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Tiny
2004-Jan-30, 06:16 PM
What is Blackbody radiation? And why they call it the balckbody? Our Sun is a shinning yellow ball, but why they call it the blackbody? ;)

Tim Thompson
2004-Jan-30, 06:54 PM
A blackbody (http://scienceworld.wolfram.com/physics/Blackbody.html) is one that absorbs all incident electromagnetic radiation, and reflects none. Such a blackbody will emit electromagnetic radiation, for no other reason than the fact that its temperature is greater than absolute zero, which is called, oddly enough, thermal radiation, or blackbody radiation (http://hyperphysics.phy-astr.gsu.edu/hbase/mod6.html). it can only be true blackbody radiation if its energy spectrum is the same as that predicted by the Planck blackbody formula (http://scienceworld.wolfram.com/physics/PlanckLaw.html).

The sun (http://www.astronomynotes.com/starsun/s2.htm), and all main sequence stars (http://imagine.gsfc.nasa.gov/docs/science/know_l2/stars.html), generate blackbody radiation as a result of the way photons of light are scattered around in the stellar interior, on their way out. But the outer atmosphere of the star, which is relatively cool compared to the rest of the star, absorbs a lot of that radiation, so that no star radiates as a true blackbody, but they can be treated as approximate blackbodies. So, when you read about the color temperature (http://csep10.phys.utk.edu/astr162/lect/stars/cindex.html) of a star, they are integrating all of the light energy, and then computing the temperature that a true blackbody would have, if it radiated all that energy, using the stefan-Boltzmann (http://scienceworld.wolfram.com/physics/Stefan-BoltzmannLaw.html) formula.

Everything has a temperature, so everything emits thermal radiation. But because of absorption, like the absorption of a stellar atmosphere, very few things emit true blackbody radiation, exactly as described by the Planck formula. Curiously, the most precise blackbody spectrum ever measured is that of the cosmic microwave background (http://www.tim-thompson.com/cmb.html).

Tiny
2004-Jan-30, 11:21 PM
Got it thanks!