The basic difference between a star and a planet is that a star emits light produced in its interior by nuclear 'burning', whereas a planet only shines by reflected light.
There seem to be an enormous number of stars that are visible to the naked-eye at a really dark site but, in fact, the eye can only see about two thousand stars in the sky at one time. We can see the unresolved light of many thousands more when we look at the Milky Way, and the light of the Andromeda galaxy, which can be seen by the eye, comes from thousands of millions of stars.
The Sun is our own special star yet, as stars go, it is a very average star. There are stars far brighter, fainter, hotter and cooler than the Sun. Basically, however, all the stars we can see in the sky are objects similar to the Sun.
The Sun (and any other star) is a great ball of gas held together by its own gravity. The force of gravity is continually trying to compress the Sun towards its centre. If there were not some other force counteracting gravity, the Sun would collapse. Outward pressure is produced by the radiation from nuclear energy generation in the Sun's interior.
Stars form from concentrations in huge interstellar gas clouds. These contract due to their own gravitational pull. As the cloud gets smaller it loses some of the energy stored in it as gravitational potential energy. This is turned into heat which, in the early days of the embryonic star, can easily escape and so the gas cloud stays cool.
As the cloud's density rises, it becomes more and more difficult for heat to escape and the temperature at the centre of the cloud rises. If the cloud is big enough, the temperature rise is sufficient for nuclear fusion reactions to begin. These generate more heat and the 'burning' of hydrogen into helium begins, as in the Sun. The object is then a main sequence star.stellar evolution.
In its early stages the embryonic star is still surrounded by the remains of the original gas cloud from which it formed. By this stage the cloud remnant takes the form of a disk around the star. The radiation from the star gradually dissipates this disk, possibly leaving behind a system of smaller objects, planets.
The star now settles down to a long period of stability while the hydrogen at its centre is converted into helium with the release of an enormous amount of energy. This stage is called the main-sequence stage, a reference to the classical Hertsprung-Russell diagram (see Figure).
The horizontal axis shows spectral type and temperature from the hottest stars on the left to the coolest on the right. The vertical axis shows the luminosity of the stars with those 1,000,000 times brighter than the Sun at the top and those only 1/10,000th of its brightness at the bottom. The curved line marks the Main Sequence – stars, including the Sun, which are fusing hydrogen into helium. The group at the top right, including Betelgeuse and Aldebaran, are Red Giants. The group at the bottom left, including Sirius B, are White Dwarfs.
Most stars lie in a well defined band in the diagram and the only parameter that determines where in the band they lie is the star's mass.
The more massive a star is the quicker it `burns' up its hydrogen and hence the brighter, bigger and hotter it is. The rapid conversion of hydrogen into helium also means that the hydrogen gets used up at a greater rate in the more massive stars than the smaller ones. For a star like the Sun the main-sequence stage lasts about 10,000,000,000 years whereas a star 10 times as massive will be 10,000 times as bright but will only last 100,000,000 years. A star one tenth of the Sun's mass will only be 1/10,000th of its brightness but will last 1,000,000,000,000 years, longer than the current age of the Universe.
Post main-sequence evolution
Stars do not all evolve in the same way. Once again it is the star's mass that determines how they change.
Our knowledge of the evolution of these stars is purely theoretical because their main sequence stage lasts longer than the present age of the Universe, so none of the stars in this mass range has evolved this far!
We believe that the evolution will proceed as for the medium mass stars except that the temperature in the interior will never rise high enough for helium 'burning' to start. The hydrogen will continue to 'burn' in a shell but will eventually be all used up. The star will then just get cooler and cooler ending up after about 1,000,000,000,000 years as a 'black dwarf'.
During the red giant phase, a star often loses a lot of its outer layers which are blown away by the radiation coming from below. The star becomes a planetary nebula (like M57, right).
They 'burn' hydrogen into helium in their centres during the main-sequence phase but eventually there is no hydrogen left in the centre to provide the necessary pressure to balance the inward pull of gravity. The core of the star contracts until it is hot enough for helium to be converted into carbon. Hydrogen continues to fuse into helium in a shell around the core, but the outer layers of the star have to expand. This makes the star appear brighter cooler and it becomes a red giant.
Eventually the energy generation will fizzle out and the star will collapse to what is called a 'white dwarf'.
There are very few masses greater than five times the mass of the Sun but their evolution ends in a very spectacular fashion. As was said above, these stars go through their evolutionary stages very quickly compared to the Sun. Like medium mass stars, they 'burn' all the hydrogen at their centres and continue with a hydrogen 'burning' shell and central helium 'burning'. They become brighter and cooler on the outside and are called red supergiants. Carbon 'burning' can develop at the star's centre and a complex set of element `burning' shells can develop towards the end of the star's life. During this stage many different chemical elements will be produced in the star and the central temperature will approach 100,000,000°K.
For all the elements up to iron, nuclear fusion into heavier elements produces energy and so yields a small contribution to the balance inside the star between gravity and radiation. Fusion of iron into heavier elements, however, uses energy rather than releasing it. Once the centre of the star consists of iron, no more energy can be generated, and there is no longer a source of pressure to counteract the crushing pull of gravity. The star's core then starts to contract rapidly, collapsing on a timescale of less than one second. The protons and electrons in the core are crushed together to form neutrons, releasing a flood of neutrinos, which carry away most of the energy from the explosion.
The core collapse in the dying star releases a vast amount of gravitational potential energy, sufficient to blow away all the outer parts of the star in a violent explosion, and the star becomes a supernova. The light of this one star is then as bright as that from all the other 100,000,000,000 stars in the galaxy. During this explosive phase all the elements with atomic weights greater than iron are formed and, together with the rest of the outer regions of the star are blown out into interstellar space. The central core of neutrons is left as a neutron star which could be a pulsar.
What is remarkable about this is that the first stars were composed almost entirely of hydrogen and helium and there was no oxygen, nitrogen, iron, or any of the other elements that are necessary for life. These were all produced inside massive stars and were all spread throughout space by such supernovae events. We are made up of material that has been processed at least once, and probably several times, inside stars.