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Why do astronomers use standard candles for galaxies too distant for stellar parallax measurements?

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Diagram illustrating how astronomers calculate the distance to a star using parallax: Earth is shown in two positions on opposite sides of its orbit around the Sun, with lines extending to a nearby star X and further to distant background stars. The apparent shift in star X's position, measured as angle A, is used to determine its distance, with the baseline labeled as 186,000,000 miles. — Parallax Calculating stellar distances.

Astronomers use standard candles for distant galaxies because stellar parallax measurements are limited to relatively nearby stars within our own galaxy.12

Trigonometric parallax relies on measuring the tiny angular shift of a star as Earth orbits the Sun, using the diameter of Earth's orbit as a baseline. Because this shift becomes infinitesimally small as distance increases, parallax is only effective for stars within a few thousand light-years.32

For more distant objects, astronomers employ standard candles—celestial objects with a well-determined luminosity, such as Cepheid variables or certain supernova explosions. By comparing a standard candle's known intrinsic luminosity to its measured apparent intensity, astronomers can calculate its distance using the inverse-square law of light. These objects are first calibrated using nearby examples whose distances have been reliably measured, providing a vital "distance ladder" to reach galaxies far beyond the range of direct parallax measurements.145