Why do habitable zones move outward as stars become brighter over time?

The habitable zone is the orbital region around a star where a planet can maintain liquid water on its surface. The location of this zone is fundamentally determined by the star's luminosity, which is the total amount of energy a star emits.1

Because a star’s luminosity increases over time as it evolves, the amount of radiation reaching surrounding planets also increases. Consequently, the boundaries of the habitable zone—defined by the distance at which a planet is neither too hot (leading to a runaway greenhouse effect) nor too cold (where surface temperatures drop below freezing)—must shift outward to maintain the conditions necessary for liquid water.1

As a result of this outward migration, planets that were once within the habitable zone may eventually become too hot to support liquid water, while planets that were previously too cold may warm up enough to enter the habitable zone later in the star's life.1