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Date: February 4, 2010

Title: Spectroscopy 101

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Podcaster: Richard Drumm

Link: Richard’s Blog – http://theastronomybum.blogspot.com/

Description: Richard Drumm talks about the basics of how spectroscopy works and why it is so interesting to professional astronomers.

Bio: Richard is the owner of 3D – Drumm Digital Design, an award-winning video production company. He was an observer with the UVa Parallax Program at McCormick Observatory in 1981 & 1982. He’s found that his greatest passion in life is public outreach astronomy and he pursues it at every opportunity.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Kylie Sturgess of the Token Skeptic podcast, investigating superstitions and the science behind them at www.tokenskeptic.org.

Transcript:

Hello! Welcome to the 365 Days of Astronomy podcast! I’m Richard Drumm The Astronomy Bum, in Charlottesville, Virginia. Like I said last time, if Bill Nye can be “The Science Guy” I guess I can be “The Astronomy Bum”… Drumm doesn’t rhyme with much, ya know?

Anyway, this month’s podcast will be a little different from my usual interview-type podcast that you’ve all become so accustomed to. You see, all the area’s professional astronomers, the folks at the University of Virginia Astronomy Department and the folks over at the NRAO, well they’re are all up in Washington DC at the AAS winter meeting (that’s the American Astronomical Society) so I’m kinda on my own this month! Well, last month as it’s the first week of January right now when I’m recording this, not February.

So I thought I’d just chat with you all for a little bit and maybe recall a blog post I made a few weeks ago on the topic of spectroscopy.
My blog by the way is at http://theastronomybum.blogspot.com/ No need to put in a www.

[1:07]

Here’s how I put it on my blog:
“It’s like I’m looking through my telescope and see that there’s a speck on a hillside. I know the speck is a person, but I can’t tell if it’s a man or woman, or if it has both legs, nothing. But I can read this person’s mind!”

For reals, people! I’m not kiddin’ ya!

You know, when we see a news item out there that talks about something of interest to astronomers, we see them talking about spectra being taken almost even before they take the time to get a picture of the thing they’re talking about.

So what gives?

Why is this spectoscopy thing so attractive to the pros? It’s almost like they don’t even want to look at a photo of anything, they just want spectra! Well, it seems like that sometimes.

Well, after this little chat, you’ll understand (I hope) that these spectra are the keys to the kingdom, you know, the cat’s meow, whatever! And they deserve all the attention they get!

I mean, we all know our colors, right? And we all have gotten the color spectrum, the “Roy G. Biv” sequence in high school classes. So everybody knows at least that much about it. And we’re told that bluer colors of light have greater energy, you know that a blue flame is hotter than a red flame. But what is the spectrum telling astronomers?

[2:26]

First a little history. Hey, hey, stop that!
I heard that groan!

Anyway…
In 1814 we took a little trip…
No! No, no, no.

In 1814 Joseph von Fraunhofer discovered that the Sun’s spectrum had some dark lines in it, it wasn’t a smooth wash of colors from red to blue. By the way, we now call these Fraunhofer lines in his honor. Then it was Kirchoff & Bunsen (yeah the same Bunsen as in the Bunsen burner) – a little later in 1859, well they figured out that these lines were a result of the energy levels of the electrons in the gas in the flames they were studying.

They’d shifted from studying the Sun’s spectrum like Fraunhofer, to looking at the spectra of flames in the lab. This was a more convenient & controllable environment. And the work could continue on cloudy days and at night, aaaand with stuff like mineral salts sprinkled into the flame they could study known substances.

Kirchoff came up with 3 laws:
1. That a solid body will put out a continuous wash of colors. When it’s heated up to incandescence, that is.
This is now called black body radiation.
2. That a hot gas (instead of the solid body like in #1) will put out discrete, bright lines of color, kinda the opposite of Fraunhofer’s dark lines.
These are called emission lines nowadays. And,
3. That a hot solid body surrounded by a cooler gas will show the dark Fraunhofer lines just like the Sun does.
These are what we call absorption lines.

[4:03]

Now a big clue for Kirchoff & Bunsen came when it was discovered that the dark lines for Hydrogen (where it was the cool gas like in law #3 a moment ago) & the bright lines for burning Hydrogen (uh, that would be law #2) well, these lines had the exact same pattern, and were the same colors. Clearly they were different sides of the same coin, you know, the same phenomenon expressed 2 different ways.

Well, to understand all this we have to make ourselves small.
Very small [pitch shifts up to higher frequency] smaller still! Keep on going!
[pitch normal again]
Ahem! There! Now we’re the size of an atom and we can see the electron in its orbit. It’s convenient for us to think of an atom as being like the solar system, you know,with the nucleus of the atom in the center kinda like the Sun is in the solar system.

The electron is actually in a spherical cloud of a sort, but let’s think of it as being in a single flat plane.

Let’s examine an atom of Hydrogen for simplicity’s sake.
I like simple, don’t you?

Now the nucleus of our atom has just a single proton in it, and there’s just a single electron orbiting it. It doesn’t get much simpler than that, eh?

[5:15]

Now, that electron has a larger orbit when it has a higher energy. Its lowest energy orbit would be like Mercury’s orbit, to use the solar system analogy,then higher energies give you Venus’ orbit and so on. Imagine that this electron is in the 3rd level, like Earth’s orbit. Now this is a hot atom, one that is bonking into its neighbors a lot! Energy, thermal energy in this case, is literally just atoms jostling around bumping into each other.

OK, if we were in the solar system we could set out from Earth and head to Venus and after a time we’d be 1/3 of the way there, then 1/2 of way there and on & on. But in the world of the atom, where quantum mechanics holds sway, there’s no part way.
Either you’re at Earth or Venus, there’s no “in between”.

So if our atom cools down it can get to a lower energy state and the electron can find itself at the energy level it needs to be in to exist at the next lower orbit.

So what happens is it jumps instantaneously down from the “Earth” orbit to the “Venus” orbit. Ping! And it’s there!
When it makes this “Quantum Leap” (so that’s where the old TV show got its name!)…
Well, it also [Ping!] emits a photon of light that has a color that is directly tied to the energy difference between the 2 orbit levels.

[6:39]

And the reverse is true too, if a photon comes along and hits the atom, it can knock the electron up to a higher energy level, but only if it has exactly the energy (or color) needed to make the electron make a jump to a higher level.
Otherwise, the photon doesn’t interact with the atom & passes right on through.

This means the atoms will selectively filter out photons of certain colors, because only those photons have the exact energy needed by the atom’s electrons for the various orbit jumps that it can do. This is what makes the dark Fraunhofer lines seen in the Sun’s spectrum.

And when our Hydrogen atom cools down and it’s lone electron jumps from level 3 back down to level 2 (like going from Earth to Venus), it emits a red photon of light that we call Hydrogen Alpha light.

Solar telescopes often filter out all the wavelengths of light except this one red color, giving us a look at interesting details of the Sun at a certain temperature level or depth on the Sun’s surface.

[7:44]

So when astronomers look at their spectra, they’re looking directly at what’s happening inside the atoms of a star!
We can’t image the star directly (except for a very few) but we can see inside their atoms!

So this is like reading the mind of that person in the telescope.
You can’t see the person, not clearly, but you can get inside their heads!

Oh yeah, did I mention it? Science rocks!

I’m Richard Drumm The Astronomy Bum, and I’ll see you next time on 365 Days of Astronomy!
[8:16]

End of podcast:

365 Days of Astronomy
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