PRE-STELLAR EXISTENCE. On the whole it seems likely that we must assign ages of 5 to 10 million million years to most or all of the stars in the galactic system. This is as far as we can probe back into time with any reasonable plausibility. The atoms which now form the sun and stars must no doubt have had a previous existence as atoms of a nebula, but we cannot say for how long. The temperatures at the centres of the spiral nebulae may be, and in all probability are, so high that atoms are stripped bare of electrons and so shielded from annihilation. We may in fact regard the gaseous centres of nebulae as a sort of “white dwarfs” built on a colossal scale. This fits on to the fact that the nebulae generate very little energy for their weights and so shine very feebly. We have seen that the weights of two extra-galactic nebulae can be estimated to a reasonable degree of accuracy. The great Andromeda nebula _M_ 31 has the weight of 3500 million suns, its total luminosity being that of 660 million suns. The nebula N.G.C. 4594 has the weight of 2000 million suns, and the luminosity of 260 million suns. A simple calculation shews that the atoms in the Andromeda nebula have an average expectation of life of 80 million million years, while the corresponding figure in N.G.C. 4594 is 115 million million years. From these two instances, we may guess that the average life, before annihilation, of the atoms in such nebulae must be of the order of 100 million million years. It cannot be claimed that this calculation is either very convincing or very exact, but it supplies the only evidence at present available as to the probable length of life of matter in the nebular state. We can say that the stars have existed _as such_ for from 5 to 10 million million years, and that their atoms may have previously existed in nebulae for at least a comparable, and possibly for a much longer, time.