Life Cycles of Stars

Stars are formed from massive clouds of dust and gas in space.

• Gravity pulls the dust and gas together
• As it falls together it gets hot. A star forms when it is hot enough for nuclear reactions to start.
• ^^^This releases energy and therefore keeps the star hot

During the main sequence period of its life cycle, a star is stable because the forces in it are balanced. 

The outward pressue from the expanding hot gases is balanced by the force of the stars gravity.

The star maintains it energy output for millions of years due to the massive amounts of hydrogen it consumes. This stable period is called a MAIN SEQUENCE STAR.

There are two paths in which a star can follow.

ALL stars start off as a protostar and then as a main sequence star, like the sun.

Eventually after the hydrogen runs out the star can go either two ways. 

Stars that are about the same size as our sun go....

Main sequence star -> Red Giant -> White Dwarf -> Black Dwarf

Stars that are much bigger than our sun go....

Main sequence star -> Red Super Giant -> Supernova and then can either go Neutron star or Black Hole 

Nuclear fission is the splitting of a nucleus to generate heat

Used in nuclear power stations in the rods. 

The two fissionable isotypes which can be used are uranium -235 and plutonium 239.

• They fire a neutron into the nucleus  
• The nucleus splits into two nuclei and releases 2 or 3 neutrons 
• Energy is released
• The neutrons released can then go on and do what the inital neutron did and split further nucleus 

CHAIN REACTION

Reaction has to be controlled 

Amount of neutrons released can be increased or decreased in the reactor using substances that can absorb the neutrons if reaction is going too fast or if its going too slow the substances can be removed.

Joining of two nuclei to make one larger nucleus 

Nucleus of one atom joining with the nucleus of another atom.

When the two 'fuse' together they create a very large amount of energy 

Problem with the process is that each atom has a nucleus with a positive charge - so they will repell.

(Inside stars there is mainly helium and hydrogen)

There are two factors that allow the two to join to join together - Lots of heat, lots of gravity

^^This means there is enough energy in the two nuclei for them to join together

If we could do this on earth (as atm it only happens in stars) we could harness the energy and use it in power stations for electricity ect. This would be good as it is non toxic and non radioactive