Stellar Evolution

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During the AGB phase of a star’s life, the star’s core is made of carbon and oxygen, but it’s no longer fusing anything. Surrounding this core are two layers where fusion is still happening. The inner layer is where helium fuses into carbon and oxygen, and the outer layer is where hydrogen fuses into helium. These two layers of fusion are called double shell burning. On top of these layers is a very large and cool outer envelope of gas that is unstable and begins to lose mass due to strong stellar winds. The star’s instability causes it to pulse, and eventually, it sheds its outer layers, leaving behind a white dwarf! Very cool.

Double Shell Burning

Instability in the protostar's surrounding disk causes sudden inflows of material onto the star. This creates a short but powerful brightening event that increases the protostar's luminosity by 100 times. This alters the growth and temperature of the protostar!

Protostellar Accretion Burst

These occur when a collapsing gas cloud does not gather enough mass to form a low mass star (less than 0.08 Solar Masses). These leads to the dwarf not reaching the critical temperature of 10 million kelvin, crucial for hydrogen fusion! Brown dwarfs glow very faintly from gravity, but never begin fusion

Brown Dwarf Formation

2 Rare Cases!

Throughout its life, a star constantly strives to maintain hydrostatic equilibrium. During the main sequence phase, this balance is achieved as the energy from hydrogen fusion generates enough pressure to fight the force of gravity. As a result, the star remains stable in size and temperature for millions to billions of years. However, this equilibrium is disrupted when the star runs out of fusion fuel in its core. Without fusion to support it, gravity causes the core to contract. This often triggers new fusion reactions (such as helium fusion), which briefly restores equilibrium and lead the star into a new phase such as the red giant phase.

This is the balance between the inward force of gravity and the outward force of nuclear fusion in the core of stars. For example, our Sun is in hydrostatic equilibrium, as it is balancing gravity and nuclear fusion during its life on the main sequence.

Hydrostatic Equilibrium

Black holes occur when the remaining core is greater than 2.5 solar masses. Black holes are not made of mass in the usual sense, they are a singularity, which is a infinitely dense point in the universe. It is surrounded by an event horizon, which not even light can escape.

Black Hole

The supernova throws the outer layers of the high mass star. Neutron stars occur when the core of the high mass star is between 1.4 and 2.5 solar masses. Neutrons are extremely dense! These stars are usually 10-15km in size, but have much more mass than the sun.

Supernova

Iron can not be fused to release energy, so the core collapses significantly. This is one of the most violent explosions in the universe.

Neutron Star

Once hydrogen runs out in the core, the core contracts and heats up, allowing for the fusion of heavier elements. Carbon, will fuse to neon, neon to oxygen, oxygen to silicon, and silicon to iron. Each fusion stage is shorter than the last, as silicon to iron only lasts a few days! The outer layers significantly expand as high mass stars enter the red supergiant phase.

Red Supergiant

The core is now fusing hydrogen to helium as it begins its stable life as a star on the main sequence. These stars can shine millions of times brighter than our sun, but only last on the main sequence for millions of years rather than billions. As long as hydrogen fusion balances gravity, these stars will remain stable.

High mass protostars accumulate mass very quickly and can even ignite hydrogen fusion before finishing accumulating mass! It emits energy, but is not yet a star in this stage. The protostar will continue to accumulate mass until nuclear fusion starts, and its life on the main sequence begins.

High mass stars form in significant molecular clouds, as they need much more mass than low mass stars to form. This big clouds accelerate the collection of mass and create bigger collapses.

The hot and dense carbon-oxygen core is leftover as fusion has ended. It will continue to glow for billions of years as it cools down.

White Dwarf

As the red giant runs out of helium, it becomes unstable. The outer layers gently get expelled into space and form a glowing shell of ionized gas. This phase is only 10,000-20,000 years, but it is very pretty!

Planetary Nebula

Hydrogen has been depleted in the core and the core contracts under the pressure of gravity. The outer layers expand and helium to carbon (and oxygen) fusion begins in the core.

Red Giant

This is the longest and most stable period of a star's life. Hydrogen to Helium fusion takes place during this stage and the balance between radiation and gravity keeps the star stable. Our Sun is currently in this stage!

Main Sequence

A dense region of this molecular cloud collapses under gravity. This heats up the core and eventually leads to hydrogen fusion in the core

Protostar Phase

A vast, cold region of space with lots of gas and dust. These clouds are lightyears big and can form thousands of stars! Low Mass stars accumulate mass slowly and do not start fusion before the Protostar phase.

Giant Molecular Cloud

Stellar Evolution

High-Mass Stars ≥8 M☉

Low-Mass Stars ~1 M☉