LUMINOSITY. A far greater range is shewn, for instance, in the luminosities of the stars—in their candle-powers measured in terms of the sun’s candle-power as unity. The most luminous star known is _S_ Doradus, already mentioned, with 300,000 times the luminosity of the sun, while the least luminous is Wolf 359 with only a fifty-thousandth part of the luminosity of the sun. The range of stellar luminosities, as of stellar weights, extends about equally on the two sides of the sun, so that the sun is rather a medium star in respect both of weight and luminosity. It is medium in the sense of being about half-way between extremes, but we have seen that there are many more stars below than above it. In comparison with the very moderate range of stellar weights, the range of luminosity is enormous; _S_ Doradus is 15,000,000,000 times as luminous as Wolf 359. If _S_ Doradus is a lighthouse, Wolf 359 is something less than a firefly, the sun being an ordinary candle. If the sun suddenly started to emit as much light and heat as _S_ Doradus, the temperature of the earth and everything on it would run up to about 7000 degrees, so that both we and the solid earth would disappear into a cloud of vapour. On the other hand, if the sun’s emission of light and heat were suddenly to sink to that of Wolf 359, people at the earth’s equator would find that their new sun only gave as much light and heat at mid-day as a coal fire a mile away; we should all be frozen solid, while the earth’s atmosphere would surround us in the form of an ocean of liquid air. So far as we know, there is no possibility of the sun suddenly beginning to behave like _S_ Doradus, but we shall see later that the possibility of its behaving like Wolf 359 is not altogether a visionary dream. SURFACE-TEMPERATURE AND RADIATION. Sirius has been found to have the highest surface-temperature of all the stars near the sun; it is about 11,000 degrees, or nearly double that of the sun. Going further afield, we find many stars with far higher surface-temperatures. For instance, Plaskett’s star is credited with a temperature of 28,000 degrees, although it must be admitted that a substantial element of uncertainty enters into all estimates of very high stellar temperatures.