At first a star forms through contraction of interstellar matter. The interstellar matter consists to a great part of hydrogen that has a temperature of 90 K. A new star ionizes the hydrogen in its neighborhood and heats it up to 10,000 K. The ionized hydrogen expands because of its greater pressure and encloses parts of the colder hydrogen. These parts become stars themselves through contraction. Many stellar clusters have formed this way.
The temperatures inside the star are now over 5 Mio K and the hydrogen fire starts. At 5 Mio K the proton – proton – cycle starts and at 10 Mio K the carbon - nitrogen – cycle.
The new born stars are on the main row of the HRD(Hertzsprung – Russel
– Diagram) at first. Their birth mass determines where exactly. Stars with
more mass are on the upper left, stars with less mass are in the middle
or further down.
The stars stay very long on the main row because here their conditions are stable.
When the hydrogen reserves in the nucleus are used up, the nucleus is
compressed through gravitation and heated to 100 Mio K. At this heat the
outer layers of unused hydrogen are pressed away. The star grows to a giant
or a super giant. The stars with more mass become a giant faster because
their reserves of hydrogen are used up faster.
As a giant the star has found a new but more instable balance on the main row of the HRD.
The giant or super giant will now go through the state of pulsation several times, depending on its mass. In its nucleus temperatures can rise up to 1 Bil K.
When the reserves of energy now are used up again, gravitation wins. Through the high pressure inside the star protons were suddenly split up into neutrons and positrons. Through that the star looses energy and collapses. This collapse leads to a heating up and the hurling away of the outer layers of the star which again uses up energy and that again leads to further heating up to up to 200 Mio K in the outer zones and to another collapse.
Now suddenly the carbon – nitrogen – cycle can start in the outer zones and use the hydrogen there. This action gives free so much energy that the outer layers are hurled away and a super nova develops.
The super nova outburst has hurled away almost all the matter of the
star and only a small very compact object is left over.
Depending on the mass the star had before this can be a white dwarf, a pulsar (=neutron star) or a collapsar (= a black hole).