The Galactic Habitable Zone (GHZ) is an annulus region located about 7 to 9 kpc (2 kpc wide) from the Galactic Center. Within this annulus, most stars have ages ranging from 4 to 8 billion years, which allow sufficient time for life to evolve to an advanced stage. It represents the most likely region to find habitable worlds within the Milky Way galaxy. The criteria for determining the GHZ is based on suitable metallicity for the creation of rocky planets, and a danger-free zone where no supernova or gamma-ray bursts occur that could produce lethal radiation (gamma-rays, X-rays, and cosmic rays) to biological organisms. The circular orbits of solar systems that lie within the GHZ are suitable such that they do not encounter the dangerous spiral arms, where disruptive gravitational perturbations occur. The spiral arms also contain significant amounts of harmful radiation produced by supernova events. Our own solar system happens to rotate at roughly the same rate as the spiral arms of the Milky Way galaxy - i.e. the synchronization between our solar system's orbit around the Galactic Center and the galaxy's spiral arms prevents the solar system from crossing a spiral arm too often. As time advances, the GHZ migrates outward due to galactic evolution. It should be noted that other galaxies would have galactic habitable zones that vary in location and width (or might not exist at all), depending on the chemical composition of the galaxy.
The stellar habitable zone is an annulus around a host star within which water (which is required by life as we know it) remains in a stable liquid phase (from 273 K to 373 K). However, calculation of the precise location of a star's habitable zone (HZ) is more complex. The location of the HZ boundaries also depends on the nature of a planet's atmosphere, such as percentage of cloud cover which can increase planetary albedo and the greenhouse effect which can provide additional warming of the planet. For example, the equilibrium temperature of the Earth is about 255 K, but the actual temperature is near 288 K. The difference is due to Earth's greenhouse effect, and if it were not for this additional greenhouse factor, climatic conditions would be harsh for humans on Earth. If life were based on alternative chemistries, for example, ammonia as the liquid solvent instead of water, the HZ boundaries would lie at different distances from the sun. The change in the HZ boundaries is due to the fact the ammonia remains in a liquid phase over different temperature ranges than water, resulting in a HZ that would lie further out than HZ for water. As a star ages, its luminosity increases as hydrogen is fused into helium within the star's core. The increase in temperature and pressure occurs faster in stars that have higher mass and lower metallicity. In response to the changes in the star's core, there is a steady increase in luminosity and changes in effective temperature to main equilibrium. Therefore, the HZ will migrate outward as the star ages and luminosity increases.
Note that it is possible for a planet, such as Venus, to have an "effective albedo". For Venus, the effective albedo is very negative relative to the Earth. This is the result of a runaway greenhouse effect operating on Venus. The effective albedo for Mars lies in the normal range and is larger than Earth's.
The diagram of different habitable zones for hotter stars, sunlike stars, and cooler stars shown on this page is courtesy of NASA's Kepler Mission.
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