When we accept the envidence of our unaided eyes and describe the Sun as a yellow star, we have summed up the most important single fact about it-as this moment in time.It appears probable, however, that sunlight will be the color we know for only a negligblySmall 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 4000 degrees Fahrenheit. Thre are searing ghorsts blazing at 100,000 degrees Fahrenheit and it 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” ill shift as the Sun evolves, and the light of the 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 dong at the spanking rate of half a billion tons a second –it will become steadily colder and redder
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