Everything you ever wanted to know about light. Join me in this tutorial to get your head around the most important concept in all of astronomy: the electromagnetic spectrum.

Making Photons
Nearly everything we know about the universe has come riding in tiny packets of light we call photons. Photons can tell us about the temperature, chemistry, and movement of any object they come from.

Think of the simple model of an atom: a nucleus jammed with positive protons and neutral neutrons surrounded by a swarm of negative electrons. The electrons orbit the nucleus at specific distances, based on how much energy they have to stay away.

Bear with me as I explain: Because positive charge wants to be with negative charge to cancel out, keeping them apart requires a lot of energy.  The closer an electron is to the nucleus, the harder it is fighting to stay there. It needs loads of energy to move father away. Likewise, it gives up some of the fight if it falls in closer.

To make a photon, an electron has to fall inward toward the positive nucleus. In other words, it has to give up some of the fight, some of the ooomph it had staying so far away. A packet of energy called a photon is what is given away. Scientists have figured out how to open this packet and learn all about the atom that released it.

Making Spectra
Opening the photon packet, or spreading open the beam of lightwave, reveals that it is a collection of energies. Astronomers call them wavelengths of an electromagnetic spectrum. The part we are used to seeing is the rainbow. Astronomers know how to break up a rainbow even more; imagine hundreds of reds, oranges, yellows...Crayola has nothing on light!

Invisible colours are also in the spectrum, such as wavelengths that are too red or violet than our eyes can see. Astronomer range them from Radio, Microwave, Infrared, Visible (your rainbow from red to violet), Ultraviolet, X-ray, up to Gamma-ray.

Seeing Temperature
Depending on how far the electrons have fallen, they can give off any of these energies. For example, objects that are not very energetic, like a cloudy nebula of gas, give off lots of the less energetic photon packets. The light arrives as long, lazy wavelengths called Radio waves.

Quasars, on the other hand, are blazing hearts of galaxies that blast out super short, high energy wavelengths called X-rays and Gamma-rays.

If the packet of light received from an object reveals more light from the redder end of the spectrum, astronomers know the object is rather cool. But, if the packet is bursting with X-rays, astronomers know what they've found is powerfully hot.

Seeing Chemistry
The atom a photon came from, and any atoms it hit along the way, leave their mark on the spectrum in the packet. Let's go back to the crayon box: if as a kid you always drew horses in fields, you'd have used a lot of browns, greens, and yellows. If your sister always drew dolphins, she used blues and greys and whites. Someone who knew you both could pick up a box of crayons and know whose box it was, based on which crayons were worn the most.

Likewise, astronomers can look at the spectra of objects and know what they are made from, based on which colours are used the most or not used at all. For example, hydrogen shows up only in certain colours that are different from those of helium. If you know to read this, then a photon's packet is filled with detailed information of its travels!

If certain colours are bright, then this tells you which atom or molecule gave off the energy to release the photon in the first place. If certain colours are gone, then this tells you which atom or molecule took some energy from the packet on its way to you. Clever, huh? Every element and even molecules show themselves this way in the exact colours that are bright or gone in the packet, like a fingerprint. Studying spectra is like the forensic science of astronomy.

Using Filters
You, too, can steal these photons whenever you want by using a filter. Filters work by being made of a material that soaks up photons of specific colours. In other words, they nab certain colours and allow the rest to come through. The main filters are red, green and blue, or RGB. Your computer monitor works by mixing these light colours! I can prove that, if you lick the tip of your finger and touch the monitor. Look for the tiny little RGB blocks, and you'll believe me. Just don't stare for too long, it can hurt your eyes.

The filter colours are the clue: a blue filter is blue because it allowed the blues to come through. So, what it did was nabbed the red and greens and didn't let them out. A red filter, then, does the opposite and allows only reds through while stealing the blues and greens. Yellow is a mixture of red and green light, so what would happen if you put a red filter over a yellow object? It would look red. What if you put a blue filter over it? It would look black. I'll let you puzzle that through!

Astronomers learn a lot about object in space by placing filters onto their telescopes. For example, by taking images of the same object in the three main filters, RGB, they can find out quickly in which colour the objects shines most bright. Depending on what they're viewing, this tells them about the temperature of the object or about what the object is made from.

Filters can be so precise in which queues they block that they can be used to view only those bits of the spectrum that an astronomer cares about. Many of the breath-taking Hubble Space Telescope images are made this way, and thatís why they look so delicate and detailed: the images are only of a very specific set of photons. The rest of the mess has been cut out to leave that very tidy image!

Seeing Movement: Redshift
I said that different atoms leave their fingerprints on light. The mark usually comes as a specific pattern of wavelengths lit or darkened, like a barcode in the spectrum. A curious thing happens when an atom gives off light while it is speeding away through space: the barcode shifts down towards redder light.

Imagine this: you and a friend play a game where your friend stands on the pavement and you are in the front seat of a parked car beside her. Every second, you throw a ball to your friend. Your friend would be catching a ball with one-second gaps in between.

Now, imagine that the car drives off, and you are moving away. As you get farther from her, the ball has to travel farther to her every time you throw it, and the gaps get longer between her catches. In this way, she could say that the length between catches was getting bigger.

In terms of light, as the object speeds away, its light has longer to travel to get to us. So, we would say that the wavelengths were getting larger. Larger wavelengths of light are redder in colour, and so we call the change a redshift.

Astronomers can measure the amount of shift in a barcode to find out exactly how fast the object is speeding away from us. From this discovery, astronomers have spotted planets sweeping around other stars,  measured how fast our Milky Way Galaxy spins, and learned that the fabric of the infinite universe is stretching. Whoah. All from opening the packet of light called a photon.