I How to approach a more perfect blackbody?

[Ron19932017]

I wonder how an object, like our sun, can approach a more perfect blackbody.
We know that by the wiki definition, blackbody is something that absorb all radiation and is in thermal equilibrium. Its spectrum only depends on T.

We also know that, our Sun's spectrum is blackbody like, while a galaxy/nebula's spectrum is not. I heard explanations like we require the gas to be dense to be a blackbody. That's where my confusion comes.

1.) Suppose our sun is twice denser, does that means it more blackbody-like? and why?
2.) Except from the emission/absorption line, our Sun's continuous spectrum is offset from a perfect blackbody. Is this discrepency due to the density of the Sun not high enough?
3.) Does hot diffuse gas have emission lines, plus a non-blackbody-like continuous spectrum? I wonder how one can calculate that non-blackbody-like continuous spectrum, assuming the diffuse gas is in thermal equalibrium.

 

[jerromyjon]

I think you are missing the point that an "ideal" blackbody doesn't reflect or emit any light except for "blackbody" radiation due to temperature.

I wonder how an object, like our sun, can approach a more perfect blackbody.

By putting it in a highly insulated box, shutting down all nuclear reactions, et cetera, anything that brings it closer to thermal equilibrium.

 

[Ron19932017]

I think you are missing the point that an "ideal" blackbody doesn't reflect or emit any light except for "blackbody" radiation due to temperature.

By putting it in a highly insulated box, shutting down all nuclear reactions, et cetera, anything that brings it closer to thermal equilibrium.

Yes, you are right. Our Sun is not blackbody-like in the fusion site, but it is blackbody like at the surface.
Maybe I should address the question like this: How can we modify the photosphere to make it a better blackbody? Can we do it by increasing the density, and why?

 

[jerromyjon]

How can we modify the photosphere to make it a better blackbody? Can we do it by increasing the density, and why?

What do you think would happen to the blackbody emissions if you increased the density? Would the temperature change?

 

[Ron19932017]

What do you think would happen to the blackbody emissions if you increased the density? Would the temperature change?

Thanks for your reply. If we put gases directly on the surface of the Sun we increases the Sun's pressure so it will be hotter.

However I think we can still just imagine a situation that, the surface shell is departed from the fusion core, like the gas sit in a shell-shaped container and are being heated from central sunlight. Therefore these gas in the shell absorbs inner radiation, and re-emit them by thermal radiation. In that case we should be able to talk about changing the density without changing the temperature, although I am not sure how to tune the volume and pressure of the shell of gas..

 

[jerromyjon]

In that case we should be able to talk about changing the density without changing the temperature, although I am not sure how to tune the volume and pressure of the shell of gas..

I don't have the necessary background to help with that, perhaps someone else can get further into the details...