# What's a Black body

1. May 4, 2014

I'm having the hardest time understanding Black bodies. I have a lot of questions about them so I can't write all of them, instead I'll just start with : Why are stars considered black bodies?

2. May 4, 2014

### Staff: Mentor

Tell us what you know about black bodies...

3. May 4, 2014

### dauto

A black body is an object with zero albedo - that is an object that absorbs all incoming radiation and reflects none of it back. That doesn't mean they will be black because black bodies produce their own radiation according with the black body radiation formula which depends on the temperature of the black body. A star really isn't a perfect black body but it's behavior is close enough to a black body that the black body formulae will describe it to a reasonably high accuracy.

4. May 4, 2014

So the radiation they emit is not within the visible spectrum, correct?

On whatis.techtarget.com
They defined a Black body as " a theoretically ideal radiator and absorber ".

In other words, how does reflection differ from radiation?

5. May 4, 2014

### Staff: Mentor

The distribution of the radiation they emit depends on their temperature. From your reading, what is the equation for the emission spectra from a black body?

6. May 4, 2014

### dauto

Black bodies do emit visible radiation. That's where the sun's visible light come from.

That page is saying the same thing I said. An ideal absorber absorbs all radiation reflecting none.

If you're a american football fun, you know the difference between reflection and radiation. It the quarterback throughs the ball to a receiver and the receiver reflects it, we have an incomplete. If the receiver absorbs it and emits it again, we have a fumble. Physically, reflected radiation preserve properties such as its wavelength while absorbed and re-emitted radiation will have properties dictated by the emitting body temperature.

Last edited: May 4, 2014
7. May 4, 2014

To determine the peak of an emission spectrum (I think I phrased that right!) You use Wien's Displacement Law :
Peak λ * T = b
where T is the absolute temperature of the Black body, and b is around 2.9*10^-3

8. May 4, 2014

### dauto

Correct, except for the missing units for the constant b.

9. May 4, 2014

So let me get this straight, a Black body absorbs the radiation of a certain spectrum and then re-emits it in a different spectrum with a different peak wavelength?

10. May 4, 2014

I keep picturing a star every time I talk about Black bodies, how wrong is that?

11. May 4, 2014

### nasu

Maybe the missing part (in the sense that it was not pointed out yet) is that the black body's emission is not conditioned by absorption. You don't need absorption in order to have emission.
As long as the BB has a temperature above zero it will emit the BB spectrum. Of course, this will make the body to cool down and the spectrum will change accordingly. Unless there is an internal source of energy which will compensate for the energy lost by radiation.
For a star I suppose the absorption is not very important in the general energy balance.

12. May 4, 2014

### dauto

Yes, that's right.

13. May 4, 2014

### dauto

Not too wrong. Stars are fairly good black bodies.

14. May 4, 2014