Saturday, February 20, 2016

Why Is the Sun Yellow?

If you ask a random person to tell you what color the sun is, chances are he will look at you like you're an idiot and tell you the sun is yellow. Would you be surprised to learn the sun is not yellow? It's actually white. If you were to view the sun from the International Space Station or the moon, you'd see its true color. Check space photos online. See the true color of the sun? The reason the sun appears yellow during the day from Earth, or orange to red at sunrise and sunset, is because we view our favorite star through the filter of the atmosphere.

The True Color of the Sun

If you view sunlight through a prism, you can see the entire range ofwavelengths of light. Another example of the visible portion of the solar spectrum is seen in the rainbow. Sunlight isn't a single color of light, but a combination of the emission spectra of all the elements in the star. All of the wavelengths combine to form white light, which is the net color of the sun. The sun emits different amounts of various wavelengths. If you measure them, the peak output in the visible range is actually in the green portion of the spectrum (not yellow).
However, visible light isn't the only radiation emitted by the sun.
There's also blackbody radiation. The average of the solar spectrum is a color, which indicates the temperature of the sun and other stars. Our sun averages about 5800 Kelvin, which appears nearly white. Rigel appears blue and has a temperature exceeding 100000K, while Betelguese has a cooler temperature of 3500K and appears red.

How the Atmosphere Affects Solar Color

The atmosphere changes the apparent color of the sun by scattering light.
The effect is called Rayleigh scattering. As violet and blue light gets scattered away, the average visible wavelength or "color" of the sun shifts toward red, but the light isn't entirely lost. The scattering of short wavelengths of light by molecules in the atmosphere is what gives the sky its blue color.
When viewed through the thicker layer of atmosphere at sunrise and sunset, the sun appears more orange or red. When viewed through the thinnest layer of air at midday, the sun appears closest to its true color, yet still has a yellow tint. Smoke and smog also scatter light and can make the sun appear more orange or red (less blue).
 The same effect also makes the moon appear more orange or red when it is close to the horizon, but more yellow or white when it is high in the sky.

Why Pictures of the Sun Look Yellow

If you view a NASA photo of the sun, or a photo taken from any telescope, you're usually viewing a false color image. Often, the color that is chosen for the image is yellow because it's familiar. Sometimes photos taken through green filters are left as-is because it's easy for the human eye is most sensitive to green light and can readily distinguish detail.
If you use a neutral density filter to observe the sun from Earth, either as a protective filter for a telescope or so you can observe a total solar eclipse, the sun will appear yellow because you're reducing the amount of light that reaches your eyes, but not changing the wavelength. Yet, if you used that same filter in space and didn't correct the image to make it "prettier", you'd see a white sun.

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