It begins collapsing, which causes gravitational heating.
If the mass of the star is below a certain threshold the collapse stops, leaving a white dwarf.
If the mass of the star is above a certain threshold the gravitational heating makes the star hot enough to ignite helium fusion and the star begins making carbon. The now very hot stellar core makes the star expand to a red giant.
When a main sequence star, such as our sun, exhausts it's supply of Hydrogen, fusion of Hydrogen to Helium ceases in the core. This results in cooling of the core and therefore causes it to contract. As the core contracts, the layers surrounding the star gain kinetic energy until the fusion of Hydrogen is possible in the outer layers. As these layers undergo fusion, they increase in temperature dramatically. The outer layers expand due to heat, and the star becomes massive; the main sequence star has evolved into a red giant. Fusion of Hydrogen to Helium continues in the outer layers, whilst the fusion of heavier elements such as carbon occurs in the core.
When a star exhausts its core hydrogen, nuclear fusion in the core stops, causing the core to contract and the outer layers to expand and cool, making the star grow larger. The increase in size is due to the star transitioning into the red giant phase as it starts burning hydrogen in a shell surrounding the core.
The amount of hydrogen in a star decreases over time because hydrogen fuses into helium through nuclear reactions in the star's core, releasing energy in the process. As the star continues burning hydrogen, it eventually exhausts its hydrogen fuel supply, leading to changes in its structure and behavior.
These are stars that have exhausted their core's supply of hydrogen by switching to a thermonuclear fusion made of hydrogen in a shell that surrounds the core.
A star's hydrogen supply runs out because of nuclear fusion in its core. As hydrogen is fused into heavier elements like helium, the star's core temperature increases, causing it to expand and cool. Eventually, the core runs out of hydrogen to fuse, leading to the star's evolution into a different phase.
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A star that has exhausted the hydrogen in its core will become a red giant or supergiant.
As a star ages and runs out of fuel in its core, it can expand in size as it transitions into a red giant. During this phase, fusion reactions occur in the outer layers of the star, causing it to expand and cool. This expansion happens when the star exhausts its hydrogen fuel and begins fusing helium in its core.
If there is no hydrogen left at the core of star then hydrogen fusion cannot occur. What happens in the core of a star before that happens is that helium begins to fuse, and then the other elements going up the periodic table until carbon. And then if the star explodes into a supernova, traces of the higher elements are fused as well.
A hydrogen shell is most likely to form around a more massive star in the later stages of its life, as it transitions to a red giant or supergiant phase. This occurs when the star exhausts the hydrogen fuel in its core and starts fusing hydrogen in a shell surrounding the core, causing the outer layers of the star to expand and cool.
The rest of the star expands.
When a star exhausts the hydrogen fuel in its core, nuclear reactions in the core stop, so the core begins to contract due to its gravity. This heats a shell just outside the core, where hydrogen remainsinitiating fusion of hydrogen to helium in the shell. The higher temperatures lead to increasing reaction rates, producing enough energy to increase the star's luminosity by a factor of 1,000-10,000.