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Do Sun Really have a Colour?

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After Sunset

As per the need and available source,with the technology of that time,Stars are classified for easier study. To be precise to our question, lets know about the most popular classification.

According to the current level, Morgan-Keenan System or MG System is the latest,most efficient classification system followed. It connects the temperature at the surface and surface gravity along with star’s visible color.

O,B,A,F,G,K,M are the letters used in this classification to represent each class of stars. It is ordered in the decreasing order of surface temperature of stars.

It’s mostly hard to remember the order of this,for which we can use a common mnemonic “Oh Be A Fine Guy,Kiss Me”
Not disappointing the other gender, we can use, “Oh Be A Fine Girl,Kiss me”

Morgan-Keenan System

Class O :

They are blue huge giants stars. They are very hot,surface temperature is nearly 30000 kelvin. They emit mostly UV radiations with some bluish ones. They are very rare to be exist,nearly 0.0001% around solar neighborhood. As these are blue or darker than that,its difficult to see.

Class B :

Aludra star from Canis Major is an example. Bluish white color. Most energetic star. Surface temperature is between 10000 kelvin to 30000 kelvin. Mostly they are confused with class B

Class A:

They are the most visible stars with naked eye. Mostly white,who emits almost all colors of visible region,shines well at night. Gamma Ursa major is an example. Surface temperature is nearly 7500 kelvin to 10000 kelvin.

Class F :

They are dull white or yellowish white. 3% of solar neighbor hood are them. Surface temperature varies from 6000 kelvin to 7500 kelvin.

Class G :

They are very yellowish. Note this : Our sun belongs to this category. Surface temperature is 5200-6000 kelvin

Class K:

They are orangish in color. Temperature varies from 3700 kelvin to 5200 kelvin. 12% of solar neighborhood fills with them. Alpha Centauri B is an example.
These category stars have a good chance to give birth to habitable planets to revolve around where life would be possible.

Class M:

Even though these fills the 76% of solar neighborhood,they are barely visible,only at certain cases. Surface temperature range from 2400 kelvin to 3700 kelvin.

Sun and Scattering

The range of surface temperature of sun is 5778 Kelvin. Hence it shows yellowish color,which is the mostly found one among the visible emissions of sun.

Apart from these,there are several other reasons for the different colors of sunlight.

Scattering of light particles in the earth’s atmosphere.

There are several methods of scattering.

When the light of sun,which is rich in yellow,enters earths atmosphere,some of them are absorbed by the atmospheric gases and particles and reflected.

The Nitrogen and Oxygen rich atmosphere scatters high frequency,shorter wavelength radiations. Already our sunlight is short in bluish radiations,which is firstfiltered for UV and higher frequency radiations by ozone layer, which then is subjected to the scattering of atmospheric gases. This atmospheric gases scatters the bluish radiations and spread them to various directions,making our sky beautifully blue. The remaining,undisturbed lengthier wavelength radiations reaches us.

Shape of Oblate Spheroid

The earth is not a perfect sphere. It is an oblate spheroid with a flat top and bulging center.

For people like me,who are near to the equator , the color of sky varies from morning to evening. During the day Sunlight happens to travel mostly through similar distances and with same angle of earth’s axis. From noon onward the scene is different.

A view across a 20-km-wide bay in the coast of Spain. Note the curvature of the Earth hiding the base of the buildings on the far shore

During noon, higher frequencies gets scattered and from yellow onward left undisturbed. The more rich yellow light makes our noon ‘Sunny’.
From there onward, the light have to travel more from the sun to reach us, as we go opposite by earth’s rotation.
As it needs to travel more, the particles it have to encounter in the sky is too more. Hence more chance of getting scattered.
As the distance increases,more lengthier wavelength radiations scatters.
The yellows starts to reduce,which makes the sky orangish, which again reduces it and makes the sky reddish.
Time to time,from noon to evening that’s why the sky changes color from Yellow to Orange to Pink to Red and Reddish pink.

Sunrise
Sunset

The different directed scattering makes those colors to coexist and makes our evenings a mixed reddish purple.

Along with this,we can add a theoretical aspect. The light particles which is given by the Max Planck Quantization equation,which relates energy of a light particle (photon) to its frequency, directly proportionally. So as the particle needs to travel more,it encounters more situations,particles and matter which can make it to lose its energy or gets scattered. As the energy reduces,frequency reduces and wavelength increases. This also accelerate the chance to see a reddish sky,especially in the evenings. Compton effect can also be added here.

In the morning time, sun is coming towards us, providing more light rays,which never makes a shortage in radiations. That’s why we doesn’t see this color variation much during day time. But After noon we and sun are going farther in opposite direction,which makes harder for light rays to reach us,with shorter number of them.

After Sunset

Original Article on LetsPhysics

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