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Astronomers do not use one ruler to measure every galaxy. They build a cosmic distance ladder: geometric measurements anchor nearby distances, calibrated stars and supernovae extend the scale, and redshift helps estimate distances across the expanding universe. Each step depends on the one before it, and turning a distant galaxy’s redshift into a distance requires a model of cosmic expansion.
Why galaxy distances use a ladder
For nearby objects, astronomers can sometimes measure distance geometrically. Farther away, the object is too faint or its apparent position too small for that approach. Astronomers instead measure an observable—such as a star’s pulsation or a supernova’s brightness—and use calibrators to infer how bright it truly is. Comparing that intrinsic brightness with how bright it appears gives an estimate of distance.
NASA describes this linked sequence of methods as the cosmic distance ladder. The key idea is calibration: a method that works at one range helps establish the scale for the next, rather than every technique being independently precise at every distance.
How the distance ladder works
1. Parallax anchors nearby distances
As Earth moves around the Sun, a nearby star appears to shift against more distant background stars. Astronomers measure this apparent change in position—parallax—to determine the star’s distance geometrically. Those distances help calibrate the intrinsic brightness of suitable stars. Parallax anchors the ladder; it is not a practical way to directly measure the distance to remote galaxies.
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2. Cepheid variables extend the scale to nearby galaxies
Cepheids are stars that brighten and dim on a regular schedule. Their pulsation period is related to their intrinsic brightness: measure the period, infer how bright the star truly is, then compare that with its apparent brightness to estimate distance. Calibrated Cepheids in nearby galaxies connect the geometric scale to distances beyond the Milky Way.
NASA reported in 2016 that Hubble observed 70 Cepheid variables in the Large Magellanic Cloud for a precise distance measurement. That is a historical observation from that report, not a current total or a standard sample size for all measurements. See NASA’s illustration and explanation of the Cepheid calibration.
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3. Type Ia supernovae reach farther
Some galaxies contain both Cepheids and Type Ia supernovae. The Cepheids help establish the host galaxy’s distance; that distance, in turn, calibrates the supernova’s intrinsic brightness. Because Type Ia supernovae are much brighter than Cepheids, astronomers can use standardized supernovae in more distant galaxies to carry the ladder farther. NASA describes this approach in its account of measuring galaxy distances with space telescopes.
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4. Tully–Fisher provides an independent cross-check for spiral galaxies
For a spiral galaxy, astronomers can use its maximum rotation rate to predict its intrinsic brightness, then compare that prediction with its apparent brightness to estimate distance. In a 1993 report on M100, NASA said this estimate agreed with the Cepheid and supernova yardsticks for that galaxy. It is a historical example of an independent cross-check, not a guarantee that the methods always agree. NASA’s M100 report describes the comparison.
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How redshift helps estimate more remote galaxy distances
Measure the shift in the galaxy’s light
Elements leave recognizable lines and other features in a spectrum at known wavelengths. A spectrograph separates incoming light by wavelength, allowing astronomers to identify those features and measure how their observed wavelengths differ from the emitted ones. As the universe expands, light traveling through it is stretched; the measured shift is called redshift.
When spectroscopy is unavailable, astronomers can estimate redshift from a galaxy’s brightness across several wavelength bands, a technique called photometric redshift. NASA describes this as less accurate than spectroscopy because it infers the shift from broad-filter measurements rather than directly identifying spectral features. The NASA Webb and Hubble explanation of redshift and its measurement discusses both approaches.
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Convert redshift to distance with a cosmological model
Redshift is an observation: it tells astronomers how much the light’s wavelength has shifted. It is not itself a distance. To infer distance from redshift, astronomers use the relationship between redshift, the universe’s expansion, and distance under a cosmological model. The inferred distance therefore depends on the expansion history assumed; it is not a model-independent reading from a universal ruler. NASA explains this connection in its overview of cosmological redshift.
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| Method | Observable | How it contributes | Calibration or limitation |
|---|---|---|---|
| Parallax | Apparent position shift as Earth’s viewpoint changes | Geometric anchor for nearby stars | Helps calibrate stellar brightness; not a direct method for remote galaxies. |
| Cepheid variables | Pulsation period and apparent brightness | Estimates distances to nearby galaxies | Period–luminosity relation is calibrated using nearer stars. |
| Type Ia supernovae | Apparent brightness of a standardized explosion | Extends distance estimates to more distant host galaxies | Calibrated with galaxies whose distances have been established, including through Cepheids. |
| Tully–Fisher relation | Maximum rotation rate and apparent brightness | Independent distance estimate for spiral galaxies | NASA reported a historical agreement with other methods for M100; that example does not establish universal agreement. |
| Spectroscopic redshift | Wavelength shifts in spectral lines or features | Measures redshift for more remote galaxies | Distance is inferred by applying a model of cosmic expansion. |
| Photometric redshift | Brightness across multiple wavelength bands | Estimates redshift when spectra are not used | NASA describes it as less accurate than spectroscopy. |
The available NASA explanations do not give a single quantitative uncertainty table that ranks every method. Accuracy depends on the object, measurements, calibration, and—in redshift-based distance estimates—the cosmological model, so a universal error percentage would be misleading.
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How to read the distance units
Distances between galaxies are often expressed in megaparsecs. NASA defines 1 parsec as 3.26 light-years and 1 megaparsec as 1 million parsecs. A light-year is a unit of distance—the distance light travels in a year—not a measure of how long the measurement took. See NASA’s guide to cosmic distances.
Distance also means looking into the past
Because light takes time to travel, light arriving from a distant galaxy shows the galaxy as it appeared when that light began its journey, not as it looks at the moment of observation. In a NASA Webb article published December 6, 2023, University of Texas at Austin postdoctoral fellow Micaela Bagley put it this way: “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.” NASA Webb’s redshift explainer provides the context.
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