This nuclear fusion process eventually converts the hydrogen in the star into helium. This process releases energy, keeps the star hot, provides energy, and finally leaves the star in the form of radiation. But a star doesn't turn all the hydrogen into helium, as other posts suggest. Only a small part of hydrogen in a star will be converted into helium; In other words, only the hydrogen in the center of the star is hot enough to fuse. The temperature of surface hydrogen is too low to carry out nuclear fusion, and hydrogen will always exist throughout the life and death of stars.
In any case, once the center of the star runs out of fuel, hot pressing will lose its energy source. The center no longer produces energy, so that the gas has no energy reserve to maintain its extremely high temperature. So the star began to cool, and the pressure on the star began to decrease. Gravitation seized this opportunity (gravity never "turned off" in the whole process, it always tried to compress the star to smaller and smaller sizes), and the center of the star began to shrink. When it does this, it will reheat and begin to release energy to the star. This process causes the surface of the star to expand, and the star expands into a "red giant" or "super giant". "When the central temperature rises, the fusion process may start again, and more hydrogen will start to condense into helium, or if the star is big enough, it will start to absorb helium and condense into other elements such as carbon, nitrogen and oxygen." The farther away from the periodic table, the more energy is needed for fusion. But once you have nickel and iron, something will happen.
Stars like the sun will condense hydrogen into helium, and then start to condense helium into carbon and oxygen. Once the helium in the nucleus is exhausted, it will never reach the temperature to restart nuclear fusion. Gravity finally won. Nothing can help keep the core hot, and gravity will make the stars more and more condensed (and the surface of the stars will expand more and more). The reason why the central star eventually becomes a white dwarf is that I won't enter here (but if you are curious), and the gas on the surface of the star is floating (this floating gas is usually called a planetary nebula, even if it is related to planets ... this is a bit of a misnomer). This is their end.
Larger stars can restart the fusion process, melting their core elements until they reach nickel and iron. At this point, the fusion stops releasing energy, that is, the star must provide energy for the fusion process to make it happen. This is exactly the opposite of what it wants to do; It wants to release energy, so that it can keep high temperature and prevent gravity from causing it to collapse. At this point, the fusion of the core stopped and a sphere composed of nickel and iron began to form. The temperature around the sphere is still enough for other elements to fuse, so the remaining elements such as hydrogen and helium continue to fuse to form heavy elements.
Once the iron-nickel ball becomes too heavy (more than 1.4 times the mass of the sun), the iron ball will collapse. Nothing can stop gravity, nickel atoms and iron atoms are more or less crowded together. This causes these nuclei to split, and protons and electrons combine with each other to become neutrons, releasing incredible energy in a short time. The result was a supernova explosion. The released energy exploded the rest of the star. Eventually there will be a neutron star (neutron ball) with high density. In extreme cases, this neutron ball will further collapse into a black hole.