astronomy question for homework: need help with Kirchoff's Laws

[MrsBrady]

"using your knowledge of the Kirchoff's laws, describe the spectra of real stars.
(hint: central cores of stars behave as hot blackbodies and are surrounded by cooler transparent gas.)"

A. continuous spectrum with absorption features
B. emission lines only
C. no light will emerge from the star at all
D continuous spectrum only

I know C is wrong, but can't be sure about the others.

pleeeeeeeaaaase help.

thanks!

[HiFructosePronSyrup]

1. A

[MrsBrady]

2. ok why....

cuz there are several of us arguing about this on my end

and this is due at 8am


thanks!

[HiFructosePronSyrup]

3. The star's hot. It's a black body.

Hot black bodies radiant photons across the whole spectrum. Radio, infrared, vis, UV.

That's what blackbodies do.

Now those photons are going to go zooming along on their way to earth. And they're going to run into some gas. Hydrogen mostly. Some helium.

Those gasses are going to absorb some of the light. But only at specific frequencies.

So the light that you observe from that star is going to be a continuous spectrum, with some absorption lines.

Think about the sun.

Is there light? Yes, so we can rule out C.

Is there just a few specific frequencies of light? No, so we can rule out emission spectra.

Is there a continous spectrum? Sure, you can split it into a rainbow with a prism. But if you look closely, you'll absorption lines.

[MrsBrady]

4. ok, so it can't be d....even though the gas is transparent?

[HiFructosePronSyrup]

6. Transparent means it transmits visible light.

Doesn't mean it doesn't absorb IR or UV.

But I think you're overthinking the question.

Transparent gases like hydrogen, oxygen, nitrogen, etc. all absorb in the visible spectrum.

[TreasonousBastard]

7. Who said the gas is transparent and why would it be...

transparent to the entire spectrum? Assume nothing to be perfectly transparent.

(And howcum I didn't see HFPS's answer before?)

[pokerfan]

9. pics

Ideal black body radiator:


representative stellar spectrum:

[TreasonousBastard]

5. What does Kirchoff have to do with wave mechanics? The only...

Kirchoff's Laws I know of are in electronic circuitry.

Anyway, the core gives out a continuous spectrum like any black body, but the gases surrounding it will absorb some frequencies, so it's A.

(Or so I would guess.)

[HiFructosePronSyrup]

8. There's another concerning thermal radiation.

[TreasonousBastard]

10. So there is. But, now that I found something about his three..

laws of spectroscopy, it seems that #2 says hot gases don't act like solid black bodies and only emit certain spectral lines. (this follows the little I know about excited gases)

So, if a star's core is hydrogen, wouldn't the answer be B? If a rock, then A.

(Or, does Law #2 not apply to stars?)

[HiFructosePronSyrup]

11. Hydrogen's a plasma in a star.

Furthermore, it's dense. Hot gases emit continuous spectra when dense, emission spectra when less dense.