RASC Calgary Centre - The Solar Spectrum

By: Larry McNish
Page last updated November 5, 2018
Page originally created: February 11, 2012

Lines in the Sun's spectrum
The Solar Spectrum
(image produced using my RGB Spectrum Generator program)

Question: What is the Solar Spectrum?

Answer (Part 1): I don't normally answer a question with a question, but did you know that our Sun is not yellow, or orange, or red?


the Sun is not yellow

the Sun is not orange

the Sun is not red

Our Sun is called a "yellow dwarf" BUT it's light is white.

I.e. all the visible light it emits would be interpreted by our eyes as the colour white.

Don't believe me? Then continue reading.


The Sun is not yellow - it's white

A lot of daytime sunlight (1) is scattered by the Earth's atmosphere making the sky appear blue (2),
and the remaining sunlight reaching our eyes (3) appear yellow.


The Sun looks redder at sunset

Even more wavelengths are scattered through the much longer light path in the atmosphere at sunset (4)
(the orange arrow) making clouds and the Sun itself appear orange or red.


Blue Sky, orange and red clouds at sunset

At sunset we see sunlight scattered off the atmosphere (blue) and clouds (yellow, orange and red).
Sunset photo Credit: Earth Sciences Picture of the Day September 10, 2004
"California Dreamin" by: Christine Churchill


So, if you can't see the proper colour of the Sun during the daytime, how can you see it?
Easy - just observe the sunlight at Night!

Sunlight reflected off the Moon shows the Sun's true colour

The Moon glows only because of reflected sunlight.
(Actually, only about 13.6% of the sunlight hitting the Moon reflects towards the Earth.)
This is not bright enough to cause the sky to appear bright blue to our eyes, so we interpret the sky as black,
and the sunlight reflected off the Moon as closer to the Sun's actual colour - white!


The Moon is not white - the Sun is

(photo by the author)
Viewing sunlight reflected from the Moon shows that the Moon is not white - the Sun is.


By the way, in the diagram just above, I show that the light from the Moon also colours the sky blue (but not as much as the Sun does). The reason we can't see the blue sky at night is that the moon is bright, and the sky is dim, and our eyes adjust to the moon and not the sky. However, the effect is real, as the three photographs below show. These were taken on the same night by the very sensitive "all-sky" camera at the Rothney Astrophysical Observatory near Calgary before and after moonrise.

1 minute time exposure of the sky before
moonrise showing mainly the colour of
light pollution from Calgary (upper left).
The brighter stars (e.g. Orion), Jupiter,
and a bit of the Milky Way is visible.
Same exposure, but the Moon has risen
(hidden behind the large dome) showing
that, to a sensitive camera, the night sky
is blue.
Same exposure, a little later, when the
Moon is no longer hidden by the dome.
Note that this photo was taken at 01:36AM
many hours before sunrise on that day.


Answer (Part 2): The next thing to understand is that "light" has no colour at all!

All "light" is made up of photons that travel from a source to a detector (e.g. our eyes). All photons are the same elementary particle (the quantum of light). There are not "red photons", "green photons", "blue photons", "beige photons" or "blonde photons". The photons only differ in their energy level, which also affects their wavelength.

Transparency of the Earth's atmosphere to wavelengths

In fact, we see only a small portion of the overall number of photons being emitted by the Sun at any time, as shown above.

Thus, it is our "sense of sight" that is affected by this narrow range of photon energies, and the complex biological structure consisting of the eyeball, the eye's retina, the optic nerves and the visual cortex in our brains that give us the sense of colour.

(We also sense light at longer wavelengths than "red" (infra-red) as heat, mainly through our skin (e.g. heat rash, heat exhaustion, and heat stroke). Photons at even lower wavelengths called microwaves can actually cook our dinners.)

(We also sense light at shorter wavelengths than "violet" (ultra-violet light, which is further divided into ranges called UVA, UVB, etc.) through our skin. UVB exposure induces the production of vitamin D in the skin. Too little UVB radiation may lead to a lack of vitamin D. Too much UVB radiation may lead to sunburn, skin cancer, and direct DNA damage. High energy UV lights are also used for sterilization (killing organisms) in air and water filters.)


Our "sense of sight" - How we "see" light.

The retinas of our eyes contain 4 types of light sensors:
  • Rods - which are mainly active only at low light conditions, give us our "night vision"
  • L Cones - active under sunlight conditions and respond to Longer wavelengths of the "visible spectrum"
  • M Cones - active under sunlight conditions and respond to Medium wavelengths of the "visible spectrum"
  • S Cones - active under sunlight conditions and respond to Shorter wavelengths of the "visible spectrum"
  • The number of millions of each of these types of receptors and their placement on the retina varies.
It is the sensitivity of the three different types of cones to differing wavelengths (energies) of light that leads to our visual system associating these energies as "colours":

Eye response to the Solar spectrum

The visual response (sensitivity) curves for S-Cones (blue line), M-Cones (green line), and L-Cones (red line)
and the overall "population weighted" response to light wavelengths (white line) plotted over a
"perceived" colour representation of a continuous spectrum. Note the logarithmic vertical axis.
(image produced by the author using my RGB Spectrum Generator program)



Answer (Part 3): And finally, the light from the Sun is not a continuous stream of photons with every possible energy or wavelength like the background colours of the image above. Some of it has been "stolen".


Here is a description of the path light takes from its creation at the Sun, to our eyes.

First, the photons created at the Sun's Photosphere (the visible surface) have to escape through the other layers of the Sun's atmosphere - the Chromosphere, the Transition Region, and the Corona. Each of these layers is either transparent at certain energies or absorbs and possibly re-emits photons at varying energies.

The layered structure of the Sun's atmosphere
Image credit: From "The Structure of the Sun", University of Glamorgan, UK.
(Additional annotations by the author)

Then, the remaining sunlight photons of various wavelengths have to travel through the Heliosphere (created by the solar wind which is a flow of superheated, charged, gas particles from the Sun called a plasma) which fills the Solar System's interplanetary space.

Interplanetary space also contains cosmic dust particles such as the "zodiacal dust cloud" which causes the "zodiacal light" a very faint glow seen along the ecliptic after sunset on moonless nights. This "interplanetary medium" also affects photons at different wavelengths through absorption, reflection or scattering.

Then, the sunlight has to travel through Earth's Atmosphere which has it's own layers - the Exosphere, the Thermosphere (including the Ionosphere and a thin layer of sodium atoms), the Mesosphere, the Stratosphere (including the ozone layer which protects us from the Sun's ultraviolet radiation), the Troposphere, and the Planetary Boundary Layer. The Earth's atmosphere contains several different gases (78.084% nitrogen, 20.947% oxygen, 0.934% argon, 0.033% carbon dioxide, and trace amounts of other gases) as well as about 3% water vapor, and suspended dust, spores, bacteria, aerosols, and pollutants - all of which blocks out a lot of sunlight wavelengths.

Finally, the remaining sunlight reaches our eyes. It also reaches observatory detectors called spectrographs which measure the brightness of photons (remember, sunlight is not coloured) at thousands of different wavelengths across a large portion of the electromagnetic spectrum.

Transparency of the Earth's atmosphere to wavelengths



Result: The passage of sunlight from the Sun's photosphere to the surface of the Earth results in many wavelengths of light having their intensity reduced partially or completely.

Normally, the spectrograph results are shown as intensity versus wavelength curves:

Spectrgraph curve of sunlight
(image produced by the author using my RGB Spectrum Generator program)

But, because wavelengths in nanometers (or frequencies in terahertz) are not that familiar, it is sometimes convenient to place these curves against a continuous "coloured" background which itself has been modified to represent the eye's response to the wavelengths of light:

Spectrgraph curve of sunlight with colours
(image produced by the author using my RGB Spectrum Generator program)

or, to use the intensity curve to modify the background colours providing a "colored line spectrum":

Spectrgraph curve of sunlight applied to colours
(image produced by the author using my RGB Spectrum Generator program)

The Solar Spectrum
(image produced by the author using my RGB Spectrum Generator program)


So, now you know that a "Solar Spectrum" is an artificial creation using a coloured background which is then modified by the light intensity values recorded from a spectrograph.


The big problem with all the dark lines in the image above (called "Fraunhofer Lines") is determining which of all the things the sunlight passed through "stole" those wavelengths of light.

One of the best "coloured spectrum" images of the sunlight arriving at the Earth's surface was produced by NOAO - the U.S. National Optical Astronomy Observatory using the FTS (Fourier Transform Spectrometer) at the McMath-Pierce Solar Facility at Kitt Peak National Observatory, near Tucson, Arizona, and made available on their High resolution solar spectrum page:

NOAO Solar Spectrum

As you can see, there are thousands of "Fraunhofer Lines" in this image. Some are very narrow and faint, others are very broad and dark.

The images NOAO provided are overlayed on a coloured background, but unfortunately do not include any annotations for the wavelengths. I produced a version of this image with the wavelengths included so that individual lines could be found, or features could be more readily identified:

NOAO Solar Spectrum - annotated
If you would like a full-sized (1600x1152 pixel) version of this annotated image please contact the author at: Please type this address into the 'To:' address of your e-mail

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