Standard candle

A standard candle is an astronomical object whose intrinsic luminosity is known or calibrated, allowing its distance to be estimated from its observed brightness.

Inverse-square law: A physical law stating that a point source’s intensity decreases in proportion to the square of its distance. It converts a candle’s luminosity and observed brightness into distance.
Cepheid variable: A pulsating star whose period of brightness variation is related to its intrinsic luminosity. Its calibrated period–luminosity relation measures distances to nearby galaxies.
Period–luminosity relation: An empirical relation linking a pulsating star’s period to its intrinsic luminosity. It supplies luminosity estimates for variable-star candles from their measured periods.
Cosmic distance ladder: A sequence of distance-measurement methods, each calibrated using more nearby methods. Standard candles bridge geometric distances to measurements of remote galaxies.
Apparent magnitude: A logarithmic measure of the brightness of an astronomical object as observed from Earth. It records the received brightness compared with the candle’s calibrated output.
Type Ia supernova: A thermonuclear explosion of a white dwarf, recognized by its characteristic spectrum and light curve. Its peak luminosity can be standardized, making it visible across vast distances.
Parallax: The apparent shift in an object’s position caused by a change in the observer’s viewpoint. Geometric parallax calibrates nearby stars that anchor luminosity relations.
Hubble–Lemaître law: The relation between a galaxy’s recession velocity and its distance at relatively low redshift. Candle distances paired with recession velocities establish the universe’s expansion rate.
Absolute magnitude: The apparent magnitude an object would have at a distance of ten parsecs. It expresses intrinsic brightness on the same scale as apparent magnitude.
RR Lyrae variable: A pulsating horizontal-branch star with a relatively consistent intrinsic luminosity. Its calibrated brightness helps measure distances within the Milky Way and nearby galaxies.

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