Emission Spectrum Vs Black Body Spectrum, Why are they different?

We use Emission Spectroscopy to determine the composition of interstellar bodies such as stars and nebulae. This works because certain elements only emit light at certain frequencies.

On the other hand, our sun emits light across a broad range of spectrums, with no prominent spikes in particular spectrums. (Solar Spectrum) This is known as black body radiation.

What is the difference between the light produced by a hot interstellar gas and the light produced by our sun? Is the light from our sun a broad range of elements producing different frequencies or am I misunderstanding the conditions for distinct emission bands?

I might as well share what got me started thinking about this stuff:
Cygnus X-1 Confirmed as Black Hole

Edit: fixed link

This post has been edited by flyingbuttressman on Jun 28 2011, 02:35 PM

For one thing, they were observing x-rays from the disc of hot gas. This represents millions of degrees. Our sun is a class G star (5,200-6,000 K).

it is a misunderstanding of the differences between a discrete and continuous spectrum. they are really two different things...Read this it will clear it up for you.

http://en.wikipedia.org/wiki/Discrete_spectrum

classical vs quantized.


This post has been edited by bar_room_physist on Jun 28 2011, 11:55 PM

Thanks for the link. Perhaps you can clear this up. The wiki article for Continuous Spectrum contains this note at the bottom:

I get that they are different things, but WHY is it discrete in one case and continuous in another?

it's how we choose to do the experiment...i think. perhaps and this is speculative it is an example of wave particle duality.

Maybe this will help.

http://www.tutorvista.com/physics/define-emission-spectra

1) The sun emits light across a broad range of wavelengths. A broad range of spectra (not spectrums) makes absolutely no sense.

2) Black body radiation a bit more complex than just this.

3) The solar spectrum is actually an absorption spectrum superimposed on a black body spectrum.

We use Absorption Spectroscopy mainly to get compositions of stars and those nebulae which are lit from within by stars. We use emission Spectroscopy mainly for nebulae which are ionized due to nearby stars but aren't directly lit by a star. Sometimes you can use both on the same nebula depending on the stage of its life. You'll get light from a star passing through some parts and ionizing other parts.

The sun is so hot that the black body spectrum overpowers any emission due to a specific transition. Instead the transitions tend to remove some of the intensity of the black body spectrum giving us an absorption spectrum via which we can get the chemical composition. Generally speaking if the gas is ionized the only light emits is the characteristic transitions due to relaxations. Also, stars are opaque and surrounded by a lot of gas that doesn't emit light. All the light emission of a star happens within a shell of gas that only absorbs light and doesn't directly emit it. In contrast those nebulae hot enough to be ionized produce light all the edge and they tend to be very dispersed. The dispersion means the atoms are more likely to ionize than emit black body radiation. It is also important to remember that inside say the sun were the light is produced the density is so high it is pretty much like a solid and as such the energy levels tend to be more continuous than the discrete energy levels we see in the rarefied nebulae.

What you use is mainly a function of what you are looking at, where you are looking at it from, and what happened to the light between emission and you.

Thank you, this is what I was looking for.

sithdarth has pretty well answered FBM's question.

Another take on the matter is that transitions of bound electrons involving the principle quantum numbers result in nice discrete lines.

In and around the sun, bound electrons can be energized to the continuous emission band extending from the limit of the discrete energy levels to the continuous region of possible energy values.