How astronomers measure distances in space, from parallax to redshift

Astronomers cannot run a tape measure to a galaxy, so they work out distances in stages. Geometry gives the distances to nearby stars. Stars and stellar explosions of known brightness carry those measurements out to other galaxies. At the largest scales, the stretching of light by the expanding universe becomes the measure. NASA describes the chain as a cosmic distance ladder, with each step building on the one before it.

The units

Ordinary units soon become unwieldy in space, so astronomers use three of their own. The astronomical unit, close to the average distance between Earth and the Sun, is formally defined as 149,597,870.7 kilometers (92,955,807.3 miles), according to NASA's glossary, and serves for distances within planetary systems: Saturn orbits about 9.5 of them from the Sun. A light-year is the distance light covers in a year, about 5.8 trillion miles (9.5 trillion kilometers). The nearest star system, Alpha Centauri, is about 4.3 light-years away, and the Sun is about 8 light-minutes from Earth.

Once distances run into the high thousands or millions of light-years, astronomers switch to the parsec, equal to 3.262 light-years. For the gaps between galaxies they use the megaparsec, a million parsecs or about 3.26 million light-years.

Step one: parallax

Parallax is the apparent shift of a nearby object against a more distant background when the observer moves. In astronomy, NASA's glossary defines it as half the angle by which a star appears to move as Earth travels from one side of the Sun to the other. The parsec comes straight from it: it is the distance at which that angle would be one arcsecond.

NASA calls parallax a basic tool of geometry. In NASA's account of how the Hubble Space Telescope refined the universe's expansion rate, the ladder begins with Hubble measuring parallax distances to Cepheid variable stars.

Step two: stars of known brightness

Once astronomers know how bright an object really is, they can tell how far away it is from how bright it looks. NASA compares the idea to judging how far off an approaching car is from the brightness of its headlights at night. Two kinds of object serve as these yardsticks.

Cepheid variables are stars that swell and shrink in regular cycles, brightening and dimming as they do. In 1912, the astronomer Henrietta Leavitt found that the length of the cycle is directly related to a Cepheid's luminosity, so timing a Cepheid reveals its true brightness. The class takes its name from the star Delta Cephei, and NASA counts Polaris as the Cepheid closest to the Sun. Once parallax has calibrated nearby Cepheids, astronomers can find Cepheids in other galaxies and work out how far away those galaxies are.

Type Ia supernovae extend the reach. They occur when a white dwarf, the Earth-sized remnant of a star, explodes, either after merging with a companion or after drawing off enough gas from one to pass its stability limit. Such explosions reach roughly the same intrinsic brightness, and they are far brighter than Cepheids, so they can be seen much farther away. Astronomers calibrate them in galaxies that contain both Cepheids and Type Ia supernovae, then use supernovae alone in more distant galaxies.

Step three: redshift

Redshift is the apparent shift of light toward longer, redder wavelengths. NASA's glossary distinguishes three causes: motion away from the observer (Doppler redshift), light climbing out of the gravity of a massive object, and cosmological redshift, written z, in which the light's wavelength is stretched on its journey as space itself expands. In 1929, Edwin Hubble found that the farther away a galaxy is, the faster it appears to be moving away from us. At the greatest distances, NASA notes, astronomers stop quoting megaparsecs and give the redshift instead.

Light takes time to cross these distances, so the most remote objects are seen as they were long ago. A pair of quasars reported on November 10, 2026, has a redshift of about 6.08, according to the team's paper, and NSF NOIRLab places it about 900 million years after the Big Bang. With the universe about 13.8 billion years old, that light has been traveling for roughly 12.9 billion years, by our arithmetic.

Where the methods disagree

The top of the ladder yields the Hubble constant, the expansion rate that links a galaxy's distance to the speed at which it recedes: by NASA's formula, the speed is the Hubble constant multiplied by the distance. Astronomers work out the distances of faraway supernovae from their brightness and compare them with how much their light has been stretched. NASA says Hubble's observations narrowed the constant to an accuracy of almost 1%. Before the telescope's launch, estimates of the universe's age ranged from 9.7 billion to 19.5 billion years; Hubble's work pinned it at 13.8 billion.

A second route uses the cosmic microwave background, the oldest light that can be detected, released when the universe was about 380,000 years old. The two do not agree. Measurements built on Cepheids and Type Ia supernovae give a Hubble constant of around 70 to 76 kilometers per second per megaparsec, while measurements based on the microwave background give about 67 to 68, according to NASA. Scientists call the gap the Hubble tension.

NASA lists exotic particles, alternative theories of gravity and early forms of dark energy among the proposed explanations, and suggests something may be wrong in our understanding of the early universe. In its explainer on the subject, published in August 2025, the agency described the tension as still a mystery that will take more research to resolve.