Stellar, Galactic, and Super-Galactic Habitable Zones

The concept of habitable zones for planets in astrobiology is briefly reviewed. Progress has been significant in the nearly century-long effort to theoretically characterize habitable planets, The Liquid Water Belt of Shapley has been revised to include a wide variety of modern concepts. These efforts are all related to the location of planets in both space and time, which might be expected to be capable of supporting both simple and complex life as we know it. In particular, circumstellar habitable zones around single and binary stars have been proposed and depend mainly on the luminosity of the host star(s). Ultraviolet (UV) habitable zones have also been proposed to locate orbital distances and spectral types with sufficient UV to promote the origin of life and with UV below destructive levels. Circumbinary habitable zones are quite varied, with some very harsh environments and others with sedate conditions. Some binaries experience tidal torques that reduce stellar rotation and magnetic activity, the so-called Binary Habitability Mechanism, and thereby possess "better than Earth" habitability conditions. Across the spectrum of possibilities, it is important to distinguish between locations allowing for the origin of life and those providing long-term viability of life. Galactic habitable zones are defined according to the availability of elements for making potentially habitable planets as well as non-habitable zones characterized by frequent threats to complex life such as a nearby supernova, gamma-ray bursts, accreting black holes, frequent asteroid or comet impacts, and high levels of Galactic cosmic rays. Usually, the Galactic Habitable Zone is represented by an annulus in the Galactic disk, but much depends on the orbital history of the planetary system and the merger and star-formation history of the host galaxy. Avoiding star formation regions for extended periods in a low star formation rate disk galaxy can be of great benefit, conducive to the development of complex life. Furthermore, we suggest that the concept may be extended to the Super-Galactic Habitable Zone (SGHZ), defined as hospitable regions within clusters of galaxies, bounded by inhospitable active galaxies and low-metallicity dwarf galaxies in the outskirts of galaxy clusters. The SGHZ - if defined just for survival and not including the birth of complex life - is a large network of connected local habitable zones. The birth places for complex life form a smaller disconnected set of islands.