What is a standard candle in astronomy?

A standard candle is any object whose intrinsic luminosity (true brightness) is known independently of its distance. By comparing the known luminosity to the observed apparent brightness, astronomers calculate distance using the inverse-square law: flux falls off as 1/distance². The most important standard candles are Cepheid variable stars (period–luminosity relation discovered by Henrietta Swan Leavitt in 1908) and Type Ia supernovae (all reach roughly the same peak absolute magnitude). The HubbleSite Cepheid article explains Leavitt's discovery.

What is the Hubble tension?

The Hubble tension is one of the biggest problems in modern cosmology. Two independent ways of measuring the Hubble constant H₀ (the rate of cosmic expansion) give significantly different values: the early-universe method (CMB + ΛCDM model, Planck 2018) gives H₀ ≈ 67.4 km/s/Mpc, while late-universe measurements (distance ladder via Cepheids + Type Ia supernovae, SH0ES project) consistently give H₀ ≈ 73 km/s/Mpc. The discrepancy is now statistically significant (>5σ). Either there are systematic errors in one or both methods, or there is new physics — perhaps early dark energy or an extra relativistic species. The NASA Hubble tension explainer covers the debate.

How do we know the distance to the Andromeda Galaxy?

Andromeda's distance is measured by multiple methods that agree well. Edwin Hubble himself used Cepheid variable stars observed in Andromeda in 1923 to prove it lay beyond the Milky Way — the proof that "spiral nebulae" were separate galaxies. Modern measurements using Cepheids, RR Lyrae variables, red giant branch tip (TRGB) stars, and eclipsing binary stars all converge on about 2.537 million light-years (0.78 Mpc). See also our parsec dictionary entry for distance unit context.

How far can we see in the observable universe?

The observable universe extends about 46 billion light-years in all directions — roughly 93 billion light-years in diameter. This is larger than 13.8 billion light-years (the age of the universe in years) because the universe has been expanding throughout the light's journey: the source has moved away since the light left it. The most distant light we can observe is the Cosmic Microwave Background, emitted 380,000 years after the Big Bang at a redshift of z ≈ 1,100. JWST observes galaxies at z > 13, corresponding to roughly 33 billion light-years of comoving distance.

Measuring Cosmic Distances — The Distance Ladder

The cosmic distance ladder

No single method works at all scales. Each technique has a range where it is reliable, and each is calibrated using the one below it. This chain of overlapping methods is the cosmic distance ladder. Errors at lower rungs propagate upward — which is why the Hubble constant is still debated.

Parallax

Up to ~10,000 ly

Nearest stars: measure apparent shift against background as Earth orbits the Sun. ESA Gaia mapped 1.5 billion stars by parallax.

Cepheid variables

Up to ~100 Mly

Stars that pulsate with a period linked to their luminosity. Measure the period, know the luminosity, compare to apparent brightness — distance follows.

Type Ia supernovae

Up to ~10 Gly

Standard candles: all Type Ia supernovae reach the same peak brightness. Used to discover dark energy in 1998.

Redshift (Hubble flow)

Up to edge of observable universe

Beyond ~1 Gly, recession velocity from the Hubble-Lemaître law gives distance. Requires calibrating the Hubble constant H₀.

Units and tools

Distances across these rungs span an enormous range. Within the Solar System, the astronomical unit (AU) is standard. For nearby stars, the parsec (defined by parallax) is natural. Cosmological distances use megaparsecs (Mpc) or are expressed via redshift. Convert between all units with our Distance Converter tool.

The JWST deep field observations — reaching z > 13 — represent the top rung of the ladder. See our Webb deep field article for the latest distance records.

Measuring cosmic distances

The cosmic distance ladder

Units and tools

Frequently asked questions