Pictured on the left is a red giant, the first stage of star death, which is formed from all stars near the end of their main sequence. In other words, when main sequence stars start running out of hydrogen to use for fusion, the core starts to become unstable. Though it might appear that the entire star is uniform in composition at this moment, but this is not the case. The chemical structure of a red giant varies significantly as radius increases. In order to investigate the characteristics of a red giant, its means of formation are important to consider. The red color is due to temperatures being between 2,200 and 3,200 degrees Celsius. Though the main sequence star is significantly smaller, the star cools down when transitioning from main sequence to a red giant. Whereas main sequence stars take close to 5 billion years to reach the red giant phase, red giants only last between 1,000 and 1 billion years before collapsing.

Its combustible energy is the initial stage...

As the heat released from nuclear fusion decreases, the core of the red giant becomes increasingly unstable and contracts. Meanwhile, the outer shell of lighter elements expands significantly from the center of the star. The diameters of red giants are significantly larger than the main sequence star they were created from, ranging in diameter from 100 million to 1 billion kilometers, which is 100 to 1,000 times larger than our sun. At the red giant stage, the star becomes radially stratified based on chemical composition. Fusion still continues using helium instead of hydrogen, but this requires more energy. At this stage, the carbon and oxygen formed from helium fusion are located in the center of the star, shown in the diagram to the left. The fusion takes place just outside of that region, and the outer shell lies farther out.

Eventually, enough iron builds up in the center of a red giant to cause gravitational collapse. Carbon and oxygen can be formed through fusion, but the energy required to undergo fusion with iron is too high. This inability to do fusion results from iron's compact and dense nature, which means that it takes a very large amount of energy to fuse the nucleus of an iron atom. The radially outward thermal forces decrease significantly as iron builds up, as fusion is no longer occurring, which creates an imbalance in forces. Around this stage, the fate of the star diverges based on its mass. Low mass stars (less than 8 times the mass of our sun) lack the gravitational force to do fusion earlier, stopping fusion only after carbon. In any star with more than around 8 times our star's mass, fusion continues through up until iron as described above. Because the mass of a dying red giant determines the next step in its life cycle, each pathway will be described individually.

For low mass stars (<8 solar masses), white dwarfs are thought to be near the final stage of the life cycle. When fusion stops at carbon in this type of star, the outer layers expand radially outwards into space, creating a very colorful display of lower mass elements, called a planetary nebula. The dense core, now called a white dwarf star, remains dense but does not collapse further. The white dwarf, being very high temperature just after fusion ended, burns bright but dims over time, as nothing is being fused at this point. The star does not collapse due to Fermi repulsions between electrons, which are related to the physical law that no two electrons can have the same quantum state. Theoretically, as time passes, the white dwarf will cool into a black dwarf and remain dark, but no black dwarfs have been found so far in space.

For medium mass stars (8-18 solar masses), the red giant undergoes a different process. Within a thousand of a second, gravitational forces cause the star to collapse from thousands of kilometers to only a few kilometers across. Almost like a spring, the nuclear force within the individual iron atoms in the core cause the rapidly collapsing shells to rebound outwards, forming a supernova explosion. Despite the massive amounts of energy released in this process, the stability of the iron nucleus is still not enough to begin fusion again, so a neutron star is formed. The star at this point is around 10 kilometers in diameter, with all of the protons and electrons fusing together to form neutrons. Nuclear repulsions keep neutron stars from collapsing. Such stars are so dim and so far away that they must be recognized using means other than the naked eye. The first neutron star discovered using radio waves is called LGM-1, in the constellation Vulpecula.