Life Cycle of a Star

Protostar

• clouds of gas and dust (nebulae) collect together due to their gravity

• they begin to spin due to conservation of angular momentum causing them to come closer to the centre making a dense core that increases in temperature creating a Protostar

• The star is surrounded by a circumstellar disc of material

• Once this temperature is great enough to overcome the electrostatic repulsion between hydrogen nuclei, fusion begins to take place further increasing the temperature of the star

• The stellar wind is produced which blows away the surrounding material

Main Sequence

• Most stable state as gravity inwards is in equilibrium with the fusion force outwards

• Hydrogen fused into helium

• When the proportion of helium in the core reaches 10%, the fusion takes place only on a shell outside the core causing the star to grow in size

• then cools but luminosity remains constant

What happens to a main sequence star if it has a solar mass of < 3 and > 3

>3 = red super giant

<3 = red giant

Red Supergiant and Red Giants

as the supply of hydrogen diminishes, the star becomes hotter allowing helium to be fused into heavier elements.

This causes the expansion of the stars outer layers and its cooling.

Red Supergiants work the same just at a larger scale and can fuse elements up to iron and collapses to become a supernova.

White Dwarfs

When a red giant ( <1.4 solar masses) collapses after losing its fuel, it becomes a white dwarf as the lack of fusion means gravity is the only force acting on the star.

Therefore, the star begins to collapse and become extremely dense.

The outer layers are thrown off to form planetary nebulas.

Cools to become a black dwarf.

Supernova

For stars greater than 1.4 solar masses, a supernova is formed. The fusion stops when fuel runs out causing the start to rapidly collapse. The core becomes extremely rigid to the point the atoms inside can no longer compress.

The falling outer layers then meet this rigid core and are shot out in a shockwave.

As the material from the outer layers shoots out, elements heavier than iron fuse together and flung into space.

Supernova have rapidly increasing absolute magnitude.

Neutron Star

If a supernova has a solar mass of 1.3-3 solar masses, it will become a neutron star.

The core collapses to a point where gravity forces protons and electrons together to form neutrons.

Incredibly dense

Pulsars are spinning neutron stars that release beams of radiation from their magnetic poles.

Black holes

This occurs in stars that have a solar mass greater than 3. This means that the force of gravity is too great for neutrons to overcome forcing them together.

This causes a great gravitational pull that not even light can escape the event horizon as the escape velocity is greater than the speed of light.

Can calculate the size of the event horizon using the schwarzchild radius.