Life of a star: from birth to death

It is curious to think that, almost as if they were living, even the stars are born and die. Yet this happens, even if it is rare to observe the birth of a star: most of the observable stars are in fact older than humanity. The fate of a star is conditioned by many factors that manifest themselves from the earliest stages of its birth.

A star “arises” from a gas cluster (interstellar nebula) in which the elements begin to interact with each other. This results in a contraction and a dizzying increase in density. The antagonism between the internal gravitational forces, which tend to make the gas cluster contract, and the very high thermal pressure that tends to make it explode, determines the next destiny: once a certain critical mass (mass of Jeans) has been overcome, the materials collapse and there is the formation of a protostar. It is located in the center of the cloud and its gravitational force allows it to retain materials and increase mass and density and to reach very high temperatures.
The elements present to a greater extent inside it are hydrogen (H) to a greater extent, and helium (He).
Since there is no nuclear reaction within the body capable of releasing energy, the protostar continues to reduce its size until the nucleus reaches a temperature of 10 million kelvins. After this threshold the protostar becomes a star.
In the core of the star (core) the temperature and pressure are so high that they transform matter into a state of plasma. Precisely in this area nuclear fusion reactions take place, which allow to release gamma rays and photons from the transformation of hydrogen atoms into helium atoms. Thanks to the energy released, the star is able to support the outer layers, avoiding complete collapse.
In this phase the star is stable and can be placed in the main sequence of the H-R diagram, with a different position depending on the mass. Just the mass is the element that allows us to predict how long the star will be in this equilibrium situation. In fact, stability is linked to the availability of hydrogen inside the core: when it ends the nucleus is no longer able to support the outer layers. A star of greater mass will consume its hydrogen atoms more quickly and for this reason it will “stop” less time in the main sequence, soon becoming unstable. New contractions will therefore occur against the nucleus.
At this point the star faces a crossroads:

• if it has a small mass, the collapse will not allow to obtain the optimal conditions for new nuclear fusions and the star will face death.
• if it has a large mass, the temperature will increase so as to allow new reactions, turning into a red giant.

In its core nuclear reactions transform helium accumulated in carbon, but when helium also ends there will be a further crossroads, dictated by the previous conditions.

If the mass is large enough, the giant will become a red supergiant, in whose core the carbon becomes the protagonist of nuclear fusions.
This situation of instability ends when the nucleus of the star becomes iron. This element does not allow to release the energy necessary to stabilize the star and the star will go towards death.
Even the last phase of a star’s life depends on the mass and the final destinies are different:

• in the case of a star smaller than 8 solar masses, the nucleus becomes a white dwarf, after having expelled the outermost layers that will constitute a planetary nebula.
• if the star is larger than 8 solar masses, it will explode spectacularly forming a supernova. The core can become a neutron star, a pulsar or a black hole.

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