How Astronomers Measure Distances to Stars

Astronomers measure distances to nearby stars with parallax, the tiny apparent shift of a star against the distant background as Earth moves around the Sun. For stars too far away for parallax, they use objects of known brightness, called standard candles, and compare how bright those objects look with how bright they really are. No single method works at every distance, so the methods are chained together into what is called the cosmic distance ladder.

Parallax: the first rung

Hold a finger at arm’s length and look at it with one eye closed, then the other. The finger seems to jump against the background. Stellar parallax is the same effect on a cosmic scale. Observers measure a star’s position now and again six months later, when Earth is on the opposite side of its orbit. The small change in direction, together with the known size of Earth’s orbit, gives the distance by simple trigonometry.

The unit built from this idea is the parsec. One parsec is the distance at which a star would show an annual parallax of one arcsecond, and it equals about 3.26 light-years. To convert, divide 1 by the parallax in arcseconds. As an example, Proxima Centauri has a parallax of about 0.7685 arcseconds, which gives roughly 1.30 parsecs, or 4.24 light-years.

A short history

According to Wikipedia, Friedrich Bessel measured the parallax of the star 61 Cygni in 1837 to 1838 using a heliometer at Königsberg Observatory, and Thomas Henderson and Friedrich Struve made early measurements of Alpha Centauri and Vega around the same time. Only about sixty parallaxes had been obtained by the end of the nineteenth century.

Space telescopes changed the situation. The Hipparcos satellite, launched in 1989, raised the accuracy to the milliarcsecond level. The European Space Agency’s Gaia mission, launched on 19 December 2013, catalogued the positions, distances and motions of about two billion stars and other objects; science observations ended on 15 January 2025. ESA’s own summary of the mission is on its Gaia overview page.

Why parallax runs out

Parallax angles shrink as stars get farther away. Eventually the angle is smaller than the errors in measurement. Wikipedia notes that parallax and related direct methods work out to about 1,000 parsecs, and Gaia’s improved precision pushes useful measurements further than any earlier survey. Beyond that range, another approach is required.

Standard candles: the second rung

If you know how bright a light bulb really is, you can tell how far away it is from how bright it looks, because apparent brightness falls with the square of distance. A standard candle is a type of object whose true brightness can be inferred.

• Cepheid variables. These pulsating stars brighten and fade with a regular period. Henrietta Swan Leavitt found in the early twentieth century that Cepheids with longer periods are intrinsically brighter. Wikipedia notes that her relation, once calibrated with nearby Cepheids whose distances come from parallax, let astronomers measure distances far beyond parallax’s reach. Cepheids have been used to reach nearby galaxies, with the farthest found at about 29 megaparsecs according to the distance-ladder article.
• Type Ia supernovae. These stellar explosions reach a fairly consistent peak brightness once corrected for how quickly they fade, with an absolute magnitude of about -19.3. Because they are so luminous, they can be seen across more than a thousand megaparsecs, although their uniformity may be less perfect than once assumed.

Redshift for the farthest objects

For very distant galaxies, individual stars cannot be seen, so distances are inferred from the redshift of their light and the expansion rate of the universe. That requires the Hubble constant, and different methods give somewhat different values, around 60 to 80 kilometres per second per megaparsec across the literature. The difference is a significant open question in cosmology.

Why the ladder is hard

Each rung is calibrated using the one below, so an error low down carries upward. A better parallax for a nearby Cepheid improves the Cepheid scale, which improves the distances to supernovae. That is why improving the first rung matters so much.

Other branches of astronomy also depend on measuring small shifts. For example, the cross-correlation example shows how a faint signal can be recovered from noisy spectra by comparing it with a model. The publications page lists research that uses high-resolution spectroscopy, and an overview of a doctoral thesis on spinning worlds gives a sense of how spectra are used to measure the properties of planets.

Frequently asked questions

What is the most accurate way to measure the distance to a star?

For nearby stars, parallax is the most direct and accurate method because it relies on geometry rather than assumptions about the star’s brightness.

How far can parallax measure?

It depends on the instrument. Wikipedia gives about 1,000 parsecs for direct methods in general. Gaia’s predicted end-of-mission accuracy runs from several microarcseconds for bright stars to a few tens of microarcseconds for fainter ones, which extends the range greatly.

What is a standard candle?

It is a class of object whose real brightness is known or can be worked out, so its distance follows from how bright it appears.

In short

Parallax anchors the scale for nearby stars, and standard candles carry it outward. The distances astronomers quote are the product of several linked methods, each with its own uncertainty.