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What is a Star?

Particle Physics and Astronomy Research Council

Royal Greenwich Observatory
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Information Leaflet No. 7: 'What is a Star?'.

What is a Star?

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 force the Sun towards its centre, and if there were not some other force counteracting it the Sun would collapse. The necessary outward pressure is produced by the radiation from the nuclear energy generation in the Sun's interior.

How do stars originate?

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 potential gravitational energy. This is turned into heat, which in the early days of the embryo star can easily escape, and so the gas cloud stays cool. As the cloud's density rises, it gets more and more difficult for the heat to get out, and so the centre gets hot. If the cloud is big enough, the temperature rise is sufficient for nuclear reactions to take place. This generates more heat, and the 'burning' of hydrogen into helium takes place, as in the Sun. The object is then a star.

The Early Evolution of a Star:

In its early stages the embryo 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.

ARVAL's Note:
See Embryonic stars emerge from Evaporating Gaseous Globules "EGGs" and The Orion Nebula in ARVAL's Gallery.

The Main-Sequence:

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 the Figure below). 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 sooner for the more massive stars than for 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.

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.

Medium mass stars: Stars similar in mass to the Sun '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 radiation pressure to balance gravity. The centre of the star thus contracts until it is hot enough for helium to be converted into carbon. The hydrogen in a shell continues to 'burn' into helium, but the outer layers of the star have to expand. This makes the star appear brighter and cooler and it becomes a red giant.

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. Eventually, in the more massive stars of the group, the carbon may be 'burnt' to even heavier elements, but eventually the energy generation will fizzle out and the star will collapse to what is called a 'degenerate white dwarf'.

Hertsprung-Russell Diagram

The Hertzsprung-Russell diagram of the nearest stars and the brightest stars. 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 10,000 times brighter than the Sun at the top, and those only 1/10,000th of its brightness, at the bottom.

Small mass stars: 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'.

High mass stars: There are very few stars with 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, the addition of more nucleons to the nucleus produces energy, and so yields a small contribution to the balance inside the star, between gravity and radiation. To add more nucleons to the iron nucleus requires energy, and so, once the centre of the star consists of iron, no more energy can be extracted. The star's core then has no resistance to the force of gravity, and once it starts to contract, a very rapid collapse will take place.
The protons and electrons combine to give a core composed of neutrons, and a vast amount of gravitational energy is released. This energy is 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 were 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.