For ages men had known that the sun was dying. In our day men can tell that within the next ten or eleven billion years it will become a closed star--not a cold star but a closed star. The energy of the sun comes from the destruction of the matter of which it is composed, which becomes floods of energy. This change is possible at a temperature of 40,000,000 degrees C., but below that it cannot take place. Thus, at the center of the sun, where this change is taking place, the matter is at that terrific temperature. As the sun grows older, more and more of the matter sinks into the center and reaches the region of awful heat. The atoms are so violently colliding with each other at that temperature, that the atoms themselves are knocked to pieces by the violence of their collision. If the molecules of a substance collide sufficiently violently, they are broken up. Thus, at 5000 degrees, the molecules of water collide so violently that they cannot maintain themselves, and the shocks break them down into hydrogen and oxygen atoms. But at 40,000,000 degrees the atoms collide so violently they are decomposed into protons and electrons. At this temperature, a further, subtle change takes place, and the electrons and the protons suddenly are gone, and in their place is an equal mass of energy. For energy in any form has mass, and mass in any form is a measure of the energy content. Thus to say "one gram" is an easier way of saying "nine hundred million million million ergs," but the two mean the same to Nature. Now an atom is something like a porcupine with his quills up; it is much bigger in looks than in fact, only an atom has much longer "quills." An atom has much more empty space than anything else. Suppose our porcupines have quills a mile long. If all those quills are on end we won't be able to pack the animals very closely, but if we can induce them to become more friendly and lay the quills down, then the density of our imaginary population of porcupines will be greatly increased. Similarly the atoms, with the electrons revolving in wide orbits, occupy a much greater space than they really need. In the tremendous heat of the Sun, the atoms are so battered, the electrons are knocked off the nuclear protons, and we can imagine the quills now lying down. The density will be far greater. This is demonstrated by the density of some stars which are now known to have a density of over 1000. This is the result of packing the electrons and protons in the center, which is gradually going on in all stars. Gravity increases four times if the distance is halved. As the matter inside becomes denser and denser, the star contracts, till finally its density reaches a tremendous figure.