Area around a star (or in some cases around two close stars) where the amount of heat from the star(s) is just right for
liquid water to exist on an Earth-like planet (or an
Earth-like moon orbiting a giant planet). As stars evolve they become
hotter and the habitable zone moves outward - the bigger a star is, the quicker this happens. The most stable habitable
zones exist around stars similar to Earth's sun:
Sol - stars with habitable zones like this are described as
Sol-like. Habitable zones around stars too big to qualify as
Sol-like move outwards too fast for a planet to have time to become
Earth-like before it leaves the inner edge of the zone, however
terraforming could be used to speed up the evolution of a few of
these planets. Although habitable zones around stars too small to be called
Sol-like move outwards much slower than is needed for planetary evolution,
they tend to be dangerously close to their stars - the entire zone can be at risk from radiation during stellar flares.
Small stars are relatively dark and individual flares can be more bright than the normal brightness of the entire star.
In extreme cases the total output can be 10 to 100 times greater than normal. Although a planet within the habitable zone
of a small star would stay in the zone much much longer than Earth took to evolve, the radiation during stellar flares can sterilise
life on the surface, causing frequent extinctions. Although radiation resistant plants could be developed to help
terraform some of these worlds, colonists would face constant risk
- even if they avoided the very dangerous 'flare-stars', they would still need to build radiation shelters and be ready
to go into them at moments notice.
Original form of fusion power. Hot-fusion occurs at high
temperatures and pressures and was not possible before the advent of
superconductors.
See also cold-fusion.
