[♪ INTRO] For most of our history, the universe was synonymous with what we could see when we looked up at the night sky.

Now, we know that that’s just the tip of the iceberg. So-called visible light is only a sliver of the electromagnetic spectrum, and what we can see is only a hint of what’s out there.

So the United States, the United Kingdom, and the Netherlands launched a groundbreaking telescope, the Infrared Astronomical Satellite, or IRAS to explore the IR part of the spectrum. And in just ten months it nearly doubled the total number of astronomical objects that had ever been discovered. So, today, let’s take a look at what makes the infrared special, why it took going to space to unlock its potential, and what all this means for the future of astronomy.

One of the fundamental principles of modern physics is that every object emits light according to how hot it is. That temperature also controls what wavelength of light gets emitted the most. Stars like our Sun shine brightest in the visible because they’re so hot.

But for objects with temperatures from a few thousand degrees to all the way down to nearly absolute zero, the bulk of the light emitted is not visible; it's infrared. Sitting here, watching this video, you’re emitting in the infrared. So is the phone you’re holding, the couch you’re sitting on, and even the Earth itself.

The fact that most stuff in the universe is bright in the infrared is its first main advantage. Its second big one is that it can be seen through clouds of dust. See, motes of dust floating in space are similar in size to the wavelength of visible light.

So, when something like a star shines bright behind a cloud of dust, all we can see is the dust because the star’s visible light gets absorbed or deflected by it. Infrared, on the other hand, has a wavelength longer than the typical size of dust, so the light sails right past it, kind of like how a tree can block your path but a blade of grass cannot. The first big breakthrough came around the 1800s, when William Herschel discovered the existence of infrared, rays of light with a wavelength longer than those we can see.

Infrared light was put to use for astronomy almost right away, but it wasn’t until 1983 that we had the kind of all-encompassing view of the universe in the infrared that we’ve always had in visible light. So, given these huge benefits, why did it take until the 1980s for astronomers to get serious about observing in the infrared? In short: our pesky atmosphere.

Water vapor and carbon dioxide are good absorbers of infrared, meaning that most infrared photons from the universe never reach us. Astronomers have done all they can to work around this, including putting infrared telescopes high on top of mountains and even aboard airplanes. But to truly see the universe in infrared, you’ve got to go to space.

That’s where the Infrared Astronomical Satellite comes in. Astronomers put IRAS, a 60-centimeter telescope, in orbit, with the goal of imaging the entire night sky in search of new sources of infrared light. To do that successfully, IRAS would have to overcome infrared astronomy’s other big challenge: seeing anything other than the telescope itself.

Remember, most of the universe emits brightly in the infrared. That means that infrared telescopes and infrared cameras shine brightly in the infrared. Think of it as kind of like trying to take a flash photo in the mirror.

You don’t get a picture of yourself; you get a picture of the flash. According to physics, the only way to not emit brightly in the infrared is to be incredibly cold...and I mean absolute zero cold. And space is cold, but not nearly cold enough, so IRAS used liquid helium to cool its optics down to just a couple of degrees above absolute zero.

Then, it began taking pictures. Before its helium ran out, the satellite managed to photograph 96% of the night sky, creating the first all-sky survey in the infrared. Perhaps its most important observation was peering through clouds of gas and dust to see our galaxy’s core for the first time.

It also detected one of the very first indications of planetary systems around another star. Inside our solar system, IRAS discovered six new comets and two new asteroids, and performed many observations of objects that were already known. But beyond observations, IRAS proved that infrared astronomy in space could be a success.

The Spitzer Space Telescope began to zoom in on individual infrared objects in 2003, becoming the fourth of NASA’s “Great Observatories.” IRAS also paved the way for NASA’s WISE mission, which, in 2009, applied the same survey technique to observe more than 750 million objects across the universe. Which brings us to today. When NASA selected the James Webb Space Telescope as Hubble’s primary successor, they chose not to observe in the visible, like Hubble did, but in the infrared.

By the time we’re filming this, Webb hasn’t launched yet. But the hope is that, like Hubble before it, Webb will reshape how astronomers see the cosmos, from the atmospheres of exoplanets to the emergence of the first galaxies. But, like IRAS, that view will be one that a human being could never have.

There’s a whole wide universe out there, why limit it to just what we can see? So to remind us that there’s more than meets the eye, we've decided to immortalize the IRAS telescope as this month’s pin! This telescope has fundamentally changed the way we see space and now you can take it home as a reminder of what’s left for humans to uncover next.

The pin will be available all month at DFTBA.com/SciShow, but only during January, so make sure to order yours soon, because next month we’ll have a whole new pin for you. [♪ OUTRO]