally available until the outer layers of the star come crushing down on the core.After one half-life has elapsed, one half of the atoms of the nuclide in question will have decayed into a "daughter" nuclide or decay product.In many cases, the daughter nuclide itself is radioactive, resulting in a decay chain, eventually ending with the formation of a stable (nonradioactive) daughter nuclide; each step in such a chain is characterized by a distinct half-life.The energy of all that falling material drives the fusion rate of the remaining lighter elements way, way, up (supernovas are super for a reason), and also helps power the creation of the elements that make our lives that much more interesting: gold, silver, uranium, lead, mercury, whatever.There are more than a hundred known elements, and iron is only #26. Long story short: iron doesn’t kill stars, but right before a (large) star dies, it is full of buckets of iron.
While the moment in time at which a particular nucleus decays is unpredictable, a collection of atoms of a radioactive nuclide decays exponentially at a rate described by a parameter known as the half-life, usually given in units of years when discussing dating techniques.
In these cases, usually the half-life of interest in radiometric dating is the longest one in the chain, which is the rate-limiting factor in the ultimate transformation of the radioactive nuclide into its stable daughter.
Isotopic systems that have been exploited for radiometric dating have half-lives ranging from only about 10 years (e.g., tritium) to over 100 billion years (e.g., samarium-147).
For most radioactive nuclides, the half-life depends solely on nuclear properties and is essentially a constant.
It is not affected by external factors such as temperature, pressure, chemical environment, or presence of a magnetic or electric field.