Life Cycle of Stars

How does star formation begin

Interstellar cloud experiences a gravitational collapse.

The GPE is converted into KE.
According to: KE = 3/2 kT, as energy increases, temperature increases.
This makes a protostar

What is a Protostar

Made from denser clumps of matter which continue to contract and heat up.
Does not give out visible light
No nuclear fusion
The core becomes hotter eventually to reach 10^7

At what temperature nuclear reactions occur

10^7 K

What happens at 10^7 K

Hydrogen nuclei fuse into helium producing thermal energy and radiation.
Mass is converted into energy in this process.
E=mc²
A High temperature is needed to overcome the nuclei's electrostatic repulsion.

Main Sequence Star

• Gravitational force compresses the star, but photons released by nuclear fusion and gas pressure from the nuclei act outward and counterbalance the gravitational contraction.

• A star is stable in hydrostatic equilibrium with an almost constant size. The more massive a star is, the hotter it is, and it converts hydrogen to helium faster; however, it will remain stable for a shorter period.
  

Relationship between star size and stability period

A more massive star will remain stable for a shorter period of time as it will burn through all its available hydrogen in fewer years

Red Giant (0.5-1.0 Solar Masses)

• Nuclear fusion stops when hydrogen in the core runs out.

• Star evolves from the main sequence to a red giant

• Core contracts and heats up as gravity works on the matter in the star’s core

• Shell hydrogen burning happens when hydrogen present in the outer surface of the core is heated from this contraction and is now available to undergo fusion

• The heat released causes the outer layers of gas to expand

• The core is inert and no nuclear fusion occurs

Shell hydrogen burning Red Giant

• Shell hydrogen burning happens when hydrogen present in the outer surface of the core is heated from this contraction and is now available to undergo fusion

• Heat released from this causes the outer layers of gas in the star to expand resulting in the cooling of the star’s outer layers and the formation of the red giant

Is the core of the red giant inert

Yes. No nuclear fusion occurs in it except the thin shell of hydrogen fusion around its surface

How is a planetary nebula formed

The heat from the core of a red giant continues to cause the outer layers of the star to expand
eventually, these layers expand so much that they form a planetary nebula a diffuse cloud of gas

What happens to the core of the nebula

The core is left behind in the centre of the nebula and forms a white dwarf

Are there any nuclear fusion reactions in a white dwarf? Why does it shine?

No therefore there is nothing preventing the gravity from causing the star to contract further
It shines as a result of the huge amounts of thermal energy still stored inside it

What does the stability of a dwarf depend on?

Its mass.
If it is below the Chandrasekhar limit (1.44 Solar Masses) collapse is prevented by electron degeneracy pressure.

If it is above the limit, it will explode into a Type 1a supernova

What happens to the white dwarf when it cools off

Becomes a black dwarf

What is electron degeneracy pressure

• In quantum physics, two electrons cannot exist in the same energy state.

• So when the core of a star begins collapsing under the force of gravity, electrons are squeezed together.

• This creates a pressure which prevents the core from further gravitational collapse.

• This pressure is the electron degeneracy pressure.

What does a star heavier than 10 solar masses turn into

• Once all the hydrogen in its core has been converted into helium the core will still be hot enough for more nuclear fusion to occur.

• The star evolve from the main sequence into a red supergiant

What reaction takes place in the core of a red supergiant

• Helium fusing to make carbon

• If hot enough, carbon fuses to make oxygen

• oxygen to make neon and so on

• Successive stages of fusion occur in shells around the core

• No fusion in outer layers of the star

What happens in successive fusions

• They release more energy heating up the core more

• They take less time

What happens when iron is formed

• No further stages of nuclear fusion occur

• Star has an inert core of iron

• No longer sufficient radiation pressure being produced to counteract the gravity

• So the star collapses

What happens when a red supergiant collapses

• The outer layer of the star rapidly falls inwards and then bounces off the core

• This results in a core-collapse supernova

What happens during the supernova

• The outer layers of the star are ejected away at very high energies

• They leave behind the core, which will become either a neutron star or a black hole depending on its mass

How do heavier elements than iron form

By rapid nuclear fusion processes in the supernovae

What happens if the remnant is less that about 3 solar masses

It forms a neutron star

What happens to produce a neutron star

• The intense pressure of the explosion causes the protons in the star to capture electrons forming neutrons

• Neutron stars are made entirely of neutrons and are extremely dense. About 10 ¹⁷ kg m^-3

• They remain stable due to neutron degeneracy pressure

Black hole is formed if

• The supernova remnant is more than 3 solar masses

• Once a supernova occurs the remains of the star form a black hole

• Its gravitational field is so strong even light cannot escape it

Summary of the life cycle of stars