Lifecycle of a Star

If you have ever looked up at the night sky, you will see millions of tiny bright dots scattered across. Each dot is a star just like our Sun. Some of these stars are around or smaller than the size of the Sun, while others can be 100,000 times bigger. Stars may seem like they will last for eternity, but that is not true. Although they live for a long time, they too have an end.

Every star’s life starts the same, but they end differently. All stars are born in stellar nebulae, which are large clouds made of dust and gas. A star’s lifetime depends on its mass, as much larger stars will use their fuel up more quickly than smaller stars. All stars go into the main sequence phase. The main sequence phase has two sections: one for low mass stars between 0.8-8 solar masses, and one for high mass stars that are greater than 8 solar masses. The main sequence stage is the longest stage of a star’s life. During the main sequence, a star’s core fuses hydrogen into helium. This nuclear fusion creates an outward pressure that counters gravity’s inward pressure. This balance between the two pressures preventing the star from collapsing is known as hydrostatic equilibrium. 

After millions and billions of years, the star’s core finally loses its hydrogen fuel. This causes the core to contract and build heat. The outer layers begin to expand and cool because the star starts using hydrogen outside of the core. Now the star enters the red giant phase. The cooler temperature of the star is what makes it turn from blue/yellow to red. The contractions and build up of heat starts nuclear fusion again in the core. This time the core is going through a process known as the triple alpha process. The triple alpha process fuses helium into carbon and oxygen. After millions of more years, the fusion stops and the red giant becomes unstable. This creates strong stellar wings that shed its outer layers. This causes the remaining core to be surrounded by very hot gas, creating a planetary nebula. The core is very dense, bright, and hot known as a white dwarf. The white dwarf gives off ultraviolet radiation which lights up the gas around it. The planetary nebula stage is quite short compared to the other stages as it only lasts tens of thousands of years. The gas slowly drifts off in space, starting the white dwarf stage of a low mass star. White dwarfs do not produce heat and light from fusion, as they do not have enough energy. Instead, they emit the heat and light that remained from previous stages. Since the white dwarf is extremely dense, it reaches a state known as electron degeneracy. Normal relationships between density, pressure, and temperature do not apply to degenerate matter. As the mass of a white dwarf increases, its radius decreases. If the mass of a white dwarf exceeds roughly 1.4 solar masses, it will become unstable and result in a type 1a supernova that turns the white dwarf into a neutron star. This mass limit is known as the Chandrasekhar Limit. White dwarfs are really small, so they lose heat very very slowly. At the end of this cooling, a white dwarf becomes a black dwarf, the final stage of a low mass star. A black dwarf is basically a white dwarf that does not emit any heat or light. This process of cooling is so long that there are no black dwarfs in the universe. This makes black dwarfs only a part of theory. Low mass stars end their life in a very quiet and small manner. However, high mass stars that enter the red supergiant phase end their life in a complete opposite way.

High mass stars enter the red supergiant phase after they lose their hydrogen fuel. The red supergiant fuses helium, then fuses carbon, and keeps fusing heavier and heavier elements until the core is completely made of iron. Fusion ends with iron because iron is so heavy that it needs more energy than it releases, therefore making iron fusion absorb energy rather than release it. After fusion is over, the star collapses from its own gravitational weight. This collapse creates a supernova explosion which is really big and bright. The supernova leaves only the star’s core left. The material that was blown away surrounds the core creating a supernova remnant. Eventually, the material will drift off and form part of a cloud of gas and dust. This cloud known as a stellar nebula is where stars are born as previously stated. This is how elements from the very first stars are recycled. The remaining core from the supernova is a neutron star. The core is so small that when the star was squashed, the atoms smashed to pieces leaving only neutrons behind. Neutrons are sub-atomic particles that have no charge. Neutron stars are extremely dense. Some neutron stars rapidly rotate and emit beams of radiation from their poles. These neutron stars are known as pulsar stars. If the remaining core of a red supergiant has a mass greater than 3 solar masses, the inward pressure from gravity will overcome the outward pressure from neutron degeneracy. As a result, the core will become a black hole instead of a neutron star. This mass limit is known as the Tolman-Oppenheimer-Volkoff Limit. If you want to know more about black holes, you can learn all about them in my other blog about black holes. Betelgeuse, a red supergiant in the constellation Orion, is very close to its end. It is expected to go supernova very soon. Hopefully it will happen in our lifetime. The high mass star will live the rest of its life as a black hole. If there is an end to the black hole stage, we do not know about it since no black holes that we have detected have disappeared.

Source:Stellar Evolution | The Schools' Observatory