When we accept the evidence of our unaided eyes and describe the Sun as a yellow star, we have summed up the most important single fact about it at this moment in time. It appears probable, however, that sunlight will be the color we know for only a negligibly small part of the Sun’s history.
Stars, like individuals, age and change. As we look out into space, we see around us stars at all stages of evolution. There are faint bloodred dwarfs so cool that their surface temperature is a mere 4,000 degrees Fahrenheit, there are searing ghosts blazing at 100,000 degrees Fahrenheit and almost too hot to be seen, for the great part of their radiation is in the invisible ultraviolet range. Obviously, the "daylight" produced by any star depends on its temperature; today (and for ages to come) our Sun is at about 10,000 degrees Fahrenheit, and this means that most of the Sun’s light is concentrated in the yellow band of the spectrum, falling slowly in intensity toward both the longer and shorter light waves.
That yellow "hump" will shift as the Sun evolves, and the light of day will change accordingly. It is natural to assume that as the Sun grows older, and uses up its hydrogen fuel—which it is now doing at the spanking rate of half a billion tons a second—it will become steadily colder and redder. The passage indicates that the change of the Sun’s yellow color is related with ______.