Stellar Evolution

Star Birth & Death

How long it lives for and what it eventually becomes, is dependent on how much mass there is when it is formed. There are many different types of stars as you have seen (or will on the next page) and how large they are will determine their fate.

Star Birth

Stars are born in molecular clouds.

A source of gas and dust develops made up of hydrogen molecules.

These start to fragment into dense clumps and accrete more material.

They group further under gravity and increase temperature.

It produces an accretion disc that spins rapidly around it, gathering more material into it.

A star develops and nuclear fusions take place.

If there is not enough matter for nuclear reactions to take place, the body will become a brown dwarf.

Excess material is blown away from the force of the initial nuclear reaction and later forms planets and other bodies around the stars.

Star Death

During the life of a star it will convert is hydrogen in its core into helium. When it runs out of hydrogen, start burning helium and other elements.

Depending on the mass of the star there are several different outcomes. It is probably best we do this as a table. For Fun. Well not really, anyway…

M☉ = 1 Solar Mass

Table of Type, Process, Outcome, Masses

Star	< 1 M☉	1-4 M☉	4-25 M☉	>25 M☉
Type	A red dwarf lives a long time before	Mid Main Sequence Star burnt hydrogen and swells to a giant.	Short lived star swells to  a sugergiant	Shorter lived star swells to  a sugergiant
Process	Slow Process of eventual change	Outer layers expel themselves leaving a planetary nebula	Star has burnt all gases. Explodes leaving a supernova remnant	Star has burnt all gases. May explode as a supernova remnant
Outcome	Shrinks into a white dwarf	Shrinks into a white dwarf	Contracts to a Neutron Star	Collapses entirely into a Black Hole

What happens after a star is a white dwarf? It is thought by some that the thermal energy will eventually fade, stop emitting any light and turn into a black dwarf. These are hypothetical stars as none have been observed as the universe is 13.5 billion years old and the process is thought to take place over a longer period.