What determines whether a star will end its life cycle as a black hole or neutron star or as a white dwarf
The main factor that determines whether a star will end its life cycle as a black hole, neutron star, or as a white dwarf is its mass.
If the star has a mass less than 1.4 times that of the sun, it will end its life as a white dwarf. This happens when the star exhausts all its nuclear fuel and shrinks down to a small, hot, dense object, supported by electron degeneracy pressure.
If the star has a mass between 1.4 and 3 times that of the sun, it will end its life as a neutron star. This happens when the core of the star collapses under its own gravity and becomes extremely dense, resulting in the expulsion of the outer layers in a supernova explosion.
If the star has a mass greater than 3 times that of the sun, it will end its life as a black hole. This happens when the core of the star collapses so much that its gravity becomes too strong for anything, not even light, to escape - creating a singularity at the center.
The ultimate fate of a star is primarily determined by its mass. The following scenarios describe the different possibilities:
1. White Dwarf:
- Low to medium-mass stars, up to about 8 times the mass of our sun, will eventually become white dwarfs.
- As the star exhausts its nuclear fuel (mainly hydrogen and helium), it will expand into a red giant and shed its outer layers.
- The remaining core, composed mainly of carbon and oxygen, will collapse under gravity and become a hot and dense white dwarf.
2. Neutron Star:
- If a star is more massive than about 8 times the mass of our sun, but less than about 20 times the mass, it will undergo a supernova explosion.
- During a supernova, the outer layers of the star are blown off, and the core collapses due to gravity.
- If the core mass is between about 1.4 and 3 times the mass of our sun (known as the Chandrasekhar limit), it will become a neutron star.
- The core collapses so much that protons and electrons combine, forming neutrons and neutrinos. The resulting neutron star is extremely dense and has a strong gravitational field.
3. Black Hole:
- If a star is more massive than about 20 times the mass of our sun, it will also undergo a supernova.
- In this case, the core mass is greater than the Chandrasekhar limit, and the collapse during the supernova is even more extreme.
- The core collapses to a point of infinite density called a singularity, surrounded by an event horizon, which defines the boundary of a black hole.
- The gravitational pull of a black hole is so strong that nothing, not even light, can escape it.
In summary, the mass of a star plays a critical role in determining whether it will end as a white dwarf, neutron star, or black hole.
The fate of a star at the end of its life cycle is determined primarily by its initial mass, or more specifically, its mass at the time it exhausts its nuclear fuel. The key factor is the balance between gravity and the forces that push back against it.
Let's break down the different outcomes:
1. White Dwarf: Stars with low to medium-mass, up to about 8 times the mass of the Sun, will generally end their lives as white dwarfs. When these stars exhaust their nuclear fuel, they go through a collapse phase, shedding their outer layers and leaving behind a dense core of electron-degenerate matter called a white dwarf. Electrons are packed so tightly that they exert pressure, preventing further collapse. Over time, the white dwarf cools and fades away, eventually becoming a cold, dead object.
2. Neutron Star: More massive stars, typically between 8 and 20-30 times the mass of the Sun, undergo a more dramatic collapse at the end of their lives. Once nuclear fusion in the core stops, gravity overwhelms the pressure exerted by electrons, causing the core to collapse further. This collapse triggers a supernova explosion, where the outer layers of the star are expelled into space, while the core collapses inward. If the remaining core is around 1.4 to about 3 times the mass of the Sun, it becomes extremely dense and forms a neutron star. Neutron stars are made up of tightly packed neutrons and have extraordinary densities and intense gravitational forces.
3. Black Hole: Stars that are even more massive, typically greater than 20-30 times the mass of the Sun, have such strong gravitational forces that even neutron degeneracy pressure cannot prevent their collapse. These supermassive cores collapse completely, forming what is known as a singularity, where the matter is compressed to an infinitely dense point. This forms a black hole, a region where gravity is so strong that nothing, not even light, can escape.
To determine which fate a star will experience, you need to know its initial mass. Stars with higher masses have more gravitational pull, making it harder for other forces to resist collapse. Thus, they are more likely to end up as neutron stars or black holes. On the other hand, stars with lower masses lack the gravitational force to form a neutron star or black hole, so they become white dwarfs instead.