Re: Where can I find a spectrum of the interstellar average radiation?

Juha-Petri,

You're right in that the amount of radiation at different wavelengths varies. Even in optical wavelengths here on Earth, the intensity of light at any given wavelength depends on your location within Earth's atmosphere (or your height with respect to sea level) and what wavelength you're looking at. You may find the following link useful for a basic overview of the electromagnetic spectrum, including a nice plot of the electromagnetic spectrum as a function of height above sea level.

But it also sounds like you already know that the electromagnetic spectrum here on Earth is different than what you would see out in interstellar space. However, in the same way that spectra vary based on wavelength and atmospheric effects here on Earth, spectra in interstellar space vary depending on wavelength and location. This is because astronomers have found that interstellar space isn't completely empty, and depending on where you are there may be more or less particles to "interfere" with the spectra. You may ask, "What do these particles have to do with light?" Well...

Light can actually form three varieties of spectra: plain spectra (such as when light is filtered through a prism), absorption spectra, and emission spectra. These latter two types of spectra depend on particles. If you follow the link I've provided, you'll find an example of each of these three types. From the plots in these examples, you can see that the average intensity of light can vary even more when you begin discussing line spectra.

What this all amounts to is that you asked a much more complicated question than you realized! I hope the links I provided help to get you started asking even more questions in the future.

Good luck,
Amber

[Moderator's Note: Everything Amber explains is absolutely correct, I just want to add one more item. There is such a thing as the "cosmic background radiation" that is sort of an average spectrum from distant objects; as such, it is perhaps the closest thing to what you are asking for. You can see a picture of the spectrum at this site, although the explanation is a bit technical.]