Is the Sun a Perfect Black Body? Investigating the Reflection of Radiation

In summary, the sun may be a blackbody, but this cannot be measured with currently available technology.
  • #1
YoungPhysicist
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Black bodies are objects that don't reflect radiation.
But If I shoot light beams to the sun, it doesn't reflect it , so does that mean the sun is a black body(or at least very close to a theoretical one)?
 
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  • #2
The sun is too far away to do reflectivity measurements of its surface, and there isn't a source around with enough light to measure the reflectivity of the sun. For the planets in the solar system, as well as the Earth's moon, the light from the sun can be used to measure their reflectivity throughout the spectrum radiated by the sun. ## \\ ## I do believe the surface of the sun may be close to a blackbody, both in the visible as well as the infrared, because of the plasma nature of its surface that makes for good electromagnetic absorption, but I don't know of any way presently that can be used to make a reflectivity measurement.
 
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  • #3
Charles Link said:
The sun is too far away to do reflectivity measurements of its surface, and there isn't a source around with enough light to measure the reflectivity of the sun. For the planets in the solar system, as well as the Earth's moon, the light from the sun can be used to measure their reflectivity throughout the spectrum radiated by the sun. ## \\ ## I do believe the surface of the sun may be close to a blackbody, both in the visible as well as the infrared, because of the plasma nature of its surface that makes for good electromagnetic absorption, but I don't know of any way presently that can be used to make a reflectivity measurement.
Can't we use temperature to predict luminous intensity, then compare that to actual to find emissivity?

See:
https://upload.wikimedia.org/wikipedia/commons/0/0d/EffectiveTemperature_300dpi_e.png

EffectiveTemperature_300dpi_e.png
 

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  • #4
@Young physicist I think the curve that @russ_watters presents above would also give reason to believe that the surface of the sun is "approximately" a blackbody without any spectral reflectivity measurements. It would be rather unlikely that the surface would have a nearly constant emissivity/reflectivity independent of wavelength for such a wide spectral range unless that emissivity was quite near 1.0.
 
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  • #5

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  • #6
I think @Young physicist might need some explanation of what @Orodruin just presented: I believe it is a spectral measurement of the approximately ##T=2.73^o ## K blackbody-like background radiation of deep space.
 
  • #7
Orodruin said:
:biggrin:
Please, no...

Anyway, the one I posted seems just a touch off point. The total area [average irradiance] appears to have been purposely set equal, but the sun's graph looks clearly shifted to the right, indicating the temperatures don't match. I suggested the opposite: match the temperatures, then compare the areas to find the emissivity.

Googling, I find values around 0.985.
 
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  • #8
Charles Link said:
I think @Young physicist might need some explanation of what @Orodruin just presented: I believe it is a spectral measurement of the approximately ##T=2.73^o ## K blackbody-like background radiation of deep space.
Also an inside joke. Ignore.
 
  • #9
Charles Link said:
I think @Young physicist might need some explanation of what @Orodruin just presented: I believe it is a spectral measurement of the approximately ##T=2.73^o ## K blackbody-like background radiation of deep space.

It is indeed. It is the FIRAS measurement of the cosmic microwave background, which is as close as anything we know to being a blackbody spectrum. It is a remnant of the early hot universe which became transparent to radiation at around 3000 K. Due to cosmological redshift, the radiation is now at around 2.73 K.

(Also, as usual, I cannot help but mentioning my pet peeves ... There is no ##^\circ## in K. It is just Kelvin, not degrees Kelvin. ... And the LaTeX for ##^\circ## is ^\circ)

russ_watters said:
Googling, I find values around 0.985.
Seems pretty close to blackbody to me.
 
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  • #10
Orodruin said:
(Also, as usual, I cannot help but mentioning my pet peeves ... There is no ∘∘^\circ in K. It is just Kelvin, not degrees Kelvin. ... And the LaTeX for ∘∘^\circ is ^\circ)

thankyou, one of mine too

saved me the effort :biggrin::biggrin:
 

1. Is the Sun a black body?

Yes, the Sun is considered a black body. A black body is an idealized object that absorbs all incoming radiation and emits radiation at all wavelengths. The Sun's surface temperature and composition make it a nearly perfect black body.

2. What is a black body?

A black body is an idealized object that absorbs all incoming radiation and emits radiation at all wavelengths. It is a theoretical construct used in physics and astronomy to study the behavior of thermal radiation.

3. How is the Sun's surface temperature related to it being a black body?

The Sun's surface temperature is directly related to it being a black body. The Sun's surface temperature is around 5,500 degrees Celsius, which is the temperature at which a black body would emit the same amount of radiation as the Sun. This is known as the Sun's effective temperature.

4. What is the significance of the Sun being a black body?

The Sun being a black body has several implications for understanding its behavior. It allows us to accurately measure its temperature and study its radiation emission, which helps us understand the Sun's energy production and its impact on Earth's climate. It also allows us to make comparisons with other stars and objects in the universe.

5. Can a black body emit all types of radiation?

Yes, a black body can emit radiation at all wavelengths. This includes visible light, infrared, ultraviolet, and even radio waves. The amount of radiation emitted at each wavelength is determined by the temperature of the black body, following a pattern known as Planck's law.

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