What Colour Is the Sun, Really?

The sun is white. It looks yellow from the ground because the atmosphere scatters the blue part of its light out of the direct beam and across the sky, leaving the light that reaches your eye directly slightly short of blue, which reads as pale yellow. At sunset the beam travels through far more air, loses even more blue and green, and arrives orange or red. From orbit, with no air in the way, astronauts see the sun as a white disc. The yellow sun in every child's drawing is a fact about our sky, not about our star.

• The sun is white; astronauts see a white disc.
• It looks yellow from the ground because the atmosphere scatters blue out of the direct beam.
• At sunset the light passes through more air, loses more blue and green, and arrives orange or red.
• The sun is classed as a yellow (G-type) star for historical reasons, not because of its light.

Here is the physics, and the one sense in which "yellow" is fair.

Because of the same effect that makes the sky blue. Sunlight contains all visible wavelengths in roughly equal measure. As it passes through the atmosphere, the shorter wavelengths, violet and blue, are scattered by air molecules far more strongly than the longer ones. That scattered blue light comes at you from every direction, which is why the sky is blue. The direct beam from the sun has therefore lost some of its blue, and white minus a little blue is pale yellow. The colour of the sky article explains the scattering in detail. Sky and sun are two halves of the same subtraction: the blue that is missing from the sun is the blue you see overhead.

Because the beam is travelling through much more atmosphere. When the sun is overhead, its light passes through one thickness of air. Near the horizon it passes through many times that, and the scattering has much longer to work. Blue goes first, then green, and by the time the light reaches you only the long wavelengths are left in strength: orange, then red. Dust, smoke and water droplets deepen the effect, which is why sunsets after bushfires or over the sea can be spectacular. The science of sunsets covers the variations.

White. With no atmosphere to scatter anything out of the beam, all wavelengths arrive together and the eye reads the result as white, or if anything very faintly green-yellow-white, because the sun's output peaks in the green part of the spectrum. Astronauts consistently describe it as blinding white. Photographs from orbit show a white disc against a black sky. The colours in space article collects what they report.

Because it emits strongly across the whole visible range, not just at the peak. A hot object glows with a broad spread of wavelengths, and where that spread peaks depends on temperature. The sun's surface is about 5,500 degrees Celsius, which puts its peak near 500 nanometres, in the green. But the red and blue on either side are nearly as strong, and when your eye receives the whole spread together it reports white. A star would have to emit green and very little else to look green, and no star does that. Cooler stars look orange and red because their spread is shifted towards long wavelengths; hotter ones look blue-white for the opposite reason.

Because of a classification system that predates our understanding of the atmosphere. Stars are sorted into spectral classes by their temperature, and the sun's class, G, was given the colour label "yellow" when the scheme was drawn up from telescope observations made through the air. The label stuck. So the sun is a G-type star, informally a yellow dwarf, and it is white. Both statements are true; one is about a filing system and the other is about light.

Because we have never seen it any other way without leaving the planet. Looking straight at it is impossible, so what we remember is the warm light it casts. Late-afternoon sunlight, the kind that is safe to look towards, is genuinely yellow-orange. Children draw what they see. What they see is filtered.

You can reproduce the mechanism. Sunlight is white; the sky removes blue; what is left looks yellow. A yellow filter does exactly that job in a single step: it absorbs blue and passes the rest. Hold the yellow face of a CMY Cube up to a white light and you are looking at midday sun through the atmosphere. Stack the magenta face behind it, which removes green as well, and the light turns red, which is sunset. Remove both and it is white again, which is orbit. Three filters, one object, and the whole day in the sky. The science page explains the subtraction wavelength by wavelength.

The sun is white. The atmosphere scatters blue out of its direct light, so from the ground it looks yellow, and at sunset, through more air, orange or red. From space it is white. It is classed as a yellow star for historical reasons. Hold a yellow filter to white light and you have made a midday sun; add magenta and you have made a sunset.

Make a sunset in your hand, and see light through a different lens.