Synthesis of heavier elements

What happens when all the hydrogen in the star's core turns into helium? For the stars with the smallest masses (about 1/10 of the mass of the Sun), this is the end of thermonuclear reactions. For Sun-like stars, the nucleus is made of helium, but a hydrogen layer has formed around it, in which conditions are favorable for thermonuclear reactions, and helium is also formed there, which sinks into the nucleus, increasing its density and temperature. When the temperature reaches 100,000,000 K, the conditions are favorable for starting helium reactions, in which helium nuclei synthesize carbon nuclei and - at higher temperatures - oxygen nuclei. For smaller masses (such as the Sun and those with slightly larger masses), this will be the end of thermonuclear reactions. But for the most massive and hottest stars, thermonuclear reactions continue until the nucleus is made entirely of iron. All of these reactions release energy that provides light to the star, but for much shorter periods of time than hydrogen reactions.

For the formation of elements heavier than iron (atomic number 56), however, it is necessary to "absorb" significant energy during the reaction. Therefore, such elements can be formed during the explosion of Supernovae.

As a summary it could be noted that the main nuclear fusion in the stellar nuclei are:

• burning of the hydrogen depends on the temperature:

• p-p chain reaction – the most important mechanism producing energy in stars like Sun and stars with smaller masses

• starts at T = 4 000 000 K

• CNO cycle – effective for the massive stars

• starts at T = 15 000 000 K

• Dominates at T = 17 000 000 K

• helium burning – occurs at the older stars

• synthesis of elements heavier than the iron – at Supernovae.