This is all the light in the universe that we can see.
It’s just a fraction of what’s out there. Most frequencies of light are actually invisible
to us.
The light we can see appears red at its lowest frequencies and violet at its highest.
This is called the “visible spectrum,” and we see it because cells in our eyes called
“cones” interpret light reflecting off of objects.
We have three different types of cones that are sensitive to long, medium, and short wavelengths
of light.
Which roughly correspond to red, green, and blue on the visible spectrum.
These are the primary colors of light. Every other color is some combination of these three.
And that combination is the guiding principle in colorizing black and white images.
This portrait was taken in 1911.
I know. You came here for space photos. We’re getting there, I promise.
It’s one of the first examples of color photography, and it’s actually three black-and-white
photos composited together.
Russian chemist Sergei Prokudin-Gorskii took three identical shots of this man, Alim Khan,
using filters for specific colors of light.
One allowed red light to pass through, one allowed green, and one allowed blue.
You can really see how effective this filter system is when you compare the red and blue
exposures.
Look how bright Khan’s blue robe is in the photo on the right, meaning more of that
color light passed through the filter.
Dyeing and combining the three negatives gives you this.
Alright, you get the idea. So let’s take it into space.
The Hubble Space Telescope has been orbiting Earth since 1990, expanding human vision into
deep space and giving us images like this one.
The thing is, every Hubble image you see started out black-and-white.
That’s because Hubble’s main function is to measure the brightness of light reflecting
off objects in space, which is clearest in black-and-white.
The color is added later, just like the portrait of Alim Khan/
Except today, scientists use computer programs like Photoshop.
Let’s use this photo of Saturn as an example.
Filters separate light into long, medium, and short wavelengths.
This is called “broadband filtering,” since it targets general ranges of light.
Each of the three black-and-white images are then assigned a color based on their position
on the visible spectrum.
The combined result is a “true color” image, or what the object would look like
if your eyes were as powerful as a telescope like Hubble.
Okay, now one with Jupiter.
See how combining the red and green brings in yellow?
And then adding blue brings cyan and magenta to fully represent visible spectrum.
Watch this animation two more times and I think you’ll see it.
Great, now let’s add another level of complexity.
Seeing an object as it would appear to our eyes isn’t the only way to use color.
Scientists also use it to map out how different gases interact in the universe to form galaxies
and nebulae.
Hubble can record very narrow bands of light coming from individual elements, like oxygen
and carbon, and use color to track their presence in an image.
This is called “narrowband filtering.”
The most common application of narrowband filtering isolates light from hydrogen, sulfur,
and oxygen, three key building blocks of stars.
Hubble’s most famous example of this is called the Pillars of Creation, which captured
huge towers of gas and dust forming new star systems.
But this isn’t a “true color” image, like the one of Saturn from before.
It’s more of a colorized map.
Hydrogen and sulfur are both seen naturally in red light, and oxygen is more blue.
Coloring these gases as we’d actually see them would produce red, red, and cyan, and
the Pillars of Creation would look more like this.
Not as useful for visual analysis.
In order to get a full color image and visually separate the sulfur from the hydrogen, scientists
assign the elements to red, green and blue according to their place in the “chromatic
order.”
Basically that means that since oxygen has the highest frequency of the three, it’s
assigned blue.
And since hydrogen is red but a higher frequency than sulfur, it gets green.
The result is a full color image mapping out the process by which our own solar system
might have formed.
The Hubble Space Telescope can record light outside of the visible spectrum too – in
the ultraviolet and near-infrared bands.
An infrared image of the Pillars of Creation, for example, looks very different.
The longer wavelengths penetrate the clouds of dust and gas that block out visible light
frequencies, revealing clusters of stars within it and beyond.
These images showing invisible light are colored the same way: multiple filtered exposures
are assigned a color based on their place in chromatic order.
Lowest frequencies get red, middle get green, highest get blue.
Which could beg the question: are the colors real?
Yes and no.
The color represents real data.
And it’s used to visualize the chemical makeup of an object or an area in space, helping
scientists see how gases interact thousands of lightyears away, giving us critical information
about how stars and galaxies form over time.
So even if it isn’t technically how our eyes would perceive these objects, it’s
not made up, either.
The color creates beautiful images, but more importantly — it shows us the invisible
parts of our universe.