The danger lies in the fact that the sun is already perilously near to the left-hand edge of the main-sequence. According to Redman’s determinations, which are probably by far the most reliable at present available, the main-sequence belt of stable configurations for stars of the same spectral type as the sun (G 0) extends roughly between stellar absolute magnitudes, 4·88 and 3·54, the former marking the dangerous left-hand edge. The sun’s present absolute magnitude is estimated as 4·85. Thus if the sun were to become 0·03 magnitudes fainter, this representing a reduction of only 3 per cent. in its luminosity, it would arrive exactly at the edge of the main-sequence, and would proceed to contract precipitately to the white dwarf state. In so doing, its light and heat would diminish to such an extent that life would be banished from the earth. The known white dwarf star which it would most closely resemble is the companion of Sirius, and this emits only a four-hundredth part as much light and heat as the sun. To put the same thing in non-technical language, the sun is in, or is not far from, a precarious state in which stars are liable to begin to shrink and in so doing to reduce their radiation to a tiny fraction of that at present emitted by the sun. The shrinkage of the sun to this state would transform our oceans into ice and our atmosphere into liquid air; it seems impossible that terrestrial life could survive. The vast museum of the sky must almost certainly contain examples of shrunken suns of this type with planets like our earth revolving round them. Whether these planets carry on them the frozen remains of a life which was once as active as our present life on earth we can hardly even surmise. This may be thought to open up a startling prospect for the earth, but we can take courage for several reasons. In the first place a 3 per cent. decrease in the sun’s luminosity can hardly occur in less than about 150,000 million years. This in itself is not too bad, but the prospect becomes enormously more hopeful when we reflect that the evolution of the stars, including the sun, takes place in a direction almost parallel to the edge of the main-sequence. The sun is not heading for the precipice, so much as skirting along its edge. Whether it is approaching the edge, and is ultimately destined to fall over, we do not know, but it is in any case unlikely to reach the edge within the next million million years.