What color is the Sun? Is the Sun red, yellow or white?

Hyperaxion October 21, 2020 4:52 am

It is very common to think that the Sun is yellow or even orange. After all, this is what we see when we look at it every day.

However, if we look at the Sun through a satellite outside our atmosphere, we will see a very different color from the one we are used to.

What color is the Sun?

We usually see the Sun represented in orange or even in shades of red. However, like all other stars, the Sun produces its own light through nuclear fusion.

During these intense reactions, all wavelengths of the electromagnetic spectrum are produced, from infrared to gamma rays, passing through all wavelengths of visible light, from violet to red (between 380 nm and 720 nm). Therefore, the Sun is white.

I know, the habit of seeing the Sun always in shades of yellow and red makes it difficult to imagine that it, in fact, is practically white, even for scientists.

For this reason, images from observatories around the world are often artificially colored.

But if the light that the Sun emits is practically white, what happens in our atmosphere that makes it look yellow and red?

The yellow and red tones that we see when looking at the Sun is due to the scattering of the Sun’s rays entering the atmosphere. Check out the photo below:

Sun seen from outside the Earth's atmosphere.
The Sun seen from outside the Earth’s atmosphere.

Why do we see the Sun in shades of yellow, orange and red?

When the light produced by the Sun reaches the Earth’s atmosphere, some shorter wavelengths, such as violet and blue, are immediately scattered by the small particles that make up the atmosphere.

In addition to changing the color of the Sun for an observer on the Earth’s surface, this scattering is also responsible for the bluish color of the sky, clouds, and the formation of the rainbow.

The scattering pattern does not occur at random. It depends on the characteristics of the particles that constitute the medium through which the radiation propagates.

The Sun seen from the International Space Station.
The Sun seen from the International Space Station.

One of these characteristics is the relationship between the size of the particles and the wavelength of the radiation.

When the particles present in the medium (particles with a radius less than 0.1λ) are much smaller than the wavelength of the incident radiation, the scattering is called Rayleigh scattering.

Longer wavelengths, such as orange and red, manage to pass through the atmosphere without being widely scattered. This makes the sky appear blue, and our light source, the Sun, appears orange or red.

With the change in the angle of incident sunlight, during sunset, for example, the distance covered by sunlight increases, and the absorption of shorter wavelengths is even more evident.

Curious fact: If you hold a CD under sunlight, it will reflect the colors of the rainbow: violet, indigo, blue, green, yellow, orange, and red. If we mix all these colors, we get white.

Have you ever noticed that during the zenith (at noon) the color of the Sun is much closer to white? This occurs because of the shorter distance traveled by the Sun’s rays and, consequently, the lower scattering of blue and violet.

For what reason, then, does the sky appear to be blue and not violet?

Well, the answer lies in our eyes. Literally. The cells in our eyes that specialize in capturing colors, called cones, respond better to the stimulus of light whose wavelength is between green and blue (about 508 nm) than to ultraviolet, so these tones are more visible to us.

But the same is not true for insects. Do you want to know how insects see the world? We have an in-depth article on this:

How insects see the world.

Rayleigh scattering

This phenomenon results in the electrical polarization of the particles. That is, a part of the electrons moves to another end of the particle. This creates a differentiation in the number of charges.

The oscillation of the electromagnetic wave on the charges within a particle causes them to move at the same frequency. Thus, the particle behaves like a small dipole and starts to emit radiation. A dipole is a set of two equal charges and opposite signals.

If the particle is bigger than the wavelength, the radiation is not scattered and consequently, there is no chromatic decomposition (color separation). Thus, all wavelengths are equally scattered.

Blue light scattering in the Earth's atmosphere.
Blue light scattering in the Earth’s atmosphere.

In our atmosphere, where there is an abundance of N2 and O2, blue light (λ ~ 0.425 µm) is more scattered than other colors in the visible spectrum.

Compared to red (λ ~ 0.650 µm), blue is scattered 5 times more. Consequently, the spectral line of the blue is removed from the direct sunlight.

The colors (spectral lines) that are not scattered, continue in their original direction.

Therefore, when we look at the sky, we see blue, which is the most scattered color in the atmosphere.

But when we look directly at the Sun (please don’t look without adequate protection), we see the yellow wavelengths, which have suffered very little scattering.

However, there are situations where the Sun acquires other colors. For example, when the Sun is close to the horizon, at sunrise or sunset.

At such times, solar radiation tends to pass through a thicker layer of the atmosphere, causing even the yellow wavelengths to be scattered.

With this, the Sun acquires other colors, such as orange and red, wavelengths that manage to pass through the thick atmosphere.

Solar spectrum

The Sun is categorized as a main-sequence star, that is, it fuses hydrogen atoms into helium atoms to produce energy.

Within this astronomical classification, the Sun is considered a yellow dwarf star. Its category, however, has nothing to do with its color, but with its diameter and the temperature of its surface, of approximately 5800 K (5500 C).

By the way, we have a really cool article about the temperature of the Sun. Be sure to check it out.

What is the temperature of the Sun?

Furthermore, although solar radiation has all visible wavelengths, the Sun does not produce them with the same intensity, that is, some frequencies are produced more often than others.

Among the wavelengths we can see, blue is the most produced by the Sun, for example.

An interesting fact about solar observations is that today we know that the Sun emits electromagnetic radiation at frequencies other than the visible, such as X-ray and ultraviolet.

Accordingly, solar physicists use several colors to represent each monochromatic frequency, as shown in the figure obtained by the SDO (Solar Dynamics Observatory) mission.

The Sun observed through different wavelengths.
The Sun observed through different wavelengths. (Credit: SDO / NASA).

Sometimes the color in which the Sun is represented is culturally determined. We usually see it represented in yellow or orange.

However, in Japan, it is culturally represented in red. Despite all artistic licenses, including from scientists, the Sun is white.


If you’ve read this far, you will definitely love the following books:

References

Fröhlich C, Shaw GE. 1980. New determination of Rayleigh-Scattering in the terrestrial atmosphere. Appl. Optics 19(11).

Liou KN. 2002. An introduction to Atmospheric Radiation. Vol2. 2nd edition.

Scherrer D. What Color is the Sun? and other mysteries. Stanford Solar Center.

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