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
blood-red 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.