Cosmic distance ladder

A sequence of astronomical methods that calibrate one another to measure distances from nearby stars to distant galaxies. Each rung extends the reach of measurements by relying on distances established by earlier methods.

Stellar parallax: The apparent shift in a nearby star’s position as Earth moves around the Sun, used to calculate its distance geometrically. Its geometric distances calibrate nearby stars that anchor later rungs.
Astronomical unit: The mean distance between Earth and the Sun, used as a unit of length in astronomy. Earth’s orbital baseline supplies the geometry for stellar parallax.
Tip of the red-giant branch: The bright endpoint of red-giant evolution, whose luminosity in infrared bands can estimate distances to nearby galaxies. It provides an independent stellar calibration rung alongside Cepheids.
Redshift: The displacement of spectral features toward longer wavelengths, often used to estimate recession speed in cosmology. Redshift is directly observed, but turning it into distance requires a cosmological model.
Cepheid variable: A pulsating star whose period of brightness variation is related to its intrinsic luminosity. Cepheids transfer parallax-calibrated distances to galaxies beyond direct parallax reach.
Parsec: The distance at which one astronomical unit subtends an angle of one arcsecond. The unit is defined directly by the parallax geometry underlying the ladder’s nearby anchor.
Tully–Fisher relation: An empirical relationship between a spiral galaxy’s rotation speed and its luminosity. It estimates galaxy distances beyond the range where individual stars can be resolved.
Cosmic distance duality relation: A relation stating that luminosity distance equals angular-diameter distance multiplied by the square of one plus redshift, under standard assumptions. It connects two cosmological distance measures rather than calibrating successive stellar candles.
Period–luminosity relation: A relationship between a variable star’s pulsation period and its intrinsic luminosity. It turns Cepheid pulsation periods into luminosities and therefore distances.
Apparent magnitude: A logarithmic measure of how bright an astronomical object appears from Earth. Observed brightness is compared with calibrated luminosity to infer distance.

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