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People have been awed by the sky ever since there were people, and today's astronomers are heirs to that millennia-long tradition. But what's surprising is that the way modern researchers study the sky hasn't really changed in the last few centuries.

Sure, our methods have gotten better, and there's the odd cosmic ray to keep things interesting. But for the most part, astronomers still study things by analyzing their light, in other words, by looking at them. They look at stars.

They look at galaxies. They look at empty space. They even look at dark matter, and you can't see dark matter!

Still, in the end, light is all astronomers need to consistently blow everyone's minds, and there are three main ways they use it. The most obvious way is just taking pictures of things, whether in visible light or another wavelength like infrared. Essentially, light comes into a camera, and the camera spits out some kind of picture.

This technique is a major way we study things like Saturn,. Pluto's famous heart, and even distant planets. But it's not just about collecting pretty pictures.

Results from direct imaging build off each other just like any other scientific method. For example, besides helping us understand Pluto's heart, this method is also a big reason why we don't call Pluto a planet anymore. That argument hinged on what else we directly saw in its orbit.

There is a lot you can't see just by looking straight at something, though, so astronomers have also had to develop other techniques. Their second trick is investigating polarization, or how light waves wiggle, or oscillate, as they travel through space. Generally, light from stars and most other sources starts out randomly polarized, oscillating in all different directions.

But a few things can change that, like if a star is spinning really quickly, if light goes through certain kinds of gas clouds, or if a star has a magnetic field. These are all things that would be hard or impossible to pick out through more direct imaging, so by measuring light's polarization, scientists can research things that would otherwise be invisible. They study polarization by stretching long, thin strips of molecules across something like a lens, making a filter that lets through some polarizations and not others.

Between these effects, astronomers have measured the magnetic fields of planets, the Sun, nebulas, interstellar dust, pulsars, galaxies, the list goes on and on. And these numbers aren't just for funsies. Since magnetic fields come from moving charged particles, those measurements tell us how matter and the charges in it are distributed throughout the universe.

Polarization measurements can also produce some of the most beautiful images in all of science, like this stunning visual of the Milky Way's magnetic field, where the colors tell you about its strength and the lines tell you about the polarization direction. Most astronomical journals aren't dominated by direct observations or polarization measurements, though. They're all about colors.

Lots and lots of colors. There are a few types of astronomy like this, and they're all fundamentally based on studying the colors that enter a telescope instead of the full picture. One method comes from black-body radiation.

The hotter something is, the more randomly its atoms move, and the more light they give off especially at higher energies. Measurements of something's black-body radiation spectrum, then, tell us about how hot it is. Stars' light usually peaks somewhere in visible light, whether that's red for the coolest stars or blue for the hottest.

But things like the accretion disks around black holes, where gas is falling in, are hotter than the hottest stars, and we know that because they emit lots of x-rays. Empty space, on the other hand, is much colder. And, again, we know because of its black-body spectrum, called the Cosmic Microwave Background.

So just using light, we're able to get a pretty good idea of the temperature of things, no thermometer required. A second way astronomers use color is through spectroscopy. Every atom and molecule absorbs and emits light from some colors much better than others.

And after years of study, scientists have figured out how many of those particles behave. They can even graph their light patterns on a chart, kind of like a fingerprint. So when they get new data from stars or interstellar dust or extrasolar planets, they can figure out what the objects are made of just by matching up sets of lines.

Well, almost. Often, when we're studying really distant objects, their spectral lines aren't the right colors. They're usually redder than we'd guess or sometimes they're bluer.

And that's because of Doppler shifts, another one of the most fundamental tools in observational astronomy. Light gets stretched or compressed by movement, and the amount it's distorted tells you how fast it's moving toward or away from you. Doppler shifts were used by Edwin Hubble to discover the universe is expanding, they've been used in all sorts of ways to infer the existence of dark matter, they've revealed hundreds of exoplanets, and they've been used for everything in between.

The fact that we've been able to do so much with light is pretty mind-blowing. And it also helps explain why, as astronomers discover different ways of exploring the universe, like gravitational waves and neutrinos, they've been so excited. We've been using nothing but light for hundreds and hundreds of years.

Imagine what we're going to be able to do learn with something new. If you want to learn even more about those gravitational waves and what new things we might be able to learn from them, you can watch our episode about it. And as always, thanks for watching this episode of SciShow Space! [ ♪ Outro ].