Red Giants after Main-Sequence
All main sequence stars are all alike in the cores as they convert hydrogen into helium through the process of core hydrogen burning. The time that a star will spend as a main sequence star is called the main-sequence lifetime.
After all the core hydrogen has been used up, this leaves a core of nearly pure helium surrounded by a shell through which hydrogen burning works its way outward in the star. The core shrinks and becomes hotter, while the star’s outer layers expand and cool. The star then balloons to a size many times its original- a red giant is formed.
This occurs when the central temperature of the red giant increases till it reaches about 100 million K. This process converts helium into carbon and oxygen.
The process is as follows: Two helium nuclei combine to form an isotope of beryllium. However, owing to its instability, it soon breaks into 2 helium nuclei again. But if a third helium nucleus can strike the beryllium isotope before it falls apart, this collision creates a stable isotope of carbon and releases energy in the form of a gamma-ray photon.
While in more massive stars helium burning begins slowly, helium burning occurs quickly in less massive stars in a process called helium flash.
The reason why this only occurs to low mass stars is because the core must be compressed heavily to become hot enough for hydrogen burning to begin. At these conditions of extreme pressures and temperatures, atoms are ionized and detach themselves.
Free electrons bind together till the limit of further compression has been reached. Closely packed electrons can resist compression as a consequence of the Pauli exclusion principle which states, in layman terms, that “you cannot have two things in the same place at the same time”. This is known as degeneracy. The core is now supported by degenerate-electron pressure.
When the temperature reaches the high level needed for helium burning, energy is released as the helium heats up. This causes the star to burn at an ever-increasing rate and the electrons in the core are no more degenerate. This culminates in the helium flash inside the core.
Life Cycles of Stars | Diffuse Nebula | Main-Sequence Stars | Red Giants after Main-Sequence | Death of a Low Mass Star | Death of a High-Mass Star | Star Families | Magnitude Scale | Measuring Stellar Distances | Classification of stars | Wien's Law and Stefan-Boltzmannn Law for a Blackbody | Stellar Spectra