This telescope sees a past older than Earth. How does Webb work?

The light began its journey long before our planet existed. It travelled through space for billions of years before finally reaching the gold mirror of the James Webb Space Telescope. What Webb captured is a real view of the distant past. And parts of that past look different from what scientists expected.

At first, we see spectacular pictures: glowing clouds, thousands of galaxies and stars scattered like jewels. But some of Webb’s greatest strengths become apparent when a photograph is no longer enough. It measures light in such detail that astronomers can investigate gas moving around a black hole or identify a molecule in another planet’s atmosphere.

How can an instrument near Earth study places we could not reach in countless generations? The answer brings together time, light and an exceptionally sensitive observatory.

Looking farther means looking back

Light does not travel instantaneously. In a vacuum, it covers roughly 300,000 kilometres every second. Space is so vast, however, that even at this speed, journeys between galaxies can take millions or billions of years.

If an object’s light took a billion years to reach us, we see that object as it was a billion years ago. We do not see what is happening there today. A telescope receives a message that has arrived extraordinarily late.

Imagine a photograph of a city posted on the day it was founded. By the time it reaches you, the city might have grown, changed or disappeared. The photograph would still faithfully record the moment it was taken. Ancient galaxies work much the same way, except the messenger is light itself.

Earth formed around 4.5 billion years ago. Webb detects light that has been travelling much longer, revealing the universe before our planet existed. All telescopes look into the past in this sense; Webb extends our ability to study exceptionally faint, distant objects. 1

The universe stretches light, too

Something else happens during that long journey: the universe expands. As it does, the wavelengths of light travelling through it stretch. Astronomers call this cosmological redshift.

Light emitted by a distant galaxy as ultraviolet radiation can reach us as infrared radiation, which our eyes cannot see. Restricting observations to visible light would leave important information about the early universe out of reach. Webb is designed specifically to observe infrared wavelengths. 2

Picture a repeating pattern drawn on an elastic strip. Stretch the strip and the gaps between the pattern’s peaks grow. This helps illustrate changing wavelength; it is not a literal description of what space is made of.

Longer wavelengths can also pass through some dusty clouds more effectively than visible light. Webb reveals stars hidden in regions that look opaque in other images. Its view of the Pillars of Creation therefore offers more than a beautiful alternative photograph: different wavelengths highlight different components of the same region. 3

The gold mirror has a job to do

Webb’s main mirror is 6.5 metres across, made from 18 hexagonal segments. Its central task is to collect faint light. Magnifying an image alone cannot help if too little information arrives from the object. 4

Think of taking photographs in a dark room. Digital zoom cannot recover details that the camera never captured. You need more photons, and a way to distinguish their signal from noise. A larger mirror helps collect those photons.

The gold coating reflects infrared light efficiently. The segments are aligned with extraordinary precision so that they work together as one mirror. Other mirrors then direct the collected light towards the scientific instruments. 4

Those instruments perform different tasks. A camera makes an image, showing where light came from. A spectrograph separates light by wavelength, revealing information hidden within it. Webb is therefore both an advanced camera and a laboratory whose samples arrive as light. 5

To observe space, Webb must suppress its own heat

Sensitivity to infrared radiation creates a problem: warm objects emit it themselves. If the telescope were too warm, its own radiation would interfere with the faint signals it is trying to measure.

Webb therefore operates at extremely low temperatures. A five-layer sunshield, roughly the size of a tennis court, separates the observatory’s hot and cold sides. Its MIRI instrument also uses active cooling to reach approximately 7 kelvin, or −266°C. 6

Its location serves a practical purpose, too. Webb operates around 1.5 million kilometres from Earth, near the Sun–Earth L2 point. It follows an orbit around L2 while travelling around the Sun with Earth. This arrangement keeps the Sun, Earth and Moon on the sunshield’s side of the observatory. 7

Webb does not observe distant galaxies by flying towards them. It stays in our cosmic neighbourhood, collecting light that completed most of its journey long before the telescope was launched.

How light reveals another world

Atoms and molecules absorb or emit radiation at particular wavelengths, leaving recognisable patterns in a spectrum. These act like signatures that scientists can compare with laboratory measurements.

This is not an automatic translator in which one mark always means one molecule. Researchers must account for noise, temperature, clouds and overlapping signals. An interpretation must survive comparisons with different models and, where possible, further observations.

That is why an unassuming graph can be scientifically more valuable than a dramatic picture. The photograph catches our attention. The spectrum helps explain what we are actually looking at. Sometimes it forces researchers to reconsider an object’s story. 5, 8

A galaxy in an astonishingly young universe

One striking example is MoM-z14, seen as it was approximately 280 million years after the Big Bang. It is a galaxy, not a black hole. The number describes the universe’s age when the observed light was emitted, rather than the precise moment the galaxy formed. 9

MoM-z14 belongs to a population of bright early galaxies that Webb is finding in greater numbers than models predicted before launch. This raises questions about star formation and conditions in the young universe. 9

The image itself may not seem impressive. A blurry patch does not explain its own significance. The surprise emerges when image, spectrum and cosmic time are combined: this object was already there, at that early stage. A mismatch with expectations does not mean the galaxy was impossible. It tells scientists to investigate which assumptions in their models need to change.

A black hole that grew too quickly?

For Abell2744-QSO1, researchers used the motion of surrounding gas to infer a black hole mass of around 50 million Suns. We see it roughly 700 million years after the Big Bang. The findings support the possibility of a very massive starting seed, while the specific formation mechanism remains under investigation. 10

The question is fascinating: how do you build such a giant in a relatively short cosmic time? Its final mass alone cannot answer that. We also need to understand what it started from, how quickly it gained material and what environment surrounded it.

Two kinds of claim matter here. Gas motion helps reveal the gravitational effect of an unseen object. Reconstructing that object’s ancient origin is another step. A strong measurement can narrow the possibilities considerably without settling every competing explanation.

Reading an atmosphere 700 light-years away

Webb also detected a clear carbon dioxide signal in the atmosphere of WASP-39 b, a hot gas giant around 700 light-years away. Announced in 2022, the result came from observing the planet crossing its star. Some starlight passes through the atmosphere, where gases absorb particular wavelengths. Those missing wavelengths help identify what is present. 8

This is neither a photograph of molecules nor evidence of life. The remarkable achievement is making a chemical measurement on a world that no spacecraft has visited.

It also offers a useful lesson for reading exoplanet headlines. Identifying one atmospheric ingredient does not reveal the entire environment. Each detection is part of a larger puzzle, to be combined with other measurements before drawing a picture of the world itself.

Are those beautiful colours real?

Webb records radiation beyond human vision. To create full-colour images, different infrared bands are assigned visible colours. The result comes from measurements, but it is not what human eyes would see through a spacecraft window. 11

Like a colour-coded temperature map, an astronomical image makes invisible differences accessible to our senses. The important distinction is whether we are seeing a processed observation, a visualisation of measurements or an artist’s impression.

Webb gives us more than a new background for our screens. It teaches us to read messages the universe sent long before we arrived. Some add detail to a familiar story. Others reveal how much of its beginning we still do not understand.

Looking deeper into space also helps us understand where the world we belong to came from.

Sources

Verified on 21 October 2026.