As previously mentioned, in astronomy, there are very few circumstances in which astronomers can obtain physical samples with which to work. Instead, our work relies primarily on objects transmitting their secrets to us in another form.
This form is electromagnetic radiation, or, as it is most commonly known, light. For those that remember, the light that we see with our eyes is just a small part of a much larger spectrum of light that we cannot see.
Looking at the above diagram, you are probably familiar with several other of the types of electromagnetic radiation listed. The low energy radio waves are the ones that we pick up our cars on the way to work (assuming of course you're not listening to a CD or something else).
You're probably also familiar with microwaves. However, this title encompasses a much broader range of light than what your microwave oven actually uses (microwave ovens use a very specific frequency that excites water molecules to heat your food while other frequencies won't).
Infrared radiation is most commonly known as heat. When heat is not transferred through direct contact, this is the method that is generally used.
Visible radiation is what we see with our eyes.
Beyond that is the ultraviolet. Bees see in this region of the spectrum, which is why flowers and are frequently marked differently when viewed in this region, in order to indicate where the pollen is:
X-rays are what we use to see through soft tissue and take images of bones. I just had a few of these at the dentist a few hours ago.
Gamma rays are much rarer as they are only generated in high energy reactions. They are quite dangerous as they can cause cancer, but fortunately, our atmosphere shields us from the cosmological sources.
Past gamma rays, and not featured in this image, is the enigmatic cosmic rays which are even more powerful, but extremely rare.
So that's a quick overview of each different region. However, this does not answer the question of what's behind all this radiation? To answer this in a complete manner requires a look at over 200 years of physics history.
Early experiments into how light works revealed that light is a wave. A classic experiment demonstrating this was done in the early 1800's in which light was passed through two narrow slits and projected onto a screen.
If you don't already know the result of this experiment, take a moment to think about what you'd expect. Inuition would tell you it should be like shining two spotlights nearby eachother. Where they overlap, you should have a brighter spot, whereas where they don't, it wouldn't be as bright.
As you might suspect though, this isn't the case. It turns out, that when the slits are made narrow enough, a strange pattern emerges:
In this pattern, we see that there is a series of light and dark bands, which is brightest towards the center, and fades as you move away in either direction.
This pattern is indicative of waves. When a wave from the right slit would interact with the wave of light from the other, the two waves merge. When they merge in such a way that the crest of one wave matches with the crest of another, then it makes a bright spot. When the crest meets a trough, they cancel out and that point on the screen is dark.
So the wave theory of light was established. However, if light was a wave, waves, like water waves and sound waves, need a medium through which to travel. That means that there should be something beyond our atmosphere though which the wave could propogate. This mysterious medium was dubbed the "ether".
Unless you're really into science, most of you reading this have probably never heard of this ether. It's likely you don't remember everything from science class, but this term is probably one you've never even heard (unless you play a lot of roleplaying games).
So why don't we teach about this ether in science courses? The reason is that it was eventually discredited in the early 1900's by a team of scientists known as Michelson and Morely. These two attempted to determine the properties of the ether. Since the Earth travels around the sun, the Earth should be moving through this ether. Therefore, waves should be deflected in some measureable amount as they were swept away by the current relative to the moving Earth.
However, their experiment was completely unable to detect any sort of variation. No matter how many times they tried their experiment, the results always showed the waves propagating at the same speed, roughly 3 x 108 meters per second.
The puzzle seemed unsolvable and it would take a genius like Einstein to solve it. In fact, it was Einstein that solved it. Although most people known Einstein for his famous equation, E = mc2, and his laws of relativity, his nobel prize was actually awarded the prize "for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect."
Huh? Photoelectric effect?
Since a comprehensive explanation would take quite a bit of time and space, I'll cut to the chase on this one and just say that this effect shows that light comes in discreet "packets". This indicates that light is a particle with a fixed energy. This particle, which travels at the speed of light and has no mass, was called the photon.
So which is it? Is light a particle, or a wave?
In reality, it's both. This discovery, and similar ones that showed all subatomic particles exhibit this wave/particle duality, gave rise to the entire field of quantum mechanics.
But that's beside the point for this post. For this topic, it's just important to understand that light can be thought of as either and while neither is wrong, one may be more convenient than another for explanation.
So now that you have a rough understanding of what light is, the next question I'll cover is "Where does it come from?" That should cover everything about light from the time it starts, until we detect it.
Then in the next section, I'll cover the history of observations of the light that crossed these vast distances, from the naked eye, to the modern CCD (more emphasis will be placed on the latter since that's what contemporary astronomers do).
Then last, I'll attempt to cover how astronomers can glean detailed information from a ray of light.