Eee-gads. It’s been a long time since I actually wrote for you – I jumped on the NSF-grant-submitting bandwagon, and the deadline is fast approaching. That took priority over AstroInfo for a while, much to my regret. I have a list of topics to address as long as my arm, so let’s start working through them, and hope that I haven’t lost all my readership!
As a birthday present to my brother, let’s talk about how birthday candles are related to rainbows, stars, and astronomers. Get out some rainbow glasses. What? You don’t have any? Well, the stores generally sell a bunch right now for looking at Holiday lights – so pick up a cheap pair  at your local … well, it’s unpredictable who will carry them.
Play with them. You’ll notice that different lights look different, and give off different rainbows. From a birthday candle you’ll get a smooth, pretty, complete rainbow – maybe some extra yellow. From a fluorescent light you’ll get a couple of single-colored “pictures” of the light you’re looking at. The birthday candle gives you a “black body” spectrum – the whole spectrum of light, released because the candle is hot. From the fluorescent light you’re getting an emission spectrum. DO NOT LOOK AT THE SUN!
The Lines (the individual colors you see when you look at a fluorescent light):
Electrons. Electrons are jumping around. When energy (light) hits an atom (like the stuff inside the fluorescent light), it excites the electrons. They jump into a higher energy state (go read the first 3 chapters of a chemistry book). When they fall back down they release energy (light). We see that energy as a specific color based on the exact amount released by the electron. The specific electrons of specific atoms release specific colors of light. Always. Therefore we know, every time we see a certain color of red, that it was created by electrons around a Hydrogen atom changing energy states.
The opposite is also true – when the electron jumps up to the higher energy state it absorbs that same specific amount (color) of energy (light). So if we see that only that very specific color of red is missing, we know that the electrons around a Hydrogen atom absorbed that energy.
Each electron can only absorb or release a certain, specific amount of energy. Not a little more or a little less, just that one amount. (Well, you have a few choices, but they’re specific choices).
The Smooth Rainbow:
The “black body” spectrum is just heat energy being released as light. Since there are no electrons jumping, it can release any amount (color) of energy at all, and so it emits them all at once – and you get all colors – a rainbow!
Now, my colleague Zeta has a really great write-up on this, to which I’ve added a few details.
If the sun is mostly hydrogen and helium, why does sunlight emit the full spectrum of visible light?
Darn good question. Here’s an answer- stick with me, we need to go through a couple steps to get there.
Ok, our sun has nuclear fusion going on inside of it, and this energy heats the gas our sun is made of to high temperatures. Some of this heated gas turns to light energy, which is the light we see. (in fact, our sun gives off all types of energy of the E-mag spectrum… but the most energy the sun gives is in the visible part of the spectrum, peaking in the “yellow” wavelengths, which is why our sun appears yellow.) All things that are hot give off a visible spectrum, and this is what we see when we put sunlight through a prism. (This is like the birthday candle, but hotter!)
This is called a continuous spectrum, because, well, it’s a continuous strip of colors. This is very similar to what you’d get from a birthday candle, but more complete.
However, part of why we see this continuous spectrum is because our eyes don’t see all the little details that are actually present. If we looked at this spectrum super closely or with cool machines that can tell us the amount of light actually present at each wavelength along the spectrum, we would see something different…
this is the sun’s spectrum actually. See the dark lines? These lines tell us that some elements present in the sun’s atmosphere are actually absorbing energy at certain wavelengths, and the result is “holes” at those wavelengths. (this in fact is called an absorption spectrum, because certain wavelengths are being “absorbed” by gases.)
And by looking at where these black lines are, and what wavelengths they match to, we can tell what gasses are present to cause these absorption lines. This is how we know the sun is mostly H and He. About 3-10 of those lines are the absorption by Hydrogen and Helium. The others are absorption of other elements – because our Sun is a bit “dirty” – it has a large number of different elements, just not very much of each. This is called metallicity. Younger stars have a higher metallicity, because they were made out of the stuff that older stars spewed out into the Universe when they died- Carbon, Iron, Uranium … etc. (Here’s a labeled version of the Solar Spectrum )
In our classes we are showing an emission spectrum of a certain gas, and that is why for H we see this:
Simply put, we don’t see just this emission spectrum for our sun, because our sun is so very hot, and because of that heat a whole visible spectrum is emitted.
Also, astronomers don’t generally use emission spectrums of stars like we see in our lesson- its an oversimplification. Astronomers do study emission spectra, but only in nebulae. Nebulae emit as excited gasses. Maybe stars do, but it doesn’t overpower the black-body spectrum.
Generally, astronomers use the absorption spectrums to see where black lines fall in the spectrum to determine compositions of stars and atmospheres of planets. However, it’s in the lab that astronomers can learn the emission spectrums of certain gasses, when is needed to determine what gasses are showing up in the absorption spectrums they are seeing.
Alice Enevoldsen & Zeta Strickland
P.S. I’m sure that most of you won’t actually read all the way through this one, so I’m writing it as a reference to hand to people when they ask these questions.