Tuesday, August 18, 2009

What is Broad-line Emission?

That is the question Stef asked me the other day. Actually it was more like "What is the difference between narrow-band spectra and broad-band?" Initially I said I don't know, but then curiosity piqued my interest and I had to look it up. Nothing but academic papers came up on Google, so I read some abstracts and intros and tried to pull out key words that I could look up in Wikipedia. Finally I found something relevant. Let me explain it to you.

I pulled everything I know from here so follow along with the pictures or just read that page yourself and skip my version. Or if you want the probably half wrong, half over simplified version then stick with me.

We basically get light because some atom becomes "excited" and then emits a photon to return to it's ground state. The atom usually becomes excited because it has absorbed a photon (or several) to move the atom into it's excited state. That is called absorption, and when a photon is emitted that is called emission. We have detectors that can detect (sorry for this awful streak of redundancy) both emission and absorption.

In stars, galaxies, etc there are lots of gases swirling around and lots of photons entering those gases. A gas is just a bunch of atoms or molecules and so there is lots of emission and absorption happening. We can see that stuff with our detectors and thus can know what kinds of gas we are looking at. Oh I forgot to mention the reason this is, is because photons have frequencies and atoms emit at specific frequencies. So if we know what frequency of light we are looking at we can match it to what atom emits at that frequency (or absorbs).

Now in special situations, like the kind that exist in the universe, gas goes through some crazy phenomena. In space it is under all sorts of crazy conditions like high pressure, temperature, density, etc. These situations can affect atoms in different ways. According to Quantum Mechanics, if you looked at the energy given off by Hydrogen (for example) you would notice bands. That is because atoms only emit in quantized amounts meaning specific numbers. In our world we see a continuous spectrum, but to atoms everything is discrete.

Anyways, as you take more effects into account, you have to structure the quantizations a little differently. Basically you are zooming in. For instance you will see distinct energy bands if you only take electron orbital level into account. You will see a different organization if you include electron spin. You would see an even different organization if you include more and more specific natures of the atom. This is way simplified, but you get what I'm saying.

Anyways there are physical effects you could see if you do other things as well. One such thing is called the Zeeman Effect and another is the Stark Effect. One happens when you apply a magnetic field to an atom and the other is when you apply an electric field (respectively). Basically these effects can change the energy from a single band in your detector to multiple bands around the same frequency as the single band you would normally have had. This spreading is where the term broad-line (or band) emission (or absorption) lines come from.

Check this out. In that picture, at E=0 (basically the y-axis only) you have discrete energy levels for various energy states. As you apply an electric field and increase the field each distinct energy level becomes several energy levels. If you are looking through a detector at a cloud of hydrogen, under normal circumstances you would see a spike at whatever frequency photon you normally get from hydrogen. But if the conditions are such that you are getting a Stark effect (for example), then you would see a smear of frequencies around your normal expected frequency. Thus you went from a narrow-line emission spectra to a broad-line emission spectra.

Get it? (Sorry for the no pictures and the really dumbed down explanation, but I don't have time to teach all of quantum mechanics.)

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