Astronomy Fundamentals: Light

 Let's Lighten Things Up

Hopefully you have some ideas about light. Thanks to Isaac Newton and Pink Floyd, we're familiar with the idea that white light can be split by a prism into a rainbow. Prisms can be the classic long glass triangles, and also water droplets - this is how we get a rainbow, of course.

Cover art for Pink Floyd's Dark Side of the Moon featuring white light being split by a prism.

So what's happening? And why do astronomers care? Well, astronomers care about light because that's kind of all we get as far as information about the Universe. When we observe starlight or collect sunlight, we're collecting photons, which are units of light. These get kind of interesting but also technical.

Getting a bit technical

So, photons are units of light, and we can kind of visualize them as packets of a wave. You might've heard light described as both a wave and a particle, so the way I think of it is as a particle with wave-like properties. Specifically, the properties are "color" (i.e. wavelength (i.e. frequency)) and "direction" (i.e. polarization orientation). 

A photon is a wave of an electric field and a magnetic field moving forward at the speed of light, and therefore has particle and wave qualities.

Right now, I'm just going to talk about the wavelength/frequency. As shown in the above image, a photon consists of an electric field interacting with a magnetic field. Electronics and magnets go hand in hand (like when your younger brother puts a magnet up on your TV screen and breaks both the TV and the VHS copy of Cinderella inside). The same effect is happening here, where the movement of the electric field influences the movement of the magnetic field and vice versa. The oscillation, or back and forth movement, of these fields is happening as fast as possible, by definition*. There is nothing technically stopping this oscillation from going as fast as possible, and so it goes at the speed limit of the Universe: the speed of light.

You'll notice, dear reader, that these oscillating fields are waves, which means they have a wavelength. If you're familiar with waves, you might also know they have an amplitude; this property, though, is constant between all photons, and so is not really worth distinguishing. The wavelength, though, tells us all we need to know about that photon. 

The other key property of a photon to talk about is its energy, which is conveniently directly related to the photon's wavelength. When a photon has a longer wavelength, it has a lower energy. A longer wavelength also indicates a smaller frequency, since they are inversely proportional to each other. The opposite is also true: a shorter wavelength with a larger frequency has more energy. 

* This definition is described in mathematical detail with the Maxwell equations

Understanding the Rainbow

When we see light, we are seeing many many many photons. How many photons we detect is directly related to how bright the source is (the technical term is intensity). A bright lightbulb is sending out more photons than a dim one. When we stare directly at the Sun, unblinking, for hours on end (ok don't actually do this), we see many many photons, of course. But each of those photons actually have a different wavelength and energy. 

The electromagnetic spectrum, showing the category of radiation a photon exibits based on its wavelength.

Above is a diagram we use to show the range of wavelengths photons have: they can be very long in the radio waves, pretty short in the visible light, or extremely short and energetic in the gamma rays. This range is called the electromagnetic spectrum, and we experience all of it. Our eyes only detect a small portion of it (named, of course, the visible spectrum), but we feel photons that are in the infrared as heat. That's right, when you feel heat from the Sun or a heat lamp, you're detecting photons. A microwave uses photons with a longer wavelength (in the microwave spectrum, no less) to energize the water molecules in your food. Broadcasting stations send out radio waves because they are long enough to not be disrupted over long distances. 

On the other side of the visible spectrum is ultraviolet, which the Sun also emits. The wavelength of these photons are big enough to be stopped by our skin cells, but they're also very energetic, and can damage our skin cells (which is how we get sunburned). X-ray photons have a wavelength that's small enough to pass through our skin, muscle, and other tissues, but can't pass through our bones: doctors use this to their advantage in order to see into a patient. Finally, gamma rays are pretty rare, and usually only found in the most extreme environments like nuclear power plants. They're extremely small and can pass through all but the densest materials, and are also extremely high energy, so they can do a lot of damage. 

Phew. Ok, so that's how the average Joe interacts with the EM spectrum.

Now back to astronomy

The Sun emits a ton of photons, but these photons have a wide range of wavelengths. We can feel the heat from the photons in the infrared; we see photons in the visible light; we get burned by photons in the ultraviolet (PSA: wear sunscreen). Remember the Pink Floyd album art? Well we can take all of these photons and split them up by wavelength using a spectrograph, which is a fancy dancy prism. Here's a really pretty, detailed image of the Sun's visible light passed through a spectrograph (i.e. the Sun's spectra)

The solar spectrum, where photons from the Sun are separated by wavelength.

The first thing you might notice is that it's very pretty. I love it. Please enjoy 🌈

The second thing you might notice is that there are a lot of splotches that are not so pretty. But that's ok, they're definitely supposed to be there. There is a ton of information in those black spots, but it's pretty complicated and deserves its own post. I'll come back to this later. (I'll just say now, these are not related to sun spots, so sorry if that's confusing. Two separate things. I'll get to both later, pinky promise).

Overall, the green part looks very bright, while the blue and red parts perhaps look a bit dimmer. There are a number of reasons this is the case:

1. The human eye has evolved to see green extremely well. The average person can distinguish more shades of green than either blue or red, and this is simply part of our biology and evolution. Something something leaves and berries. So our eyes are going to see the green more vibrantly here than the blue or red.
2. The types of devices used to collect this data work in "bands," meaning that they are very efficient at collecting photons that are, say, green, but maybe less efficient at collecting photons that are, say, red. Astronomers use a variety of instruments in order to collect a more accurate representation of the source, but with only one spectrograph, the fading in and out at the top and bottom of the image can be due to the instrument itself.

Putting all of that aside, I will now show a plot. Brace yourself...

The solar spectrum as a plot of intensity vs. wavelength. Labelled are sections of UV, visible, and IR light

Let's go through this together...

The x-axis is wavelength, so UV is on the left and outlined in purple, the visible spectrum is the rainbow part outlined in yellow, and the IR part is on the right outlined in red. This image points out that the area under the curve changes based on the wavelength region. What this means is that, overall, there are very few photons from the Sun that are UV photons (but still enough that you should wear sunscreen). There are a good amount of photons from the Sun that are in the visible spectrum, but the largest area belongs to the IR spectrum. 

Now. I would argue that this is simply how we define these regions. We put limits on the visible spectrum not because of anything physically going on with the photons or Sun, but because that's how human eyes developed. One could imagine that a mantis shrimp, which has way more cones that we have and can see in the UV, would extend their "visible spectrum" further into the UV. Or snakes that can detect heat signatures would extend their "visible spectrum" into the IR. So I don't actually like this whole "area under the curve" thing. But this was the best image I could find for now.

I need to wrap this up.

Conclusion: The Sun is a green star

The black curve in the above plot shows the intensity of the Sun as a function of wavelength, or, how many photons of a given color come out of the Sun. If you collect solar photons for an hour, what would be the mode of the color? Which color would have the most photons?

GREEN!!!!

Our Sun gives off green photons more than any other color in any other region of the EM spectrum! It's a green star!! We see it as white, though, because those green photons are mixing in with the blue and red photons that are also coming out, and there's almost as many blue as there are red, which are also just a bit lower than green. Mixed altogether, we have white light, and need to use a prism or spectragraph to separate them. Our eyes might see green brighter than red or blue, and our detectors might collect green photons better than red or blue, but our Sun does in fact emit more green photons than red or blue, and so is brightest in the green part of the spectrum!

Thank you for getting hyped up about this with me. It takes a lot of background to get to this fun fact, and I gave way more information than necessary because I also find all of that information fun too. There's so much more to dive into, like what those black spots are in the spectrum, what Sun spots are, why other stars are red or blue, and so much more. See you in the next post! 🌠