The Death of Stars

 

ESA/Hubble, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons

It is estimated that there are “about 100 thousand million stars in the Milky Way alone” ^[1], our Sun being one of them. All life on Earth is powered by the Sun which generates energy through nuclear fusion: the process in which light nuclei combine to form heavier nuclei. The protons and neutrons in the resultant nuclei are more tightly bound than in the initial, fusing nuclei. This movement to a more stable, lower energy state means that energy is released during fusion.

In order to achieve nuclear fusion, the strong nuclear force must overcome the electrostatic repulsion between the positively charged nuclei. Since the strong nuclear force is a short-range force, this only occurs when nuclei are very close together. An increase in kinetic energy of the nuclei is required to overcome the electrostatic repulsion and bring the nuclei together.^[2] Therefore, stars like the Sun have ideal conditions for nuclear fusion to occur. The high temperatures and pressures allow nuclei to come close enough to fuse. More specifically, the Sun (and other main sequence stars – about 90% of stars in the universe ^[3]) generates its energy through the fusion of hydrogen nuclei into helium.

 

Running out of fuel

Stars do not have an endless supply of hydrogen and so eventually, nuclear reactions will halt in their cores.^[4] Stars, throughout their lifetime, are fighting against the inward force of gravity. The outward pressure generated by nuclear fusion prevents this from happening. However, once all the hydrogen has been used up, the star can no longer prevent its own collapse. The energy generated when stars start to collapse is radiated outwards, pushing the outer layers of the star further from its core.^[5] For stars of a sufficient mass, the collapsing core becomes hot enough for helium (produced from the fusion of hydrogen) to fuse and produce heavier elements.^[4] This temporarily slows down the stars demise until the helium also runs out.

 

Death of low/average mass stars

Stars with a core of up to 1.4 solar masses (the Chandrasekhar limit) will become white dwarfs: the outer layers of the star, which are being pushed away, are eventually ejected leaving behind the stellar core. Astronomers of the past were confused as to why these white dwarfs did not collapse further. The solution came from the Pauli exclusion principle: “no two electrons in an atom can have identical quantum numbers.” ^[6] A consequence of this is that electrons of the same spin cannot occupy the same energy state in the same region of space ^[7] Thus, during stellar collapse, as electrons are squashed into the same region of space, they must fill higher energy states and travel at higher speeds. This exerts an outwards electron degeneracy pressure which prevents further collapse. Over time white dwarfs will cool down until it no longer emits heat or light. At this stage, the star is known as a black dwarf. Black dwarfs remain a theoretical concept since none have been detected yet.

 

Death of high mass stars

The electron degeneracy pressure is not strong enough to prevent gravitational collapse of stars with cores above 1.4 solar masses. However, there is one final lifeline for stars with cores between 1.4 and 3 solar masses. For these stars, protons and electrons which are squeezed together can fuse to form neutrons, producing neutron stars. It turns out that neutrons also obey the Pauli exclusion principle such that no two neutrons can occupy identical states. This generates a neutron degeneracy pressure which withstands gravitational collapse. This pressure is not strong enough for stellar cores above 3 solar masses. As a result, these stars are destined to collapse completely and form black holes.

 

Rebirth of stars

When high mass stars die, they go out with a bang. As they collapse, the temperature of the core reaches a high enough temperature that the helium can fuse to form carbon which can then fuse to oxygen and so on (Neon and then Silicon) through to Iron.^[5] After this, nothing can prevent gravitational collapse. The outer layers also begin to collapse but then they rebound outwards violently releasing vast amounts of energy in an explosion known as a supernova. After this, the star either becomes a neutron star or black hole (as mentioned above). The debris left behind by explosive events such as supernovae combine with surrounding gas and dust to provide the building blocks for new stars.^[4]

Recent evidence (e.g. https://www.pnas.org/content/117/3/1240) suggests that not all massive stars die via supernovae and that they can form black holes or neutron stars more quietly instead.

 

Sources:

1. European Space Agency: https://www.esa.int/Science_Exploration/Space_Science/Herschel/How_many_stars_are_there_in_the_Universe

2. Lumen Learning: https://courses.lumenlearning.com/boundless-chemistry/chapter/nuclear-fusion/

3. Nola Taylor Redd, Space.com: https://www.space.com/22437-main-sequence-stars.html

4. NASA Science: https://science.nasa.gov/astrophysics/focus-areas/how-do-stars-form-and-evolve

5. Professor Dave Explains YouTube Channel: https://www.youtube.com/watch?v=4xIQGbYur9Q

6. Carl Nave, HyperPhysics: http://hyperphysics.phy-astr.gsu.edu/hbase/pauli.html

7. SAO Encyclopedia of Astronomy: https://astronomy.swin.edu.au/cosmos/E/Electron+Degeneracy+Pressure