How is a galactic habitable zone different from a stellar habitable zone?

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Habitable zone The habitable zones (green) for stars that are like the Sun (middle), hotter than the Sun (top), and cooler than the Sun (bottom). The red areas are those in which liquid surface water would be lost as a result of a runaway greenhouse effect, and the blue areas are those in which liquid surface water would be completely frozen.

The concepts of stellar and galactic habitable zones both define regions conducive to life, but they operate on different scales and rely on distinct environmental factors.

Stellar Habitable Zone

A stellar habitable zone is the orbital region around a specific star where a planet can maintain liquid water on its surface. Its boundaries are primarily determined by the star's luminosity and the planet's atmospheric conditions, such as the presence of greenhouse gases.1

Galactic Habitable Zone

The galactic habitable zone extends this concept to a planet's location within a galaxy, such as the Milky Way Galaxy. Instead of stellar radiation, its boundaries are defined by galactic-scale hazards and chemical availability:1

Inner Region: Near the galactic centre, stars are densely packed, increasing the risk of life-threatening events like supernovae.1
Outer Region: In the galaxy's outskirts, there is a lower concentration of chemical elements necessary to form terrestrial planets.1

While a stellar habitable zone is relatively well-defined by orbital mechanics and radiation, the boundaries of a galactic habitable zone are significantly more difficult to quantify.1