JWST sees light invisible to the eye. These scientists decide how to color it

When it comes to appearances, the universe is a complicated place. Visible light occupies only a sliver of the electromagnetic (EM) spectrum. To study the cosmos in its entirety, scientists must look beyond visible light using specialized instruments, including radio telescopes and X-ray telescopes. And the James Webb Space Telescope (JWST) senses infrared (IR) radiation, seeping through dusty curtains that block visible light.

Researchers and developers tasked with processing this data are presented with a unique challenge: What color is light that humans cannot see? It takes a dazzling blend of art and science to answer this question and bring these cosmic scenes to life.

The process of decoding data from non-optical telescopes is often called false color, but the word “false” does it a disservice. The technique has been used for decades to produce color pictures from raw data, including some of Hubble’s most famous images, which combine the telescope’s optical capabilities with its ultraviolet (UV) and IR data. Like any other camera, this processing creates pictures from meaningless series of zeros and ones.

“Representative color is a more accurate term,” says Joe DePasquale of the Space Telescope Science Institute (STScI) in Baltimore, Maryland, which manages JWST and its images. As one of only two space imaging developers at STScI, DePasquale is passionate about dispelling any public skepticism created by the term “false color.”

“It’s a subtle blend of art and science, leaning more towards science,” he explains. “We work with scientists to make aesthetic judgments and highlight key scientific features without altering the data. The whole point is to show what the telescope observes in the best possible way.”

Nothing fake about fake color

The representational coloring process involves assigning colors based on relationships modeled on human perceptions of visible light. Since JWST detects light in the near- and mid-IR bands, that range of wavelengths must be shifted—or mapped, as image processors call it—into the visible color space.

“The color assignment is pretty solid,” says Alyssa Pagan, STScI’s other space visuals developer. Typically, this refraction follows the same order as the visible part of the spectrum, “making shorter wavelengths bluer and longer wavelengths redder,” she explains. “But there is a lot of flexibility. We can make the mid-wavelengths a little greener or greener just to get a fuller color representation.

JWST’s different filters—29 in the mid-IR and 10 in the near-IR—reveal different aspects of an object’s structure. A nebula may have filaments of dust overlapping clouds of hot gas, each highlighted by different wavelengths. Adjustments have been made to highlight these features.

“You do your best to go down or up,” Pagan says. “Although scientists can use six filters to get an image, you don’t necessarily want to use them all. This can neutralize features that overlap, such as too much paint mixing and browning. You have to be careful, but you want to get as wide a range of colors as you can.”

Including the spectrum

In addition to JWST, astronomers use a wide range of observatories to look at the rest of the EM spectrum. Starting with low-energy radio waves and moving on to microwaves, then IR, UV, X-rays, and finally gamma rays, these telescopes reveal the unimaginable, including the cosmic microwave background left over from the Big Bang, holes in supermassive blacks, the inner structures of nebulae and exploding supernovae. (A key observatory in this fleet, the Chandra X-ray Observatory, was recently tagged for retirement by a NASA committee with no replacement in sight—much to the consternation and protests of scientists.)

The

Although the same basic rules apply in interpreting data from this diverse range of telescopes, combining their data across the EM spectrum is challenging. But the results – known as multiwavelength imaging – can be spectacular and revealing.

Pagan compares the process to trying to combine a normal photo of a human arm with X-rays of the bones inside. “They don’t match very well, but you can assign one color to the X-ray and another to your skin to see what’s really going on. It’s like drawing a graph with five different colors to show different numerical information.”

Visualizations for the public eye

In the spring of 2022, on the eve of JWST’s first science images, STScI gave itself a short period of time to select and present some of the most spectacular targets the telescope had imaged up to that point. “The team met every day for six weeks,” says DePasquale. “We had scientists, writers, graphic designers, and then Alice and I.”

The resulting images became instantly iconic. Pagani worked on cosmic rocks — the nickname the team gave to a billowing wall of gas and dust surrounding the star cluster NGC 3324, near the Carina Nebula. DePasquale encountered the Tarantula Nebula (30 Doradus), a massive stellar nursery in the Large Magellanic Cloud, among others.

DePasquale says he doesn’t take the coloring task — or its impact — for granted. “It’s a tall order to interpret what we get from a telescope and create images that shape the public’s perception of the universe.”

---

If you want to try your hand at representational staining, STScI makes the JWST data and software available mission website. For more information, see Warren Keller’s story in our September issue on how to process your JWST images.