I ofen looked up at the sky an' assed meself the question - what is the moon, what is the stars?
- Captain Boyle, from Sean O'Casey's play Juno and the Paycock

Did you ever look up and wonder at the stars? Why are some brighter than others? Why are they different colours? And will they keep shining for ever? There are many pages on the web explaining these things, but they tend to be very technical, quoting stellar core temperatures, and referring to Hertzsprung-Russell diagrams. There are even some such entries on h2g2. If you understand that stuff, then go and read them. If you want a more gentle introduction, this is the place.

Stage 1: Condensation and Ignition

Throughout the galaxy are clouds of gas and dust. The gas is mainly hydrogen, with some helium; these are by-products of the Big Bang in which the universe began. The dust is left over from explosions of old stars. Gravity pulls the atoms of the cloud together. Gradually the gas cloud condenses into a big lump.

Now for some practical work. Take a bicycle pump, fill it with air, put your finger over the hole at the end so that no air can escape, and press the pump in hard. When you've compressed the gas down to about one 20th of the original size, feel the sides of the pump; you'll find that they are hot. This phenomenon of the heating of a fluid through compression has many manifestations, including for example the ignition systems of diesel engines. Another manifestation is the genesis of stars: the condensing gas cloud, compressed by gravity, heats up as its volume reduces.

If the lump is not big enough, it will form into a sphere, but will not ignite. This is known as a 'brown dwarf'. Such 'failed stars' have been found in space, orbiting around successful stars. There may be lots of these, but they're extremely difficult to detect.

If the lump is big enough, the temperature will be hot enough to start fusion - a nuclear reaction takes place turning the hydrogen in the star to helium, and giving off heat and light. This heat is enough to encourage more fusion, and the whole process becomes self-sustaining. A star is born.

Stage 2: Burning Hydrogen

The star now settles down into what astronomers call the main sequence, because it includes most stars. The star does what it is going to be doing for most of its life: burning hydrogen in its core. This is not a chemical reaction such as when you set fire to some hydrogen in the school science lab. It is a nuclear reaction something like the one in a hydrogen bomb, fusing hydrogen into helium, and it produces far more heat and light than normal burning does.

The burning takes place at the core of the star, where the pressure is greatest. The energy given off heats the outer layers of the star to thousands of degrees, and as a result they glow brightly.

Time for another experiment. Take a thin piece of metal such as a pin or hair-clip and hold it in a strong flame such as a Bunsen burner. When the metal gets hot it will start to emit a dull red glow. As it gets hotter, the colour will change from red to orange, then yellow, then white. Hot gases will also glow with these colours, and so the stars glow with different colours depending on what temperature they are.

The coolest stars will glow red - their temperature is at least 2,000°C - but that is cool compared with the hotter stars, which will glow orange, yellow, white or blue depending on their temperature. We're used to thinking of blue as being a cool colour and red a hot one; in the world of glowing gases it is the other way around. The coolest gases glow red while the hottest ones glow blue.

The table here shows the different types of star. We've included the surface temperatures for reference, but you don't need to worry about them much. It's enough to note which ones are hotter and which are cooler. The letters for the different types are traditional and their order doesn't make any sense, but they're the ones that all astronomers use.

The appearance and behaviour of the star depends entirely on its mass, that is, the amount of hydrogen in it. Small stars burn cooler, redder and dimmer, for longer, while big stars burn hotter, whiter, brighter and burn themselves out quicker.

Red Dwarfs

The coolest stars are red in colour. Astronomers are an imaginative lot: because the stars are small and red, they are called red dwarfs. They are the most abundant type of star in the universe, making up about 75% of all stars, but you won't have seen any. Red dwarfs, as well as being small and cool, are also rather dim, so they can't be seen unless you are really close to them. The closest star to our sun is in fact a red dwarf, and it is so dim that it is invisible to the naked eye. This particular red dwarf is called Proxima Centauri.

Red dwarfs burn their hydrogen so slowly that none of them have yet run out of fuel - the universe, at a mere 13.7 billion years old, is not old enough yet for that to have happened.

Orange, Yellow and Yellowish-White Stars

Stars like our Sun are much bigger than the red dwarfs, and burn with orange, yellow or yellowish-white colour. Most of the stars we see in the sky are of this type; they are far less common than the red dwarfs, but they are visible over long distances while the red dwarfs are not. Such stars seem to last about 10 billion years - that's the estimated length of the hydrogen-burning phase for our sun. Cooler stars will last longer, while hotter stars will finish the fuel sooner.

Our own sun is a little less than half way through its hydrogen-burning phase. There's probably another five billion years or so to go.

White, Blue-White and Blue Stars

These stars are the biggest, hottest and brightest. They burn through their fuel fastest, so that some of these had already run through their entire life before our sun was even born. In the Tarantula Nebula in the constellation of Dorado, we can see stars of this type which are only 40 million years old and are already reaching the end of their lives.

Stage 3 - Expansion

When eventually the star runs out of hydrogen in the core, whether it is after 40 million years or 20 billion years, the star will change dramatically. We don't know what will happen to red dwarfs as none of them have yet reached this stage, but other stars expand until they are enormous in size, although their mass remains the same. At the same time, their surface temperature drops and their colour becomes redder. These are known as red giants, although their colour is more often orange than red.

Here's how it happens. While a star is burning hydrogen in its core, there is a constant flow of energy out from the core. This blows the gas of the star outward. When there is no more hydrogen at the core to burn, this outward pressure stops, and the gas of the star collapses inwards. The pressure builds up to even greater values than before and the core becomes hotter, eventually reaching the temperature where helium will burn. Again this is a nuclear fusion reaction rather than a chemical burning. Three helium atoms combine to make a carbon atom, giving off lots of energy in the process. This energy heats up the shell of hydrogen around the core, and it too starts to burn. The centre of the star is now giving out much more energy than it did in the plain hydrogen burning phase. The outer parts of the star are blown outwards and the star expands enormously. Although it is giving out more energy, the energy is spread over a much greater surface area so the surface cools and turns redder.

This expansion can take quite a while, anything from 10 to 100 million years. This sounds like a long time, but when compared with the 10 billion years that the star has been in a stable hydrogen-burning state, it is quite short. The star Capella (Alpha Aurigae), the sixth brightest in our night sky, is a yellowish star which is in the process of expanding into a red giant.

Stage 4 - Red Giants

Examples of red giants are Arcturus, Gamma Crucis, Antares and Betelgeuse. The orange or red colour of these stars is clearly visible to the naked eye. In fact the name 'Antares' means 'Rival of Mars' because its appearance is comparable to that red planet.

• Arcturus is a prominent bright star in the Northern Hemisphere, the brightest star in the constellation of Boötes and the fourth brightest star in the night sky. It is a fairly typical giant. It is approximately the same mass as our Sun, but has a radius 27 times as big. If Arcturus was at the centre of our solar system, its surface would reach one third of the way out to Mercury, and it would look 27 times as big in the sky as the Sun does.

• Gamma Crucis, sometimes known as Gacrux, is the third brightest star in the constellation of Crux, the Southern Cross. It is clearly visible throughout the Southern Hemisphere. It has a mass three times that of the sun and a radius 113 times the sun's radius.

• Betelgeuse and Antares are so big that they are known as supergiants. They started out as very big stars and now that they are in their red giant phase, they are enormous. Betelgeuse has a radius 630 times that of our Sun, and Antares tops 700 times the Sun's radius. If either of these monsters were positioned in the centre of our solar system, their surface would be out beyond Mars, and our Earth would be inside the star.

Although the core of a red giant is very dense, the outer parts are so spread out that they are extremely thin - much thinner than the Earth's atmosphere. In fact they are as attenuated as the vacuums that are produced in laboratories on Earth. This means that the star does not have a clear surface, but fades off into space gradually. You may have seen pictures of our sun, with its clear circular shape and the corona outside of this. A red giant will have no clear dividing line between star and corona.

Red giants don't burn steadily. They tend to be very variable, and the redder they are, the more unstable they are. Betelgeuse, for example, can vary between a magnitude of 0.3 and 1.2. At its brightest it is the seventh brightest star in the night sky. At its dimmest it drops to only the 19th brightest.

Stage 5 - The End of the Giant Phase

Red giants are burning helium in their core and hydrogen in their outer layers. This process happens faster than the stable hydrogen burning of Stage 2, so stars only spend a short part of their life as red giants, and the bigger they are, the faster they burn.

What happens when they run out of fuel depends on just how gigantic they are. We'll divide them into big giants and little giants.

Stage 5a - Little Giants

Stars of about the size of the sun or smaller, when they reach the end of the Red Giant phase will shed their outer layers and turn into ultra-dense small white stars called white dwarfs.

What happens is that when the star stops burning helium at its core, the outward pressure of the energy produced suddenly disappears. The outer layers of the star collapse inward and then bounce outward off the denser core. These shockwaves cause the centre of the star to be compressed and the outer layers to be stripped off. The star throws off a shell of matter, which expands outward and appears to us as a planetary nebula - nothing to do with planets, it gets its name because the early astronomers could easily confuse such a nebula with a planet when they were planet hunting. The throwing off of shells of outer material may happen a few times in the life of a red giant. Eventually, all that is left is the dense core, which carries on glowing as a white dwarf.

Stage 5b - Big Giants

Really massive stars keep on fusing elements at the core into heavier elements until the core is solid iron. Iron is the most stable element, and can not be fused to produce larger elements without actually absorbing energy, so the core remains as iron. Once all the fuel has run out at the core, there is no outward pressure from the core to keep the outer layers out. Gravity pulls them inwards. The pressure on the core increases, crushing the core inwards. It gets denser and denser until the protons and electrons are compressed together and turn into neutrons. The core becomes a block of solid neutrons. Suddenly the core becomes totally incompressible, as neutrons can't be compressed into anything smaller. This is like the sea suddenly hitting a brick wall. The inward rushing gas suddenly hits the incompressible spherical core, and bounces back outwards causing a shock wave to resound through the star. This tears the star apart - the star explodes, and all the outer layers are shed outwards in a violent blast of energy. The solid neutron core is left behind. We see the explosion as a type II supernova, a sudden bright star in the sky which can outshine all the other stars in the galaxy put together. Although the 'nova' in the name means 'new', what we are seeing is in fact the death of a star.

In the year 1054 AD, astronomers in China recorded a 'guest star' which was so bright that it could be seen during the day for 23 days. At night, it was visible for nearly two years, then it faded away. Looking now at the place where the astronomers recorded the star, in the constellation of Taurus, we find M1, the Crab Nebula, a spectacular interstellar splat of dust and debris which is all that is left of the star which exploded so many years ago.

The wreckage of the explosion of a supernova normally leaves the superdense neutron core of the star behind; such neutron stars may produce flashes of energy as things fall onto them: there is one such flashing 'pulsar' in the middle of the Crab Nebula. Such a neutron star, however, is basically dead.

In extreme cases, theory predicts that the neutron star may be big enough that its gravity overcomes the forces holding the neutrons together and it collapses further into a black hole.

Stage 6 - Life as a White Dwarf

Big giants are destroyed by the explosion. Little giants survive, but with much of the outer layers thrown off. They enter the final phase of their life - as a 'white dwarf'.

White dwarfs are small, very dim stars which shine with a white light, indicating that they are still very hot. A good example of a white dwarf star is Sirius B, the companion of Sirius A, the brightest star in our night sky. Sirius B is almost exactly the same mass as our sun, but has a radius which is less than 1% that of the sun. This puts the star as only three times the radius of the Earth. For it to have all that mass in such a small volume, it must have a density about 125,000 times that of water. While we think of stars as balls of gas, this is far too dense to be gas. It is in fact known as 'degenerate matter', where the atoms have been squashed so that the gaps between the electrons and nucleus have been eliminated.

White dwarfs shine dimly, but they are not actually burning any fuel. They are just glowing because they are hot. It is thought that they continue to shine for a long time, but eventually just cool down and stop shining. This has not yet been verified by experiment, because nobody has ever observed a white dwarf extinguishing.

One Last Fling

Some white dwarfs have a more exciting life. Many of the bright lights in the sky are not in fact single isolated stars but are pairs of stars, orbiting around a common point in space. In any pair, it is likely that one star will be bigger than the other. It will go through its life faster than the smaller star; it will reach the red giant phase and then shed its outer layers to become a white dwarf while its partner is still steadily burning hydrogen in its core. Sirius, the brightest star in the night sky, as has already been mentioned, has a white dwarf companion. Procyon, the brightest star in the constellation of Canis Minor, also has a white dwarf companion.

In cases where the two stars are very close together, this will result in the white dwarf orbiting within the outer atmosphere (corona) of the other star. The intense surface gravity of the white dwarf will pull in the atmosphere of the larger star and the mass of the white dwarf will increase.

If this flow of matter onto the white dwarf is slow, it will cause the white dwarf to temporarily flare up, burning off the excess matter. The star becomes much brighter but then fades back to its former state. Such a flare-up is known as a nova, literally 'new star', because observers centuries ago thought that these were new stars being created. The most famous nova in recent times was in 1975 in the constellation of Cygnus. For a few days at the end of August, the cross of Cygnus had an extra bright star in it, of magnitude 1.7, but then the 'new star' faded away. Novas like this are reasonably common, although not usually as bright as the one in Cygnus.

If the flow of matter from the normal star onto the white dwarf gets too much, then the white dwarf star will grow rapidly. When it reaches a mass of about 1.4 times the mass of the Sun, the so-called 'Chandrasekhar limit', the white dwarf will heat up rapidly and undergo a runaway fusion reaction. The details of this are still being studied by astrophysicists, but the result is that within a few seconds the entire star ignites in a fusion reaction which blows the star apart. This is known as a type I supernova. It is the brightest event in the history of a star, outshining an entire galaxy and even more luminous than the type II supernova mentioned earlier. The white dwarf star is destroyed and its last gift to us appears as a bright star in our sky for six months to a year.

In early November 1572, such a star appeared in the sky in the constellation of Cassiopeia. It was as bright as Venus, and gradually faded over the next two years; some time in 1574 it was no longer visible to the naked eye. The astronomer Tycho Brahe studied this star intensely and as a result it is often known as 'Tycho's Star'. Recent studies have shown the explosion took place about 7,500 light years away from the Earth. For it to have rivalled Venus at such a distance, it must have truly been spectacular.

Star Stuff

We are made of star stuff.
- Carl Sagan, space exploration champion and visionary

The Big Bang created only two elements, hydrogen and helium. Here on Earth, we have 92 naturally occurring elements from hydrogen all the way through to uranium. So where did those other 90 elements come from?

We've already seen that in large red giant stars the reactions in the core can turn helium into carbon. Other elements are produced as well, and the bigger the star, the heavier the elements. When the star goes supernova, its constituent matter is spread across the galaxy, later to condense into new stars and into planets. Our sun and the Earth are formed from the leftovers of previous supernovas.

During the life of the red giant, only elements up as far as number 26, iron, are produced. Heavier elements require a net input of energy for their production and such a process would not be self-sustaining. But the inconceivable violence of the stellar explosion throws many elements together and in these high-pressure chance encounters, the remaining elements from 27 to 92 are formed. It is this process we can thank for the utility of copper, the disinfectant properties of iodine, and the beauty of gold.

It's a sobering thought that we owe the air we breathe, the ground we walk on and the very stuff we're made of to those intense reactions inside red giants and to the explosions in which they died.