Stars

We all know roughly what a star is from school and stuff:
It's a big burning ball of gas in space.
Our sun is a star.
It creates our heat and radiates energy on to the earth.
Without it, nothing on the planet could live. There would be no such thing as life.
We revolve around it in an orbit with the rest of the planets nearby because it is huge and has a large mass which gives it a large gravitational pull.
The Latin word for our sun is Sol hence the term Solar System and the development of our Solar Calendar.
A galaxy is a big group of stars, each with their own solar systems.
There are billions of stars in a galaxy.
There are billions of galaxies in the universe.

A Star

A star is not actually made up of gas but rather plasma.
Plasma is effectively a state or matter phase (like liquids and solids) which is a gas with a proportion of its atoms ionized.
Ionisation is when a neutral atom gains or loses electrons creating anions (More electrons than protons) and cations (More protons than electrons).
Plasma and gas have only a few differences, the main one probably being the ability to conduct electricity.
Air is a really good insulator which is why it is used for insulation in houses between bricks and between panes in double glazed windows, but plasma has a practically infinite conductivity.
Plasma makes up the majority of matter in the universe by a long shot.
Every star is plasma (Which as we know is a lot of stars) and the majority of the vacuum between everything in space (although sparsely spread) is plasma as well.

Stars are born when loads of big clouds of dust and hydrogen and helium collide together and create a huge dense mass and the elements ionize and it makes a star.

Stars 'burn' through thermonuclear fusion where atoms collide and each nucleus joins together which creates a lot of energy. Almost every element that isn't man made was created by a star.

Our Sun

Our sun is the biggest thing in our solar system by quite a lot.

It is obviously the most prominent as we all see it almost every day and we don't see much else of the solar system except the planet we are on.

The sun itself makes up 98% of the entire mass of the solar system. That gives you an idea of how big it is. 

It would take 109 planets the size of earth to cover the solar disk (the face of the sun that we see - the bit the moon goes in front of to make an eclipse).

In terms of overall mass the sun is equivalent to just over 1.3 million planet earths. 

This diagram shows the structure of the sun. The middle of the sun is called the core and this is where the thermonuclear reactions take place as described earlier. Thermonuclear sounds complicated and sciencey but actually 'Thermo' just means heat and it is called a 'Nuclear' reaction because it is literally the nuclei of the atoms reacting. 

The temperature at the core of the sun is 15,000,000° C which is quite hot. The pressure in there is 340 billion times earth air pressure because of the huge mass and gravity field that holds it all together. 

It is this huge temperature and pressure that makes the nuclear reactions. 

What happens is that 4 hydrogen nuclei fuse together which creates a helium nucleus. 

This finished helium particle has about 0.7% less mass than the original 4 nuclei and that remainder of energy gets released up through the layers of the sun through convection. Once it reaches the outside it is released as heat and light energy. The time for the energy to convect up through the layers of the sun is 1000000 years. 

Every second the thermonuclear process converts 700 million tons of hydrogen into helium. Out of this process, 5 million tons of energy is produced. 

That means that the sun is gradually getting lighter. 

The photosphere is the outside of the sun and it is the bit that we can see from here. The temperature is 6000°C on the photosphere.

The layer above this is called the chromosphere and that is where stuff like solar flares happen.

Finally there is the corona which is like the glow of the sun caused by luminescent gas from the chromosphere. 

Our sun is about half way through its life time at the moment. It has been active for around 4.6 billion years and has enough fuel for about another 5 billion.

Towards the end there will be less fuel and the sun will start fusing into denser elements and it will start to swell up and get bigger. Eventually it will be so big that it will swallow the earth completely. 

This stage is called a 'Red Giant' and after a billion years of that it's mass will become too great and it will collapse in on itself and become a 'White Dwarf' 

After that it could take a trillion years to fully cool off.