Expert: Jayendra Upadhye - 4/27/2008

Question
a star does not have a black body spectrum as its continuous spectrum

Answer
Hi,
I do not know the scope of your question, as it is truly "laconic".

Such extreme brevity some times is counter productive, as it leaves the person answering the question a bit mysitified as to its "scope".

You see, in Engineering, what may be "true and sufficient" may not be so in actuall science.

Engineering deals with "practical and sufficient" approximations.

Science has to deal in accuracies, with known approximations.

For example when applying black body physics to stars and planets we actually deal with apporoximations "as assumed truths".
(since the % errror is around 3%, which is acceptable in engineering practices).

Most importantly, the scope includes location.
If measured in space in the earth's vicinity at right angles to incident solar radiation (solar constant @ 1.4 kilowatts/square meter), one would see a continuous spectrum corresponding to 5250 deg C. But down below at the earth's surface, absorption lines would appear due to gas in the atmosphere namely ozone (Ultra-violet absorber), Oxygen and water vapor and CO2, the major constituents of the air.

Likewise, actual theoretical black bodies are best approximated by "point sources" whereas the sun is a large enough spherical body even at 98,000,000 miles.

Plus it has its own light absorption areas. (This absorption is what heats up the sun. First occuring at Gamma levels deep in the core and then successively lower levels in the convective and outer layers.

links:-
1 - http://en.wikipedia.org/wiki/Solar_radiation
under the heading 'Climate effect of solar radiation'.
If you look at the spectrum, pictured there, you will see how close is the approximation to a black body at 5250 deg C.
Also you will see what i meant by "scope". At earth's surface, discontinuities appear in the scontinuous solar black body spectrum due to absorption by Oxygen, Water vapor and Carbon-dioxide.

2 -
refer:- http://en.wikipedia.org/wiki/Black_body
under the heading 'Temperature of the sun'.

There is an interesting "aside" in the same link, (middle of the 'explanation' para), where it says..
["Real objects never behave as full-ideal black bodies, and instead the emitted radiation at a given frequency is a fraction of what the ideal emission would be. The emissivity of a material specifies how well a real body radiates energy as compared with a black body. This emissivity depends on factors such as temperature, emission angle, and wavelength. However, it is typical in engineering to assume that a surface's spectral emissivity and absorptivity do not depend on wavelength, so that the emissivity is a constant. This is known as the grey body assumption"].

Hope that suffices.

Please do rate the answer.
Jayen