The first images released from the James Webb Space Telescope (JWST) revealed new views of the cosmos in exquisite, never-before-seen detail. This was thanks in large part to the careful construction of the telescope. There will be a lot to learn from JWST during its mission, from how galaxies evolve to the composition of exoplanet atmospheres.
The data from this telescope have already started going public. Eventually, all of its measurements will be available for anyone to use. “Anybody can actually go and explore the universe; we’re not keeping any secrets here,” says Susan Mullally, deputy project scientist for JWST at the Space Telescope Science Institute in Baltimore. That’s the wonderful thing about NASA projects: the data are open-access. “Science is a very open process,” Mullally says. “It’s through a collective knowledge that we reach our understanding of our place in the universe.”
These first gorgeous images and data are just a small piece of what the team running JWST hopes will come out of the mission. “This was just a demonstration. We’re going to get [huge amounts of data] every day with Webb,” Mullally says. As scientists begin to drink from the JWST firehose, she anticipates a plethora of interesting new information to be discovered about our universe. Some findings will confirm what we already suspect while other discoveries may be paradigm-shifting. “Keep your eye out,” she says. “This is just the beginning.”
JWST’s Deepest Field—So Far
The first image released from JWST is colloquially referred to as the observatory’s first “deep field,” referring to a technique where astronomers target seemingly barren regions of sky for long telescopic stares to reveal hidden faint objects. Although this picture is described as a deep-field image, this is actually a misnomer, says Becky Smethurst, an astrophysics researcher at the University of Oxford. “It’s not a deep field,” she says, because the image’s target was not empty sky but rather SMACS 0723, a galaxy cluster more than four billion light-years away. The observatory’s first true deep-field image, Smethurst says, will likely be released in January or February 2023, after the telescope’s scheduled survey of the same section of sky where the Hubble Space Telescope captured its Ultra Deep Field image. When that image is released, she anticipates many “oldest galaxy” records will be smashed thanks to the telescope’s ability to see deeper into the cosmos than Hubble ever could.
Although not a true deep-field image, JWST’s view of SMACS 0723 is the deepest, sharpest picture of this galaxy cluster to date. Yet it only took 12.5 hours of observation time to generate—roughly an order of magnitude less than the time required to produce a similar but inferior image using Hubble. So much mass is packed within SMACS 0723 that it dramatically curves the surrounding fabric of spacetime, acting as a “gravitational lens” to warp and amplify light traveling around it and boosting far-distant background galaxies into view. Some of the light captured from these even more distant galaxies was emitted less than a billion years after the big bang.
Stellar Spiders, a Galactic Warp and a Cosmic Hall of Mirrors
The shape of the mirrors and struts on a telescope can change the appearance of bright objects, such as the red-tinged star seen close to the center in the image at left. Eighteen individual hexagonal mirrors make up JWST’s large segmented primary mirror, forming a giant honeycomb-shaped aperture. The primary mirror reflects infrared light—which humans can’t see but can feel as warmth radiating from a light source—onto a smaller secondary mirror. In turn, the secondary mirror sends the light to detectors inside the telescope to make an image or collect other data. The primary mirror shape creates a six-pointed diffraction pattern for sufficiently bright sources: each “spike” in this pattern stretches toward one of the points of a hexagon. The struts that hold the small mirror away from the primary mirror add six more diffraction spikes: Two visibly radiate horizontally from the center of a bright object. And four overlap with the spikes caused by the primary mirror shape. Together, this makes eight visible diffraction spikes surrounding the brightest stars in JWST images, giving them an almost spiderlike appearance.
Remarkably, the telescope’s optics are so sensitive that diffraction patterns even appear for bright galaxies that are spike-free in Hubble images. “If you zoom in on some of the galaxies, you can actually see that shape very faintly in the center,” Smethurst says. This could be a “very cool identification tool,” she adds, because it could signify a bright, growing supermassive blackhole at the center of such a galaxy.
The orange-red galaxy in the zoomed-in portion of JWST’s SMACS 0723 deep field shown in the center image appears stretched and warped because of gravitational lensing. The mass of an enormous celestial object, such as a cluster of galaxies, can bend spacetime enough to change the path light follows. The light from this galaxy travelled for 8.4 billion years before being captured by JWST, putting the galaxy’s formation at about 5.4 billion years after the birth of the universe. Scientists can assign distances and approximate ages to galaxies by carefully measuring offsets in the color of their emitted light. All galaxies across the sky, save for a few that are relatively quite close to our own, are moving away from us at high speeds, carried by the expansion of the universe itself. The more distant a galaxy is from us, the faster it moves via cosmological expansion—and, thanks to the finite speed of light, the older its light that reaches us is. This light is “redshifted” by the expansion, its wavelengths stretched out to become longer and redder as it travels across intergalactic space. Precisely quantifying this redshift yields a distance and age estimate.
Gravitational lensing doesn’t just cause distant galaxies to appear warped; it can also make galactic mirror images. In the image at right, the center arc appears to be the same red galaxy mirrored and stretched around massive foreground galaxies in SMAC 0723. Such mirror images arise when an object’s light takes multiple paths around a gravitational lens. In this case, the arcs could instead conceivably be two distinct but similar galaxies, each experiencing its own instance of gravitational lensing. But without more data, it is difficult to know either way. Luckily some of JWST’s instruments can collect spectra of light for many, if not all, celestial targets in a given field of view. These spectra are crucial diagnostics, revealing not only the motion, distance and age of a target but also its composition, because different atoms and molecules each create their own spectroscopic imprint on a body’s emitted light. Careful spectral analysis can reveal which arcs in deep-field images are actually mirror images and which are instead mirages. Comparing the spectra in this arc confirms this speculation: it represents a single galaxy, warped and mirrored by gravitational lensing.
A Spectral Sniff of Hot Air
Collecting high-resolution spectra from celestial bodies also allows JWST to probe distant exoplanets in more detail than ever before. “One of the things that excites me is trying to peer into atmospheres of terrestrial planets,” Mullally says, “We really don’t know what we’re going to find when we look there.” Key questions about exoplanet atmospheres, such as what they’re made of, can be answered using a transmission spectrum such as the one shown here. It was collected from the gas giant exoplanet WASP-96 b, which orbits hellishly close to a star 1,150 light-years from Earth. A transmission spectrum is collected as an exoplanet “transits” in front of the star it orbits, allowing starlight passing through its upper atmosphere to be isolated and studied. Molecules in the atmosphere absorb different wavelengths of light and act as wavelength-specific filters. Comparing the spectrum of a host star’s light before and during a planet’s transit can thus reveal the atoms and molecules prevalent in that world’s atmosphere. This is the first transmission spectrum to collect such a broad range of infrared wavelengths for a transiting exoplanet’s spectrum in a single observation. It reveals the presence of water vapor and other molecules in WASP-96 b’s extremely hot atmosphere. The background illustration is based on astronomers’ best guess about that world’s appearance, based on the cumulative available data.
First Glimpse of a Dynamic Duo
There are two portions of the electromagnetic spectrum in which JWST most excels: near- and mid-infrared light. The same target can look very different when viewed side by side in both varieties of light, as shown by these images of the Southern Ring Nebula, which is about 2,500 light-years away. The telescope’s Near Infrared Camera (NIRCam) captured the one at left, and its Mid-Infrared Instrument (MIRI) generated the one at right. Although the NIRCam image might make it look as if only one star sits in the center of this ring of dust, two stars are actually present. The MIRI image reveals the second star, a white dwarf, which is hidden by the diffraction spikes of its neighbor in the NIRCam image. This confirmed assumptions that a binary system created the nebula. “We’d never seen that [white dwarf] before,” Smethurst says. “Webb essentially revealed it for the first time.”
Such insights arising from multiple views of a single target are part and parcel of Webb’s broadband infrared capabilities. Whereas near-infrared light is transparent to dust, star-warmed dust thermally emits light in the mid-infrared, causing the dust-shrouded white dwarf to appear brighter and larger in the MIRI image. This star is the one that created the ringlike shells of material surrounding the pair. Before becoming a white dwarf, the star was much like our sun. But as it aged into stellar senescence, it ejected much of its outer layers of gas into space, creating the nebula. Its orbiting neighbor helped spread the material, resulting in the lovely display captured by JWST.
At the Edge of the Cosmic Cliffs
An image of a stellar nursery in the Carina Nebula about 7,600 light-years away shows massive young stars enrobed in swirling gas and dust. The youngest stars appear as red pinpricks of light in the cloud. This image was taken by JWST’s NIRCam, allowing it to capture previously hidden features within and behind the occluding dust. High-energy ultraviolet radiation—the same kind of light that causes sunburns—and stellar winds from hot newborn stars eroded some of the surrounding material, sculpting what astronomer dub the Cosmic Cliffs. In fact, what appears to be white steam rising from the “cliffs” is hot dust and ionized gas streaming away as the ultraviolet radiation interacts with the nebula. This is just a small part of the edge of a bubblelike offshoot of the whole Carina Nebula, which stretches across more than 200 light-years of space. In comparison, this image is only about 16 light-years across.
A Cliffside Close-Up
In this zoomed in MIRI-NIRCam composite image of the Cosmic Cliffs, planet-forming rings of dust show up in pink and red around stars, and hydrocarbons give off a diaphanous glow much like clouds seen in Earth’s twilight sky. Just left of center, a newborn star is identifiable by its golden tail. Although it looks like a comet in the preceding NIRCam-only image, the addition of MIRI data reveals the dusty infant spewing out a conelike protostellar jet. Right of center, another star erupts in a blowout of dust and gas, highlighted in gold.
An Intergalactic Dance Party
The largest image JWST has captured so far is of a galaxy cluster known as Stephan’s Quintet, named after the man who first spied it in 1877 through a far more modest ground-based telescope. Of the supposed quintet of galaxies, only four are in fact close enough to gravitationally interact with one another. The remaining fifth galaxy (leftmost) sits some 250 million light-years closer to Earth. The four close enough to be caught in a cosmic dance may give us a better understanding of how that interplay can drive galactic evolution. This composite image shows both near- and mid-infrared light. And like many other images, it reveals previously veiled details of each galaxy, including shockwaves generated as the galaxy at the top of the central pair smashes through the others in its region of the cluster. The shockwaves around that pair are highlighted in red and gold. The black backdrop is speckled with eight-pointed stars and distant galaxies.
Spotlighting Supermassive Black Holes
When you see spiderlike diffraction spikes emanating from a celestial body in a JWST image, you know you’re looking at something bright. In this case, the MIRI-only image of Stephan’s Quintet at left reveals a brilliant monster lurking in one of the group’s galaxies. The diffraction spikes here come from a feeding supermassive black hole that contains more than 24 million times the mass of our sun. Dust, gases and other material trapped in the black hole’s gravitational grip generate friction and heat up to enormous temperatures as they swirl around its maw. Although no light escapes from the black hole itself, the incredibly hot material spiraling into it emits huge amounts of mid-infrared light. In contrast, the NIRCam-only image of this galaxy at right reveals only a vanishing fraction of such details in a swirl of wispy white light that emanates from the galaxy’s stars, some of which you can see as red pinpricks.
