Spectrum of the Sun vs black body

[fluidistic]

In wikipedia I've read that the Sun's surface temperature is about 5700K.
The emission spectrum graph can be seen there: http://en.wikipedia.org/wiki/File:EffectiveTemperature_300dpi_e.png.
I don't understand why the irradiance (or intensity I guess) of the Sun is greater than the one of a black body at 5700K. This would imply that the emissivity of the Sun is greater than 1, which is impossible. What am I missing?

 

[chemisttree]

Must be that the Sun's emission spectrum has more going on than just blackbody radiation.

 

[JeffKoch]

In wikipedia I've read that the Sun's surface temperature is about 5700K.
The emission spectrum graph can be seen there: http://en.wikipedia.org/wiki/File:EffectiveTemperature_300dpi_e.png.
I don't understand why the irradiance (or intensity I guess) of the Sun is greater than the one of a black body at 5700K. This would imply that the emissivity of the Sun is greater than 1, which is impossible. What am I missing?

It's just that the sun is not a blackbody. In other portions of the same curve, the ratio is less than 1. The 5700 number comes from a single-parameter fit to the actual spectrum, and does not mean that the sun really is a blackbody with a temperature of 5700K. At short wavelengths especially, it looks nothing like a blackbody.

 

[fluidistic]

It's just that the sun is not a blackbody. In other portions of the same curve, the ratio is less than 1. The 5700 number comes from a single-parameter fit to the actual spectrum, and does not mean that the sun really is a blackbody with a temperature of 5700K. At short wavelengths especially, it looks nothing like a blackbody.

I understand. But the Sun's surface is at 5700K, right? If I consider the Sun as a gray body then I should expect its irradiance to be lesser than the one of a blackbody at the same temperature, for all wavelengths. Am I right on this? If so, I still have trouble understanding the graph.

 

[pmacias]

I can explain the difference between the spectrum of the Sun and that of a black body at 5700K. First of all, it is important to understand that the Sun is not a perfect black body. A black body is an idealized object that absorbs and emits all radiation that falls on it, with a perfectly smooth and continuous spectrum. However, the Sun is a complex and dynamic object with a variety of processes happening on its surface and within its layers.

The Sun's surface temperature of 5700K is an average value and does not represent the temperature of all the layers of the Sun. In fact, the temperature of the Sun's corona (outermost layer) can reach millions of degrees, while the temperature of the inner layers can be much lower. This temperature variation leads to a broadening of the emission spectrum, resulting in a wider range of wavelengths being emitted by the Sun compared to a black body at 5700K.

Additionally, the Sun's surface is not a perfect emitter. It consists of different elements and materials that have varying emissivity values. Emissivity is a measure of how efficiently an object emits radiation compared to a perfect black body. Therefore, the Sun's emissivity is not equal to 1, and this also contributes to the difference in the intensity of radiation emitted by the Sun compared to a black body.

Furthermore, the Sun's emission spectrum is also affected by absorption and scattering of radiation by the atmosphere and other particles in space. This can lead to certain wavelengths being absorbed or scattered, resulting in a different spectrum than that of a black body.

In conclusion, the Sun's emission spectrum is different from that of a black body due to its complex nature, temperature variation, and varying emissivity values. It is important to consider these factors when comparing the two spectra.