Re: What is the color of the Sun?

This is a pretty hard question to answer with physically correct facts because there is no known object in the Universe with a size and general constitution like that of the Sun that does not emit light. Thus there are no facts that can tell what the Sun would look like if it did not emit light. If the Sun stopped emitting light, it would collapse instantly into a solid body slightly bigger than Earth. Such stars exist. They are called white dwarfs, and even those emit bluish white light. Therefore we have to take guesses based on what we know about the matter that makes up the Sun.

The outer layer of the Sun is a very tenuous gas composed of 90% hydrogen, 9% helium and less than 1% of other elements at a temperature of 5,770 degrees. In laboratories on Earth hydrogen and helium are described as "colorless". That means that when they are illuminated by white light, they reflect the light without changing its color. However those observations have been made on small laboratory flasks containing hydrogen and helium. The Sun has an atmosphere where light can travel back and forth along large distances, and we must take into account what happens to that light after it travels some distance inside the solar atmosphere.

First we must clarify what white light is and what color is. We can always produce light of different colors and mix the colors in different amounts to produce light. Sunlight separates in different colors when going through a glass prism. Since there is no physical definition of white light that is useful for artists, white light has being defined as light from the Sun. That means that white light is composed of different colors. Sunlight emits maximum light at the yellowish--green color, but it has reasonable amounts of red and blue lights. (NEVER LOOK DIRECTLY AT THE SUN. IT CAN CAUSE PERMANENT DAMAGE TO THE EYES!). An object at 5,770 degrees emits those colors. But you are not interested in the light emitted by the Sun. OK.

When we want to determine the color of an object, we illuminate the object with white light and look at the light reflected from it. For example, red objects tend to absorb blue and green light, and reflect red light. If you shine white light over the Sun, that light will enter its atmosphere and will be reflected and absorbed by the atoms in the gas. However atoms reflect and absorb light differently depending on the color of the light. Blue light tends to be more reflected than red light. The sky is blue because the blue light from the Sun is more reflected than the red light by the atoms in the atmosphere. The Earth atmosphere is 78% nitrogen and 20% oxygen, which are also colorless gases in the lab, yet they do look blue in the atmosphere when illuminated by white light, don't they? What we are seeing here is the effect of making white light traverse a large distance into a gas, and looking at the light reflected by the atoms in that gas. This gives us a hint of where all this leads.

If, and this is a big if, the hydrogen and helium in the Sun atmosphere behave like the nitrogen and oxygen in Earth's atmosphere, we can guess that when you shine white light over the Sun, the reflected light, that is its color, will be mostly blue light.

Greetings,
Vladimir Escalante Ramírez
Center for Radio Astronomy and Astrophysics
UNAM, Morelia, Mexico