An aging star undergoes a series of nuclear fusion processes in its core, consuming elements in a specific order based on the star's mass. For a massive star that is destined to become a black hole, the sequence of fusion generally proceeds as follows:
- Hydrogen: The star primarily fuses hydrogen into helium in its core during the main sequence phase of its life.
- Helium: Once the hydrogen is depleted, the core contracts and heats up, allowing fusion of helium into carbon and oxygen.
- Carbon and Oxygen: As the helium runs out, the core continues to contract, reaching higher temperatures that allow it to fuse carbon into heavier elements, such as magnesium and neon, and then subsequently fusion of oxygen into silicon and sulfur.
- Silicon: After the carbon and oxygen are depleted, the star begins to fuse silicon into iron. Iron fusion does not yield energy, which leads to a critical point in the star's lifecycle.
- Iron: Once the core is predominantly iron, fusion stops being an energy-generating process. The core cannot support itself against gravitational collapse because fusion reactions no longer produce energy to counteract gravity.
The final stages of a massive star's lifecycle involve rapid collapse when the core becomes unsupported, leading to a supernova explosion and leaving behind a black hole if the remnant core is sufficient in mass.