Sunlight in space is a combination of every wavelength of electromagnetic radiation (EMR), including those wavelengths that human vision cannot sense and the Earth's atmosphere absorbs at high altitude. Humans perceive various combinations of full spectrum wavelengths as white. Sunlight is so very "white" that it can actually cause blindness by damaging photoreceptors in the retina. The World Book at NASA online states that this radiation peaks in the yellow-green visible spectrum. Yellow-green is the color that humans have the knack of sensing most intensely. The very hottest stars, such as Sirius, emit radiation that peaks in the blue spectrum, but our star is not that hot.
Origin of Light in the Sun
The mass (weight) of the sun is about 70 percent hydrogen and 28 percent helium, but about 94 percent of the total number of atoms in the sun are hydrogen that is superheated to a ionized plasma. An ionized hydrogen is one positive charged proton, with no associated electron. When two hydrogen nuclei fuse to create one helium atom, nuclear energy is released as waves of EMR. Eight minutes later, the EMR reaches Earth's atmosphere, which absorbs 60 percent percent of that radiation before it reaches the human eye. Short wavelength gamma-rays and x-rays are completely absorbed by the atmosphere, and most of the ultraviolet radiation is also. Light in the visible spectrum and light that humans cannot sense (radio waves, microwaves and infrared heat waves) pass through the atmosphere to reach Earth's surface.
Sunlight in the Atmosphere
Inside the atmosphere, sunlight is scattered as atmospheric gases absorb and reradiate the "color" wavelengths differently. Blue light is the shortest visible wavelength and therefore is absorbed, held longer and scattered more than other wavelengths, making the sky appear to be blue. At sunrise and sunset, sunlight passes through a thicker transect of atmosphere to reach the human eye. As the atmosphere scatters the shorter wavelengths, the longer wavelengths of light at the red end of the visible spectrum persist, making the sun and sunlight on objects appear golden yellow, orange, pink and red. Scattering of light by gases or particles shorter than the wavelengths of the light is called "Rayleigh scattering" and was first described by John William Strutt, 3rd Baron of Rayleigh, of Cambridge University, in 1871.
Yellow Dwarf
In the late 19th century and early 20th centuries, several observatories and universities proposed classification systems for stars that assigned them positions on graphs or charts. The classifications were strictly taxonomic and were based on gross characteristics such as temperature, luminosity or magnitude (brightness), mass (weight) and radius. At that time, observation of most stars was not possible, even for smaller or darker stars that are in our own neighborhood. Our star, the sun, was classified as a dwarf, although we now know the sun is larger than most stars and will, in about 5 billion years, become a carbon-producing red giant. The 19th-century classification of our sun as "yellow" was in comparison to the "color" of other stars, all of which were acknowledged to be actually white. In the 20th century, our star, which was once thought to be a flaming chariot driven by a god, became instead merely a yellow dwarf.
Yellow
Nineteenth-century physicist James Clerk Maxwell demonstrated that concretely measurable "spectral" light is electromagnetic radiation and differs from light "perceived" by the brain. Electromagnetic radiation in the 570 to 580 nanometers wavelength range is yellow. Following Maxwell: When sunlight contacts an object, some wavelengths of radiation are absorbed as energy into the electron orbits of atoms in the object, but other wavelengths are reflected. A "black" object absorbs all wavelengths. A "white" object absorbs no wavelengths. A "yellow" object absorbs all wavelengths except 570 to 580 nanometers. Therefore a "yellow" sun (a source of EMR that peaks in the yellow range) is not the same as a "yellow" paint pigment that absorbs all wavelengths except yellow. James Clerk Maxwell's work with color wheels was reported as "Experiments on Colour, as Perceived by the Eye, with Remarks on Colour Blindness" in the Transactions of the Royal Society of Edinburgh in 1855 and led to his work in early color photography. James Clerk Maxwell was first to demonstrate that light projected through several primary color filters onto the same space will appear white, whereas combining paint pigments of those same colors will produce a gray paint.
Sunlight in Photography
Changes in dust and humidity from region to region on the Earth's surface affect how yellow the sun and sunlight appear in photographs. When a photographer wants outdoor color photography to present a true white light effect, then a lens filter is used to adapt the sunlight at that locale. In areas of high humidity, where atmospheric scattering has reduced enough blue light to skew daylight toward yellow on film, then a yellow filter is used to make white items appear true white in the photograph.
Tags: absorbs wavelengths, Clerk Maxwell, James Clerk, James Clerk Maxwell, object absorbs, object absorbs wavelengths