While light propagates extremely fast – 299,792 km (186,282 miles) per second – its finite speed means that when we observe an extremely distant object, we see the state of the object when the light left the object, not the object's state today. When viewed through a sensitive telescope, distant galaxies will look as they did at their birth – transforming the telescope into a “time machine” to study how the Universe evolved.
The James Webb Space Telescope (JWST) is the most powerful infrared space telescope in history. JWST's unprecedented improvements in sensitivity and resolution are documenting 13.5 billion years of cosmic history covering the first light after the Big Bang, the formation of galaxies, the evolution of supermassive black holes, the lifecycle of stars, and exoplanet composition.
Credit: NASA/Chris Gunn
A joint NASA/ESA/CSA mission, thousands of skilled scientists, engineers and technicians from 14 countries contributed to the design and fabrication of Webb. Launched on December 25, 2021, JWST is deployed in a halo orbit at L2. This orbit – approximately 1.5 million km (~1 million miles) from Earth – enables the telescope to remain in line with Earth as it orbits the Sun.
Planning for the JWST began almost three decades ago, as scientists discussed the successor to the groundbreaking Hubble Space Telescope. While Hubble determined the age of the Universe, could a telescope actually see its formation?
Infrared detection enables Webb to peer through massive clouds of dust that are opaque to visible-light observations – such as those collected by Hubble – revealing awe-inspiring images of star-birthing. Credit: Producer: Greg Bacon & Frank Summers (STScI), NASA’s Universe of Learning, NASA's Goddard Space Flight Center. Visualization: Greg Bacon, Ralf Crawford, Joseph DePasquale, Leah Hustak, Danielle Kirshenblat, Christian Nieves, Joseph Olmsted, Alyssa Pagan, & Frank Summers (STScI). Author of Original Release: Christine Pulliam. Narrator: Jacob Pinter. Support/Editor for Shortened Version: Paul Morris. Images: NASA, ESA, CSA, STSci
As the Universe expands, the wavelengths of light propagating through the universe get stretched. When stars are formed, they emit ultraviolet and visible light. However, light emitted from the first stars and galaxies has been stretched so that when their light reaches Earth, it is “redshifted” into the infrared. To study the distant universe, we need to observe it in infrared light – or "heat".
To ensure that JWST can collect all possible infrared light from distant objects, engineers developed a giant mirror over 6.5 meters (21 feet) wide – the largest ever deployed in space. This mirror is composed of 18 smaller mirrors that fit together like a honeycomb, and coated in a thin layer of gold to maximise infrared reflectance.
Furthermore, Webb needs to keep itself as cold as possible to avoid emitting any infrared radiation that would interfere with observations. The size of a tennis court, a five-layer sun shield blocks heat from the Sun, Earth and even the Moon, enabling the primary mirror to radiatively cool to 50 degrees Kelvin (-370 degrees Fahrenheit).
This collage of images from the Flame Nebula shows a near-infrared light view from NASA’s Hubble Space Telescope on the left, while the two insets at the right show the near-infrared view taken by NASA’s James Webb Space Telescope. Much of the dark, dense gas and dust, as well as the surrounding white clouds within the Hubble image, have been cleared in the Webb images, giving us a view into a more translucent cloud pierced by the infrared-producing objects within that are young stars and brown dwarfs. Astronomers used Webb to take a census of the lowest-mass objects within this star-forming region. Credit: NASA, ESA, CSA, STScI, Michael Meyer (University of Michigan), Matthew De Furio (UT Austin), Massimo Robberto (STScI), Alyssa Pagan (STScI)
Instruments
Revisiting Hubble’s deep-field survey, this image – captured by NIRCam – uses false-colour coding to convey galaxy redshift, dust content, and age. In false-colour, red/orange galaxies often indicate dust‑enshrouded star formation or active galactic nuclei (emitting longer IR), greenish-white dots represent very high‑z galaxies, while blue/cyan galaxies are lower‑redshift and brighter in near‑IR. Orange/red tones highlight mid‑IR bright galaxies with dust or active nuclei. Green/white points are likely galaxies at the highest redshifts in the field, their UV/optical light shifted into JWST’s 5–7 µm band. Blue/cyan objects are closer and luminous in shorter IR bands. The colour mapping makes research-friendly distinctions visually intuitive. Thanks to this colour-coding, scientists can visually sort early, dusty, and more mature galaxies at a single glance. Credit: NASA
Webb’s mirrors and instruments are designed to have the wavelength range, sensitivity, and resolution to detect extremely faint red to mid-infrared light to capture detailed images and spectra from objects as close as Mars to as distant as 13.5 billion light-years.
JWST uses 15 of Teledyne’s H2RG 2048×2048 pixel infrared detectors – totalling 63 million IR pixels. Teledyne also supplied the SIDECAR ASIC focal plane electronics that operate each H2RG array. To completely eliminate dark current, the detectors are operated at 37 degrees Kelvin (-393°F). The detectors and electronics must operate at very low power due to the difficulty of removing heat at the very low temperature of operation.
Each detector is made of HgCdTe. During fabrication, Teledyne engineers varied the ratio of mercury to cadmium, tuning the material to sense longer or shorter wavelengths. Less mercury optimises the detector to 0.6 - 2.5 μm, while a proportional increase optimises for 0.6 - 5 μm.
It is possible to read the pixels in an H2RG detector more than once before resetting them. This provides two benefits. The read noise can be further reduced by averaging multiple non-destructive reads together. Another advantage is that by using multiple samples of the same pixel, it is possible to see the "jumps" in signal level indicating that a cosmic ray has disturbed a pixel. Once confirmed, ground-based processing recovers much of the scientific value of the affected pixel.
NIRCam
Near Infrared Camera (NIRCam) H2RG infrared detector. Credit: Teledyne
Four H2RG detectors mounted into a focal plane module. This shows the black optical baffle that admits light onto the detectors while blocking it from hitting any reflective surfaces. Credit: NASA/U. Arizona
The Near-Infrared Camera (NIRCam) is JWST’s primary imager in the wavelength range from 0.6 to 5 μm. It consists of two, nearly identical, fully redundant modules, which point to adjacent fields of view on the sky and can be used simultaneously. Each module uses a dichroic lens to simultaneously observe in both short, and long wavelength channels.
NIRCam detects light from the earliest stars and galaxies in the process of formation, as well as the population of stars in nearby galaxies, and young stars in the Milky Way, and Kuiper Belt objects.
In addition to imaging with a wide range of narrow, medium, and broad filters, NIRCam also offers wide-field slitless grism spectroscopy and coronagraphic imaging modes, as well as time-series and grism time-series observing modes for high-accuracy photometric monitoring and spectrophotometric monitoring, respectively.
NIRCam also obtains wavefront sensing measurements critical for periodic alignment and phasing of the segments of JWST's primary mirror. Webb’s wavefront sensing and control system detects and corrects for slight irregularities in the shape of the primary mirror or misalignment between mirror segments, giving the telescope the ability to focus clearly on objects near and far.
NIRSpec
A NIRSpec detector from the engineering test unit in the cleanroom at Goddard. The person holding it is on the European Space Agency team. Credit: NASA/Catherine Lilly
JWST's Near-Infrared Spectrograph (NIRSpec) is a versatile spectroscopic instrument operating in the MWIR 0.6 to 5.3 μm wavelength range in order to determine the chemical composition, temperature, and velocity of observation targets.
To capture the faintest objects, Webb's giant mirror must stare at them for hundreds of hours to collect a full spectrum. NIRSpec is designed to simultaneously observe 100 objects – the first spectrograph in space with multi-object capability. To make it possible, engineers at NASA’s Goddard Space Flight Center had to invent a microshutter system with magnetically controlled shutters the width of a human hair. This multi-object spectroscopic mode will maximise the data collection on different stars, exoplanets, and distant galaxies.
Additional spectroscopy modes:
Slitted Spectroscopy - measures the spectrum of a single object in a wide field of view. Single slit spectroscopy is also used to analyse the spectrum of a small area of an object that is large in the field of view, such as a galaxy or planet.
Integral Field Unit Spectroscopy (IFU) - combining imaging and spectroscopy, the instrument captures an image of the field of view along with individual spectra of each pixel in the field of view. IFU observations allow astronomers to investigate how properties – such as composition, temperature, and motion – vary between different objects, such as stars in a crowded star field, or from place to place over a large region of space, such as a galaxy or nebula.
Time-series Spectroscopy - records the spectrum of an object or region of space at regular intervals in order to observe how the spectrum changes over time. Time-series spectroscopy is used to study planets as they transit their stars.
Fine Guidance Sensor (FGS)
An engineering test unit of the James Webb Space Telescope's Fine Guidance Sensor (FGS) about to undergo cryogenic testing at the David Florida Lab in Canada. Credit: CSA, COMDEV
The Fine Guidance Sensor (FGS) is a star tracker that uses star patterns to aim the observatory to ensure Webb’s instruments are pointed correctly. The three Teledyne 2048 × 2048, 5 μm pixel, H2RG detectors in FGS point and stabilise the telescope to an accuracy of 1 milli-arcsec in MWIR 0.6 - 5 μm. Once an observation begins, FGS can further compensate for small drifts in the observatory's alignment to help the telescope maintain accurate pointing.
Major Scientific Focus Areas
First light and reionisation
This image highlights the region of study by the JWST Advanced Deep Extragalactic Survey (JADES). This area is in and around the Hubble Space Telescope’s Ultra Deep Field. Scientists used Webb’s NIRCam instrument to observe the field in nine different infrared wavelength ranges. From these images, the team searched for faint galaxies that are visible in the infrared but whose spectra abruptly cut off at a critical wavelength. They conducted additional observations (not shown here) with Webb’s NIRSpec instrument to measure each galaxy’s redshift and reveal the properties of the gas and stars in these galaxies. In this image, blue represents light at 1.15 microns (115W), green is 2.0 microns (200W), and red is 4.44 microns (444W). Credit: NASA, ESA, CSA, Mahdi Zamani (ESA/Webb); Science: JADES Collaboration, Brant Robertson (UC Santa Cruz), Sandro Tacchella (Cambridge), Emma Curtis-Lake (UOH), Stefano Carniani (Scuola Normale Superiore)
After the Big Bang, the universe was essentially a dark, superheated cloud of subatomic particles. The only direct observations of this era have consisted of microwave emissions, as there were no light sources at that time. Furthermore, light would be incapable of transmission as it would be scattered by an abundance of free electrons.
As the Universe expanded and cooled, protons and neutrons combined into ionised atoms of hydrogen, in turn attracting electrons. These newly formed neutral atoms allowed light to travel freely – ending the “dark age” of cosmic history. Over millions of years, this slow process accumulated enough matter to form the first light-emitting objects. The interaction of radiation from these early stars would ionise the neutral hydrogen gas – a process known as reionisation.
Reionisation produces heat, increasing the temperature of the surrounding gas cloud. This creates an imbalance as the bigger and hotter galaxies can absorb more hydrogen, hoarding it from smaller galaxies and making it harder for them to grow. This effect may even explain the properties of some of the Milky Way galaxy's smaller neighbours.
Assembly of galaxies
Present-day disk galaxies typically feature a thick, star-filled outer disk and an embedded, thin disk of stars. Astronomers have proposed three major theoretical scenarios to explain how this dual-disk structure forms. Using archival data from JWST, a team of astronomers is closer to understanding the origins of disk galaxies and the formation process of their stellar thick and thin disks. The team carefully identified, visually verified, and analysed a statistical sample of more than 100 edge-on disk galaxies at various periods — up to 11 billion years ago (or approximately 2.8 billion years after the Big Bang). The results of their analysis suggest that galaxies form a thick disk first, followed by a thin disk. The timing of this process depends on a galaxy’s mass: high-mass, single-disk galaxies transitioned to two-disk structures around 8 billion years ago, while low-mass, single-disk galaxies formed their thin disks about 4 billion years ago. Credit: NASA, ESA, CSA, STScI, Takafumi Tsukui (ANU)
JWST is tracing the formation of galaxies – a process guided by gravity that took place over billions of years. The sprawling elliptical and spiral galaxies – such as our own Milky Way – started as small and clumpy fields of dust, often with a majority of star formation occurring in dense knots.
Those knots clump into a network of intersecting sheets and filaments, separated by large regions with relatively little matter. These “empty” spaces probably contained dark matter. Stars and heavy matter collected at nodes while dust strung into filaments forming a kind of cosmic web – the skeleton around which galaxies develop. As these networks intersected, they gathered into vast clusters of thousands of galaxies.
Over time, the contrast between the sheets and filaments of the cosmic web grew – the gravity of the extra matter in the web continued to collect more and more matter – and the “voids” in between grew emptier and emptier. As the stars multiplied, gravity sculpted the cosmos, swirling galaxies into vast spirals that collided and merged with other galaxies.
This figure shows the NIRCam (top) and NIRSpec (bottom) data for now-confirmed galaxy MoM-z14: the most distant galaxy known to date as of May 2025 – 33.8 billion light-years from Earth. Completely invisible at wavelengths of 1.5 microns and below, its light is stretched by the expansion of the Universe. Emission features of various ionised atoms can be seen in the spectrum, below, as well as the significant and strong Lyman break feature. Credit: R.P. Naidu et al., Open Journal of Astrophysics (submitted)/arXiv:2505.11263, 2025
Before JWST, scientists had confirmed only a single galaxy from the first 500 million years of cosmic history: GN-z11, discovered with the Hubble Space Telescope. Several other ultra-distant galaxy candidates existed, but it would take a superior tool, such as JWST, to find and confirm them.
The telescope has officially observed the four most distant – therefore oldest – galaxies known. Webb observed the galaxies as they appeared about 13.4 billion years ago, when the universe was about 2% of its current age. The oldest galaxy yet discovered by Webb appears as it was 282 million years ago. With the high sensitivity of JWST’s instruments, researchers have discovered thousands of new galaxies more distant and ancient than any previously documented. More detailed data on the earliest formation of galaxies increases the understanding of why the Universe appears as it does today.
Birth of stars and protoplanetary systems
This area – called the Cat’s Paw Nebula – is of great interest to scientists, having been subject to previous study by NASA’s Hubble and retired Spitzer space telescopes, as they seek to understand the multiple steps required for a turbulent molecular cloud to transition to stars. Webb’s view reveals a chaotic scene still in development in this star-forming region: Massive young stars are carving away at nearby gas and dust, while their bright starlight produces a bright nebulous glow, represented in blue. The Cat’s Paw Nebula is located approximately 4,000 light-years away in the constellation Scorpius. Credit: NASA, ESA, CSA, STScI
A star is a massive ball of gas bound together so tightly by gravity that it heats up to enormous temperatures – tens of millions of degrees Celsius at its centre.
The material inside the compact core of a star experiences a strong gravitational force. To counter this force and prevent a star from imploding, its structure needs to be supported by an intense field of pressure, which can only be generated by the extreme heat of nuclear fusion. At such high temperatures, the atoms in these stars undergo nuclear fusion, in which light elements (like hydrogen) collide and stick together as heavier elements. During each of those reactions, a tiny bit of matter is converted into energy, providing the extraordinary luminosity of the Sun.
The larger the star, the greater the force of gravity and its corresponding demand for a bigger generator with a higher rate of fuel consumption. Once a star runs out of fuel, it has no way to maintain a high temperature, resulting in either a supernova explosion, in which nearly the entire star explodes with enormous energies, or the formation of an equally massive black hole. Additionally, the first stars that exploded as supernovae may have collapsed further to form black holes or mini-quasars, which, upon consuming more matter from surrounding gas clouds and stars, may have eventually grown into the very large black holes found at the centres of nearly all large galaxies.
The first stars were supergiants that consumed their fuel supplies in just a few million years. The titanic forces they generated released the heavy elements – the base material that would eventually form into more stars, planets and smaller objects.
Planets and origins of life
From left, Jupiter, Saturn, Uranus, and Neptune as seen in near-infrared by JWST. Saturn's upper atmosphere is so faint that it has long been one of the hardest regions in the solar system to study. The sensitivity of JWST has produced a detailed profile of the planet's upper atmosphere for the first time. Credit: NASA, ESA, CSA, STScI
In addition to other planetary systems, JWST will also study objects within our own solar system. Webb is analysing the atmospheres of Mars and the giant planets, as well as the mineralogy of smaller targets, such as asteroids, comets, and objects in the Kuiper Belt.
To learn more about the atmosphere of distant exoplanets, Webb employs the transit method. When a planet crosses in front of its host star, elements in the planetary atmosphere present gaps in the spectrum – absorption lines – as atmospheric molecules absorb light at the characteristic energies of corresponding elements. Thus, if a planet’s atmosphere contained oxygen, scientists would expect that oxygen signature would be missing from a spectrograph of the light emitted by the host star.
Atmospheric analysis greatly advances the search for alien life. The ultimate goal is to detect the building blocks of life on an exoplanet, particularly one with an atmosphere similar to Earth's. In fact, JWST identified carbon dioxide in the atmosphere of a planet called WASP-39b – the first time the gas has ever been found in an exoplanet's atmosphere.
For more information on the James Webb Space Telescope, check out this article by Teledyne's Chief Scientific Officer, Dr James Beletic.
Works Cited
Cooper, Keith. (2025, August 4). James Webb Space Telescope revisits a classic Hubble image of over 2,500 galaxies. Space.com. https://www.space.com/astronomy/james-webb-space-telescope/james-webb-space-telescope-revisits-a-classic-hubble-image-of-over-2-500-galaxies
Editors of Science News Today. (2025, July 26). The Cosmic Timeline: A Journey Through the Universe’s History. Science News Today. https://www.sciencenewstoday.org/the-cosmic-timeline-a-journey-through-the-universes-history
Foord, Adi. (2025, June 30). How can the James Webb Space Telescope see so far? The Conversation
https://theconversation.com/how-can-the-james-webb-space-telescope-see-so-far-257421
Ibid. (2024, April 68). https://theconversation.com/could-a-telescope-ever-see-the-beginning-of-time-an-astronomer-explains-221568
Furlanetto, Steven. (2021). Cosmic Dark to Cosmic Dawn. https://cosmicdawn.astro.ucla.edu/.
NASA. (n.d.). Early Universe. https://science.nasa.gov/mission/webb/early-universe/
Ibid. FGS/NIRISS: Fine Guidance Sensor / Near Infrared Imager and Slitless Spectrograph (FGS/NIRISS) Instrument. https://science.nasa.gov/mission/webb/fine-guidance-sensor-near-infrared-imager-and-slitless-spectrograph-fgs-niriss/
Ibid. Galaxies Over Time. https://science.nasa.gov/mission/webb/galaxies-over-time/
Ibid. Near Infrared Camera (NIRCam) https://science.nasa.gov/mission/webb/nircam/
Ibid. Near InfraRed Spectrograph (NIRSpec). https://science.nasa.gov/mission/webb/nirspec/
IIbid. Other Worlds. https://science.nasa.gov/mission/webb/other-worlds/
Ibid. Star Lifecycle. https://science.nasa.gov/mission/webb/star-lifecycle/
Lea, Robert. (2025, May 30). 'Cosmic miracle!' James Webb Space Telescope discovers the earliest galaxy ever seen. Space.com. https://www.space.com/astronomy/cosmic-miracle-james-webb-space-telescope-discovers-the-earliest-galaxy-ever-seen
Ibid. (2022, September 19). James Webb Space Telescope's 1st images of Mars reveal atmosphere secrets. https://www.space.com/james-webb-space-telescope-first-images-mars
Slegan, Ethan. (2025, May 21). JWST breaks its own record with new most distant galaxy MoM-z14. BigThink. https://bigthink.com/starts-with-a-bang/jwst-breaks-record-most-distant-galaxy-mom-z14/
Teledyne Technologies. (2024, February). The James Webb Space Telescope https://www.teledyne.com/digital-imaging-space-science-monthly/the-james-webb-space-telescope
