14. The process that occurs at the center (core) of all active stars is nuclear fusion, specifically hydrogen fusion. In this process, hydrogen atoms fuse together to form helium, releasing vast amounts of energy in the form of heat and light. This energy is what powers the star, and it is also what creates the outward pressure that prevents the star from collapsing under its own gravity. The process of nuclear fusion requires extremely high temperatures and pressures, which are only found at the cores of stars. As the hydrogen in the core is consumed, the core will eventually begin to shrink and heat up, which can trigger the fusion of heavier elements and lead to further energy production. This process continues until the star runs out of fuel, at which point it may either collapse and form a white dwarf or explode in a supernova.
15. A star is held together by the balance of two opposing forces: gravity and pressure. The balance between these two forces determines the stability and size of a star. If the pressure is too weak, gravity will cause the star to collapse and form a black hole or neutron star. If the pressure is too strong, the star will expand and eventually dissipate into space. Only when these forces are in balance can a star maintain its size and stability over time.
16. An old star would have nearly no hydrogen left in its core.
17. Hydrogen molecules can be forced together by high temperatures and pressures, causing them to fuse and form heavier elements. This process is known as nuclear fusion.