Astronomers usually do not measure the distance to the most distant galaxies by surveying their positions directly. They measure how much the galaxies’ light has been stretched by cosmic expansion, estimate each galaxy’s redshift, then use a cosmological model to infer a distance or a lookback time. The redshift is an observation; the distance derived from it depends on the model and on which distance convention is being reported.
What astronomers actually measure
For a very distant galaxy, the first direct evidence is its light. That light carries clues about the wavelengths at which it was emitted. As the universe expands while the light travels, its wavelengths are stretched toward the red end of the spectrum. Astronomers call this shift redshift.
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Redshift is not itself a distance. It is an observed property of the light. To turn it into a distance, astronomers use a cosmological model—a description of how the universe expands and of the properties that affect that expansion. The result is therefore an inference rather than a direct ruler measurement, and it should be reported with its distance convention and model context.
The same observations also tell us that we are seeing the galaxy as it appeared in the past. Light takes time to cross space, so the light arriving now left its source earlier. As Micaela Bagley, a postdoctoral fellow at the University of Texas at Austin, explained to NASA in December 2023: “Light takes time to travel through space. When light from a distant galaxy (or any object in space) reaches us, we are seeing that galaxy as it appeared in the past.”
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This is why two related questions should not be collapsed into one: how long the light has travelled, and what distance is assigned to the galaxy under a particular cosmological convention. The observations establish the redshift; the model supplies the framework for translating it into a distance or lookback time. A headline that gives only a distance number can leave out an important part of what astronomers actually measured.
How astronomers find distant-galaxy candidates
1. Search images taken through multiple filters
Space telescopes such as the James Webb Space Telescope can image a patch of sky through several filters. Each filter records the average brightness of incoming light over a particular wavelength band. A distant galaxy may appear in some filters and become faint or disappear in others because its light has shifted into longer wavelengths.
Researchers compare those measured colors and brightnesses with galaxy models to estimate a photometric redshift. This is a probability-based estimate from broad-band imaging, not a precise spectral measurement. It is useful for identifying promising candidates, especially because the faintest galaxies can be difficult to observe with a spectrograph.
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2. Treat a photometric redshift as a candidate estimate
Broad filters combine light across wavelength ranges. That makes them effective for finding objects, but the resulting color pattern may not uniquely identify a galaxy’s redshift. A photometric estimate should therefore be understood as a distribution of plausible redshifts, not a single definitive distance. The estimate helps researchers decide which objects deserve more detailed follow-up.
In other words, the image-based result is a way to prioritize targets, not a final verdict about how far away every object is. NASA’s explanation of galaxy distances distinguishes these photometric candidates from more precise spectroscopic measurements. When a report describes an object as a candidate, that wording matters: it signals that the redshift estimate still needs stronger evidence where such follow-up is possible.
How a spectrum reveals redshift
Compare observed features with their emitted wavelengths
A spectrograph separates light by wavelength, giving astronomers a more finely sampled view than an image made through broad filters. They look for features such as emission lines or spectral breaks, then compare the observed wavelength of a feature with the wavelength at which it is known to be emitted. The shift between those values gives the redshift.
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One useful feature is the Lyman break, an ultraviolet drop associated with hydrogen absorption. In light from a distant galaxy, cosmic expansion shifts this feature to longer observed wavelengths. It can then be identified in infrared observations, where instruments designed for those wavelengths can detect the galaxy’s light.
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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsWhy spectroscopy matters
Spectroscopy can make the redshift estimate more precise by resolving the light into narrower wavelength samples and identifying the evidence behind the estimate. It can help distinguish a high-redshift galaxy from an object whose broad-filter colors only resemble one. But spectra of very faint galaxies are challenging to obtain, so imaging remains valuable for finding and prioritizing candidates.
In practical terms, the two methods have different roles: imaging efficiently produces candidates, while spectroscopy can test the spectral evidence and refine the redshift. A candidate selected from images is not automatically a confirmed distance measurement.
The observational sequence is therefore straightforward in principle, even when difficult in practice: first locate possible galaxies in images, then estimate redshift from their filter colors, and, when the object is bright enough for follow-up, inspect its spectrum for identifiable features. NASA’s March 2016 explanation of redshift and remote-galaxy distances describes the spectral comparison at the heart of the method; NASA’s December 2023 account explains why photometry remains useful when a distant source is too faint for spectroscopy.
From redshift to a distance or lookback time
Once a redshift has been estimated, astronomers use a cosmological model to infer how the galaxy’s light relates to the expansion history of the universe. The conversion depends on such features as the universe’s expansion rate, geometry, and energy content. Different distance conventions describe different aspects of an expanding universe, so a reported value should specify which one is meant.
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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11Lookback time is a related but distinct way to express the observation: it describes how long the light has been travelling to reach us, or how far into the past we see the source. A distance in light-years and a lookback time are not interchangeable labels. In particular, a redshift measurement should not be casually restated as a number of light-years without specifying the cosmological model and distance measure used.
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This distinction is why headlines about a “most distant” galaxy need context. The spectral redshift is the measured quantity, while the quoted distance is model-dependent. A reader comparing two claims should check whether they report the same kind of measurement and the same distance convention, rather than comparing bare numbers as though they came from a direct survey.
The redshift-to-distance conversion is not a separate observation made by the telescope; it is an interpretation of the observed light within a description of cosmic expansion. This is why it is useful to ask whether a source is reporting redshift, lookback time, or a particular distance measure. Those quantities are connected, but they answer different questions and should not be treated as interchangeable figures.
A dated example: JADES-GS-z14-0
On May 30, 2024, NASA reported that spectroscopy with JWST’s Near-Infrared Spectrograph (NIRSpec) measured a redshift of about 14 for JADES-GS-z14-0. NASA described it as the most distant known galaxy at the time of that announcement. The figure is a reported redshift, not a distance in light-years.
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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Repair Windows errors before they cause bigger problemsFix Now →“Most distant known” is time-sensitive: it describes the state of observations when a claim was made, and later observations may change the record. The May 2024 report is a dated example of the method, not by itself evidence of the current record. When reading a newer record claim, look for its publication date, the reported redshift, and whether spectroscopy or imaging supports it.
The example also illustrates why it is important to name what the number represents. NASA’s figure for JADES-GS-z14-0 was a spectroscopically measured redshift of about 14. It should not be rewritten as a distance in light-years without a stated cosmological model and distance convention. If a later object is claimed to be farther away, readers should compare the evidence and definitions rather than assuming that a newer headline uses precisely the same measurement basis.
Where the cosmic distance ladder fits
The cosmic distance ladder is a related chain of techniques used to calibrate distances, especially for nearby galaxies and measurements of the universe’s expansion rate. Geometric measurements in our Milky Way neighborhood help establish distances to objects that can serve as calibrators. Astronomers can then use objects with known intrinsic brightness to extend the reach of distance estimates.
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Cepheid variables and RR Lyrae stars are examples of standard candles: their brightness behavior allows astronomers to estimate intrinsic brightness. Comparing that with apparent brightness gives a distance estimate. Calibrated Cepheids and Type Ia supernovae can extend measurements to more distant galaxies.
The basic logic is calibration in stages: geometric measurements establish distances nearby; calibrated stars provide distance indicators beyond that neighborhood; and calibrated Cepheids and Type Ia supernovae extend the chain to more distant galaxies. This context is important for understanding how astronomers build distance measurements and study expansion, but it is not a claim that every faraway galaxy is measured by finding a standard candle inside it.
This ladder provides important calibration context, but it is not the direct method described for the earliest, most distant galaxy candidates. For those objects, the central route is to estimate redshift from the light and infer distance using a cosmological model. It would be misleading to imply that a standard candle is directly observed in every early galaxy.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to evaluate a claim about a distant galaxy
When comparing candidate galaxies or distance estimates, check the evidence and the definitions before comparing the headline numbers:
- Method: Was the redshift estimated from multi-filter imaging, or measured spectroscopically?
- Spectral evidence: Does the report identify a line or break, such as the Lyman break, and explain the confidence in that identification?
- Reported redshift: Is the value presented as a candidate estimate or as a spectroscopic measurement? Is an uncertainty reported?
- Distance convention: If a distance is given, does the source say which cosmological model and distance measure it uses?
- Date: Is “most distant known” attached to a publication date, rather than presented as a timeless fact?
These checks separate what the telescope observed from what astronomers inferred. They also help explain why a highly promising image-selected candidate may later receive a more precise or revised redshift after spectroscopy.
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A useful comparison therefore has several parts, not just a contest between redshift values. Check whether each object has photometric or spectroscopic evidence, what line or break is identified and how confidently, and whether the reported redshift includes an uncertainty. If the reports give distances, check whether they use the same model and distance convention. Finally, check the publication dates before treating either object as the current record-holder.
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What a consumer telescope can—and cannot—do
The measurements discussed here rely on professional infrared observatories, sensitive imaging, and, where possible, spectroscopy. An ordinary consumer telescope is not a way to reproduce the redshift measurement of one of the faintest, most distant galaxies described in these reports. The central challenge is not simply magnification: astronomers need enough light, wavelength coverage, and spectral information to identify and measure the galaxy’s features.
The accessible takeaway is conceptual rather than a home observing procedure: distant-galaxy distances come from interpreting light with calibrated instruments and a cosmological model. Images help locate possible targets; spectra provide finer evidence; cosmology connects the redshift to a distance or a lookback time.
The essential distinction
Astronomers measure the stretching of a galaxy’s light and use that redshift as the observational starting point. They infer distance by applying a model of cosmic expansion, while the light’s travel time means the galaxy is seen in the past. Keeping those three ideas—redshift, model-dependent distance, and lookback time—separate is the key to understanding claims about the farthest galaxies.
Frequently Asked Questions
How do astronomers tell how far away a galaxy is?
For very distant galaxies, they identify spectral features or breaks and compare their observed wavelengths with their known emitted wavelengths to estimate redshift. They then use a cosmological model to infer a distance or lookback time; redshift alone is not a distance.
What does a galaxy’s redshift tell us?
It tells astronomers how much the galaxy’s light has been stretched toward longer wavelengths as the universe expanded during the light’s journey. Redshift is an observed property of the light, while a distance derived from it depends on the cosmological model and distance convention.
What is the difference between a photometric and spectroscopic redshift?
A photometric redshift is estimated from a galaxy’s colors and brightness across multiple imaging filters, so it is generally a probability distribution used to identify candidates. A spectroscopic redshift comes from a more finely sampled spectrum and comparison of identifiable lines or breaks with their emitted wavelengths; it can refine the estimate, though very faint galaxies can be difficult to observe spectroscopically.
Was JADES-GS-z14-0 the most distant galaxy?
NASA reported on May 30, 2024 that JWST/NIRSpec measured its redshift at about 14 and called it the most distant known galaxy at that time. “Most distant known” is a dated claim, so that announcement alone does not establish the current record.
Can a regular telescope measure the redshift of the most distant galaxies?
The measurements described here rely on professional infrared observatories, sensitive imaging and, where possible, spectroscopy. An ordinary consumer telescope cannot reproduce these faint-galaxy redshift measurements.
The Bottom Line
A telescope observes the galaxy’s light, not its distance directly. Astronomers use redshift evidence from images and spectra, then apply a cosmological model to infer distance or lookback time. Always distinguish a candidate estimate from a spectroscopic result, and treat “most distant known” as a claim tied to a publication date.
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