THE LIFE OF A STAR

During the life of a star the pressure of the overlying layers of gas consisting of hydrogen with an admixture of up to 20% helium, causes the temperature in the nucleus of the star to rise to more than 10 million degrees Kelvin, (10 x 10⁶)°. At this temperature and pressure the individual protons of the hydrogen atoms are stripped of their electrons. Normally each proton which carries a positive electric charge is accompanied by an electron which carries a negative electric charge; the opposite charges thus producing a neutral atom. The mass of the proton is 1836 times that of the electron and the proton occupies the central point of the atom. One must not think of the electron as if it revolves around the proton in the same way that a planet revolves around the Sun. Rather, one must postulate the space around the Positive proton as being curved by the gravity brought about by the mass of the proton and having a wrinkle or wave in the space-time; here at one moment and there at the next moment pulsating all the time. Wave mechanics describes the electron by equations which show that the electron can be fully described by a wave motion. When the atom absorbs energy the electron or wrinkle in space-time jumps further away from the central proton but this jump occupies no time and when the atom radiates energy, it is because the wave of negative electric charge has jumped to a position nearer the central proton. Wave mechanics has shown that the "orbits" of the electrons can only exist at particular distances from the proton, these distances being in the ratios 1², 2², 3² . . .

In the nucleus of a star, therefore we have a plasma consisting of free positive charges, charges (protons) and free negative charges (ELECTRONS). The plasma is considered as being the fourth state of matter, the other three being solids, liquids and gases.

Because of the pressure under which the protons of the plasma occur two protons are literally squeezed into each other, despite the repelling forces which the positive charges of the two protons exert on each other. This comes about because one of the protons sheds its positive charge which is shot out as a positron - a fuzzy wave of positive electricity having the same mass of the electron:

Here H stands for the proton; the figure at the top indicates the mass which is taken to be one atomic mass unit (1 amu), and the lower figure as indicating 1 positive charge of electricity. e+ stands for the positron. When two protons fuse together a deuteron is formed. It has a mass of 2 amu but still only one positive electric charge; one of the protons having shed its positive charge by shooting it out as a positron. This proton therefore becomes a neutron. The neutron was discovered by J Chadwick of Cambridge University, England in 1932. For this advance he received the Nobel Prize for physics. Whilst the mass of the proton is 1,00758 amu, that of the neutron is 1,00893 amu, thus slightly heavier than the proton. Although this difference in mass is very small, the energy involved when these particles react, can be calculated from Einstein's equation E = mc². When the mass m is multiplied by the square of the speed of light, we see that the energy involved is enormous.

The nucleus of mass 2 amu is the deuteron. This nucleus readily attracts another proton, forming which contains 2 protons and 1 neutron. Because the electric charge on this nucleus is 2 instead of 1 it is a nucleus of helium whose mass is 3 and which has two positive electric charges. Another proton gets crushed into this nucleus and another positron is shot out, thus forming a nucleus of helium of mass 4 amu and positive charge 2: . In effect 4 protons have been converted into helium of mass 4 amu and positive charge 2. The amount of mass which is converted into energy in these reactions is 0,02863 amu or 0,7%. When this is multiplied by the square of the speed of light, we find an amount of energy sufficient to boil away almost 10 million kilograms of ice cold water (a dam 1 metre deep and 35 metres in diameter) from 4,03252 grams of protons which are converted into helium; but only 0,7% of the mass of the 4,03252 grams of protons are annihilated. The nucleus is very stable. It is the "ash" left over from the fusing (burning) of hydrogen (protons). In this nucleus the four separate particles have been packed very tightly together. Physicists speak of the packing fraction to indicate the amount of crushing together which the particles have undergone when 4 separate protons change into one helium nucleus.

All atoms that have masses that are multiples of 4 are very stable. Their nuclei apparently consist of 2 or 3 or 4. . . helium nuclei.

During the lifetime of a star in the Main Sequence (10 milliard years in the case of the Sun and other similar stars but only 31 million years in the case of a massive star) nuclei heavier than helium are formed, such as carbon of atomic mass 12; oxygen 16; neon 20;magnesium 24; silicon 28; sulphur 32; calcium 40; titanium 48; chromium 52 and iron 56. Many other atoms of various masses are formed. Atoms having the same charge but different masses are called isotopes. Many of the isotopes are radioactive. Up to the stage when iron is formed, the packing fraction consistently increases so that iron has the highest packing fraction of all atoms. When lighter atoms (up to iron) are fused together, energy is set free. In order to fuse atoms heavier than iron energy has to be supplied.

When a certain amount (about 3/4) of the hydrogen of a main sequence star has been consumed, the outward pressure of the radiation from the centre decreases radically so that the pressure of the overlying layers causes the star to collapse catastrophically on its centre. A star like the Sun will reach this stage after about 10 milliard years. The collapse brings about such a great increase in temperature (up to 100 million degrees) that the helium ash undergoes fusion into carbon 12. This new increase in liberated energy blows the upper layers of the star into space. The star is now on its way to become a red giant -- red because the temperature of the upper layers decreases.

the red giant has reached a size large enough to fill the orbit of Jupiter. The core of the star is now a white dwarf which can go on existing almost to infinity eventually becoming a black dwarf. The white dwarf, surrounded by its overlying gas layers is what is called a planetary nebula. The temperature of the white dwarf is about 10 000 degrees.

Jan Eben van Zyl