Everything we know about space comes from radiation.
Now wait just a moment here. That statement explains how astronomy is such a successful field of science—it’s based entirely on the information we can glean from radiation, after all. But how does that make sense?
I mean, it’s one thing to study radiation. It’s quite another thing to study matter, the “stuff” in the universe. How does one have anything to do with the other?
Well…that’s where atoms come in. Radiation does, in fact, have a lot to do with the “stuff” it comes from. And if it weren’t for that basic principle, astronomy as a science wouldn’t work.
Thankfully for astronomers, it does. So what’s the secret, then? What does radiation have to do with matter?
Consider this. The universe is absolutely full of radiation.
Radiation bombards us every day of our lives. Anywhere in the universe you go where you can see at least one star, radiation is hitting you. And if you see something that doesn’t emit its own light, radiation is hitting you, too.
You can know this because the only reason you can see anything, regardless of what it is, is because it either produces or interacts with radiation.
How, then, does matter interact with radiation?
It all has to do with atoms.
Atoms are mysterious things. Thankfully, on the most basic level, we understand them pretty well. And we understand what happens when a bit of radiation hits them.
You see…atoms are made up of three basic particles, which I’ve explained in detail in my past three posts. These are protons, neutrons, and electrons.
Let’s consider a hydrogen atom, since it’s both the simplest and the most abundant element in the universe.
A hydrogen atom is one single proton, a positively charged particle, orbited by one single electron—a negatively charged particle.
As I described in my last post, electrons orbit in certain permitted orbits. They’re like steps in a staircase. Electrons can be found on any one of an atoms permitted orbits, but not in between—it would be standing on stair step number one-and-one-fourth.
Now…if hydrogen is the most abundant element in the universe, and the universe is full of radiation, it’s not too surprising if some of that radiation hits a hydrogen atom, is it?
Let’s imagine if that happened.
Here, you see a hydrogen atom. The dotted circles represent permitted electron orbits. Remember, we’re only dealing with one single electron. Let’s assume that it begins on the lowest permitted orbit—n=1.
Imagine that you’re the nucleus of this hydrogen atom, and you’re holding onto that electron. If it’s on n=1, then you’ve got your arms wrapped tight around it and you’re holding onto it with all you’ve got.
But what if a bit of radiation comes along? This bit of radiation is a single photon carrying just enough energy to tear that electron away from you.
Oops—your grip slips a bit. But you’re not letting that electron go without a fight. It only slips out to the n=2 orbit. But now it’ll take less energy to pry it away. And the same goes for if the electron is on any of the other permitted orbits.
Here’s the idea: because each permitted orbit is associated with a certain amount of energy, they’re called energy levels. That explains why you see those words on the diagram above.
An atom whose electron has moved is called an excited atom, and one whose electron hasn’t moved is in its ground state.
So what’s up with the red/blue-green/violet stuff?
Well…let’s consider the photon that came along and bumped the electron to a different energy level.
A photon is a bundle of radiation waves. All radiation travels in the form of waves. These are literal waves, just like ocean waves or wavy lines on a paper. You don’t see them as waves. But trust me, that’s what they are.
Now, these waves have different amounts of energy. See that arrow that points to “increasing wavelength”? It’s showing you that as you get closer to radio waves on this spectrum, the waves are going to get a bit longer from one crest to the next.
And see how, just below that, “increasing energy” is labeled as going in the other direction?
That’s because you can think of each single wave as carrying the same amount of energy. But if you can fit more waves into the same distance, you can fit more energy. So…smaller wavelengths of radiation will carry more energy.
Now you know how a single photon can carry the right amount of energy to move an electron.
The atoms we’re observing are all extremely far away. But we can tell that an electron got moved because it doesn’t stay moved—it falls right back where it belongs. But it also emits a photon of the same energy as the one that hit it.
But here’s the crux of it all.
We can tell what atom we’re dealing with from how the energy levels are arranged. The energy necessary to move any one electron will be different depending on the atom, and depending on which specific energy levels we’re talking about.
That means that if we know which photon hit that atom, we know exactly what element we’re looking at. We know what that object in the sky we’re looking at is made of.
So how do we know which photon hit that atom?
Easy. As I’ve mentioned in a previous post, spectroscopy is the most powerful tool in astronomy. It’s the practice of spreading light from an object out into a rainbow, so we can study each individual wavelength.
Just to be clear, that means each individual photon.
Coming up soon, I’ll talk about exactly how we can know which photon we’re dealing with and what that even means—because this is just the beginning.