Sun as a radiation source

In difference to the planets and other little objects of our solar system (asteroids, comets,..) the sun is a self radiating object.

But what does self radiating or radiation mean?

Out of normal life we know radiation in the context of radio waves (radio, television, mobil phones) medical diagnostics (x-ray) and also the so called radioactive radiation (gammarays). But also our normal light ia a form of radiation. Actual it is nothing else than a propagating electro magnetical wave. This wave is similar to normal water waves. The difference is only that the electro magnetic field changes instead of the deflection of the water molecules. Waves are now described by frequency, wave length and velocity of propagation.

here we can use the following:

    λ * ν = c     

    λ... wavelength    ν... frequency     c... speed of light

This figure shows the electro magentic spectrum.     (Von Horst Frank / Phrood / Anony - Horst Frank, Jailbird and Phrood, CC BY-SA 3.0)

The interesting thing about radiation is that the kind of the evolving radiation is a function of the object. The most important quantity is temperature. This connection is known in everydays life. If you increase temperature of steel it beginns to radiate red light. If you increase the temperature further the color is changing to brighter ones (dark red, bright red, yellow, white,....). That is why we call it red heat or white heat. So if we know the "color" of our sun, we know something about the surface temperature.

Furthermore you can imagine that the originated radiation from Sun is changed during the way through the suns atmosphere and also through earth atmosphere. The kind of change is of course a function of the composition ot the atmospheres and so we can learn a lot about the composition. Every atom has special wavelengths it absorbs and so the radiation in this lines becomes weaker. In physics this phenomenon is called "Frauenhofer" absorption.

This figure shows the visual spectra with "Frauenhofer" absorption lines. In normal life such features are used for colorfilter. (Public Domain, Wikipedia).

At the end. The most important things for a second time:

In astrophysics we can only measure the radiation flux of interesting objects. Radiation is a kind of energy and can be illustrated by a spectrum. Due to the spectrum of radiation one can deduce interesting information like: chemical composition, temperature , velocity,....