# Can the Sun be treated as a black body radiator?

1. May 25, 2010

### Major_Disaster

Hello,

I am trying to revise for my Solar System exam and going through a past paper i have a question relating to something that we dont seem to have covered:

"By estimating the energy output of the Sun's corona (in watts), comment on whether can be treated as a blackbody radiator. (Assume a coronal radius = 2Rsun)"

I understand what a blackbody is, and i understand how i can estimate the energy output of the Sun by calculating it luminosity http://en.wikipedia.org/wiki/Luminosity#Computing_between_brightness_and_luminosity"

However, i dont see the link between luminosity and black body. By getting a value of x Watts how can i say yes this is black body or no it isnt?

Thanks for any help

Last edited by a moderator: Apr 25, 2017
2. May 25, 2010

### nicksauce

Well presumably you should know know (or you can look up) what the actual luminosity of the sun is, and you can compare it to the value you get.

3. May 25, 2010

### Matterwave

Or you can just intuitively think that you get close to the Solar luminosity if you treat the Sun as a 6000K black body...while the corona (with more surface area since it's farther out than the photo-sphere) is more on the order of 1 million K.

What does this suggest?

4. May 26, 2010

### Major_Disaster

Thanks for the replies.

Yeah, its given.

I calculated the luminosity of the corona to be 1.38 x10^32
The value of the solar luminosity is given as 3.9 x10^26 - then what?

That it cant be treated as a black body...? Not really sure

5. May 26, 2010

### Matterwave

So, by treating the Corona as a black body you get the answer that the luminosity would be hundreds of thousands of times brighter than the actual luminosity.

So...if the answer we get is ludicrous, then one of our assumptions must be wrong (or our math is wrong, but presumably you've checked that). Which assumption should we throw out first?

6. May 27, 2010

### Major_Disaster

Ohh yes i see. Ive only just realised that im using the "Stefan Boltzman BLACK BODY Law" (i assumed that was the lumonosity for any body). So by using this law and getting a ridicolous answer, one of the assumptions for using the law must be wrong - ie the body is not a black body.

Cheers