Life cycle of stars
A nebula made from interstellar dust, including hydrogen gas, forced together under gravity until it collapses, forming a protostar. The potential energy is converted to kinetic and thermal energy, so the protostar is very hot and dense. Nuclear fusion does not occur yet at this stage.
If the protostar is under enough pressure, and the temperature has reached roughly 1 × 107K fusion of hydrogen nuclei begins, forming helium nuclei and releasing energy, which creates an outwards radiation pressure. This outwards radiation pressure from the energy released during hydrogen fusion eventually balances the inward force of gravity, creating a main sequence star. A star can stay in the main sequence for millions to billions of years. It is the most stable stage in the star’s life cycle.
Once the hydrogen in the main sequence star runs out, the outward force from fusion decreases, and the force from gravity forces the star inwards to collapse. At this stage, there are two routes the star can proceed down depending on its mass (much larger than the Sun or roughly the same size as the Sun):
0.5 - 10 solar masses (roughly same size as the Sun)
When the hydrogen runs out, the star will collapse, so gravitational energy is converted to thermal energy, leading to increasing pressure and temperature in the core of the star. This means helium fusion can occur. When this happens, the outer layers of the star cool and expand, forming a red giant.
Once all the helium has been fused, the outwards radiation pressure will once again be less than the force of gravity, and the star will collapse.
This collapse causes the gravitational force to increase, but this is not balanced by radiation pressure from fusion, but from the electron degeneracy pressure.
If the star is less than the Chandrasekhar limit (1.44 solar masses), the electron degeneracy pressure will balance the gravitational collapse and a white dwarf will be formed.
The outer layers left from the red giant drift away as planetary nebulae, with the white dwarf at the centre. The star will remain as a white dwarf for the rest of its life, cooling down gradually to eventually form a black dwarf (lost all heat energy).
If the star has a mass greater than the Chandrasekhar limit (1.44 solar masses), then the electron degeneracy pressure will be less than the force of gravity and the core will collapse further into a neutron star. The radiation pressure that balances out the gravitational force is the neutron degeneracy pressure.
Greater than 10 solar masses (much larger than the Sun)
When the hydrogen runs out, the star will collapse, so gravitational energy is converted to thermal energy, leading to increasing pressure and temperature in the core of the star. This means helium fusion can occur. When this happens, the outer layers of the star cool and expand, forming a red supergiant.
Heavier elements can also form through fusion in layers around the red supergiant. Fusion can only form elements up to iron, as this is at the top of the binding energy per nucleon curve. As the red supergiant expands and contracts, these layers cool and drift away as planetary nebulae.
Once the outwards pressure from fusion no longer balances gravity, the red supergiant collapses instantly, imploding as a supernova. Elements heavier than iron are formed in this process and spread about the galaxy.
After the supernova, if the star was on the lighter side (between 1.44 and 3 solar masses), it will form a neutron star. The radiation pressure that balances out the gravitational force is the neutron degeneracy pressure.
If the star was on the heavier side (greater than 3 solar masses), after the supernova, the force from gravity is much larger than the neutron degeneracy pressure, so the star will collapse into a black hole, with a singularity at the centre.