Stars are essentially huge balls of gas (mainly hydrogen). The extaordinarily amounts of bright light and heat they emit is caused by nuclear reactions happening within the star. Perhaps the easiest way of describing what is happening inside a star is to explain how they are born:
Stars are born in vast clouds of gas and dust. Dense clumps start to form in these clouds and these clumps start to exert gravitational pull. They start to suck in more gas from the surrounding cloud and this increases their gravitational pull which sucks in more gas. This cycle continues and just like a haystack in summer the inside of the protostar starts to get very hot. If enough material is is hoovered up from the surrounding gas and dust, the protostar will hit a critical point in its life. As the core temperature reaches about 10 million degrees centigrade it ignites, nuclear reactions begin and as they say in Hollywood a star is born.
With its own gravity trying to collapse the star inwardly and the nuclear reactions trying to blow the star apart it manages to find an equilibrium and will settle down to burn for anything between a few million years and thousands of million years. (Our sun is about half way through its 10,000 million year life cycle.)
Why do stars twinkle ?
Stars twinkle because we are viewing them through thick layers of turbulent air in the Earth's atmosphere that slightly distort our view. The closer the star is to the horizon the more it appears to twinkle. This is because the light of stars near the horizon has to travel through more of the earth's atmosphere than the light of stars overhead and so is subject to more distortion. If you were to view the stars from the moon the starlight would not have to travel through an atmosphere and the stars would appear rock steady.
How many stars can be seen in the night sky ?
On a clear night there are roughly 1500 to 2000 stars visible to the unaided eye, although this varies hugely depending on where you are. Cities and large towns cause light pollution that masks many of the fainter stars. If you want to see a really good starry night you need to get away from the cities and towns. The best view I've ever had of the night sky was on a river boat down the Amazon surrounded by millions of square miles of jungle. A cheaper option is to use a pair of binoculars to scan the night sky. You will then see many more stars than you ever could with the unaided eye.
How many stars are there in the universe ?
Okay it's number crunching time. The universe is made up of anywhere between 50 billion to 200 billion galaxies. Our galaxy contains about 100 billion stars so at a rough guess you are looking at:
100 billion multiplied by 100 billion.
I can't possibly get my head around these numbers but a nicer way of putting it is:
"There are more stars in the heavens than grains of sand on all the beaches on earth".
Why is our sun so different from the other stars ?
It isn't. The sun is a rather ordinary star. In fact astronomer's rather unkindly relegate it to the status of a yellow dwarf. The reason the sun dominates our sky is purely down to distance. At a mere 150 million km away the sun is sitting on our doorstep. After the sun the next nearest star is a red dwarf called Proxima Centauri which is over 250,000 times the distance away. If you were to travel out to Proxima Centauri and look back at the sun you would see it as a small faint yellow star.
Is it true that stars are different colours ?
Yes. Stars can be white, bluish-white, yellow, orange or red. It's colour depends on the temperature of the star. We normally associate red with hot and blue with cool but with stars it is the other way round. The hottest stars are white or bluish-white, then yellow, orange and the coolest stars are red. (cool being about 3,000 degrees in this case).
Stars also differ greatly in size from giants that would swallow up half our solar system to superdense dwarf stars. When stars are in main sequence it is the larger stars that burn up their fuel more quickly and die. Smaller stars like our sun tend to conserve their fuel and last for much longer.
What is a supernova ?
Stars that have managed to suck in enormous amounts of material at birth erupt into life as supergiants. These stars as well as being mammoths burn incredibly fiercely and after a relatively short lifetime will run out of fuel. As this happens the vast mass of the star collapses in on itself and the core becomes incredibly upset and volatile. This causes a tremendous reaction and the star blows itself apart. This explosion is called a supernova: the most cataclysmic event in the universe.