Distance and luminosity

The brightness of the stars in Earth's skies depends on their true luminosities, and also on their distance from Earth. A star's brightness in the sky is termed its apparent magnitude, while its true brightness or absolute magnitude is defined as its magnitude at a distance of 32.6 light-years. The only direct way of measuring a star's distance uses the parallax effect caused by Earth's orbit around the Sun each year, but this only works for relatively nearby stars.

the parallax effect

Direct measurement of the distance to stars relies on parallax—the movement of nearby objects against a more distant background as the observer's point of view shifts. The farther away an object lies, the less it will appear to move against the "background" stars.

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