A habitable zone is an approximated distance to a star where life would theoretically exist. This distance varies up to high levels, because of 3 categories of factors: hosting star, hosted planet and additional technologies used to terraform the planet.
Host star Edit
The hosting star can be of many types. Some of them might be very uncommon and might require extreme engineering in order to transform the planet. Stellar age or lifetime is not important for settlers, since human lifetime is almost nothing compared to a star's lifetime anyway.
O - type stars - they are the hottest blue stars, blowing deadly X-ray waves and having the strongest solar winds. A habitable planet is unlikely to exist, but future cutting-edge technology might be able to create one.
B - type stars - like those in the Pleiades, they are hot white-blue stars, probably the most common stars seen in the visible sky. Even if they generate very strong UV and X rays, still some of them might possess planets .
A - type stars - like Sirius A, they are white stars, with very strong luminosity, but with an acceptable radiation level. Many are found to have planets and some scientists argue that they might be stable long enough to create some microbes on their planets.
F - type stars - like Procyon A, yellow-white, these stars might be considered heavier candidates to our sun, Sol. Good destination for future settlers.
G - type stars - like Sol, our sun, they generate the best quality of light, consisting of all kind of radiation, in good amounts.
K - type stars - like Epsilon Eridani, these stars are fainter then the sun, but some people might consider them one of the best destinations for future settlers.
M - type stars - like Barnard's Star, known as red dwarfs, the most common type of star in the Universe. They are faint, have a long lifetime and many scientists argue that they are good enough to support life.
Multiple Star Systems - with complex conditions for any hosted planet.
Other celestial bodies
Brown Dwarfs - might be called failed stars. They are too small to fuse hydrogen, but might be able to fuse deuterium. Even if their light is too dim for Earth-type plants, they can provide a planet with enough infrared radiation to keep it warm.
White dwarfs - like Sirius B, are highly-compressed stars, with a size comparable to Mars, but with a mass of usually 50% of the Sun. Even if they have strong X-ray radiation emissions, some scientists argue that they can host habitable planets.
Black Dwarfs - for the moment only hypothetical celestial bodies, dead white dwarfs.
Red giants - like Betelgeuse, are dying stars, just like our sun will one day become. Despite their strong solar wind and short lifetime, they might have planets that are suitable for terraforming.
Variable Stars - stars that don't give the same amount of energy output all the time. Some have a predictable variation, while others are unpredictable.
Neutron stars - highly compressed stars, with a diameter of roughly 15 km and over 1.5 Solar masses. They can support an Earth-type planet, protected with a cutting-edge technology shield against their deadly gamma ray bursts.
Quasars - supermassive black holes, that generate enough light to illuminate a planet many light-years away, safe from their deadly gravitational pull.
Hot planets - slowly cooling, can generate a limited amount of heat, like a brown dwarf, but not for long.
Rough planet - a planet that is free floating across the galaxy.
Artificial sun - this is something theoretical for the moment.
Black holes - could be possible with very advanced technology to use their power to heat a planet.
Hosted planet Edit
It is important to note that there are two habitable zones:
Classic habitable zone can be defined as the area where an Earth-like planet can experience suitable temperatures for Earth-like life without managing temperature protection or changing luminosity parameters. The best, earth-like orbit, is named comfort zone by Internet Stellar Database.
Artificial habitable zone will be considered the area where an Earth-type planet can experience suitable temperatures for Earth-like life with the help of some technology (like greenhouse gasses for outer planets of shielding mirrors for inner planets).
Size is important. The hosted planet can be of any possible type, but an Earth-size planet should be a good candidate. A smaller planet (Mercury size) should require less resources to terraform, due to its smaller size, but a too small planet (Rhea size) would be too small to hold an atmosphere for long. A super-Earth will have a denser atmosphere and so it can stay further away. Terraforming a gas giant like Neptune is possible in theory, but at giant costs.
Luminosity is of high importance for the hosted planet. O, B and A stars generate much of their visible light in blue, while M stars generate almost all visible light in red. Plants need both red and blue light, while green light is mostly useless. So, if agriculture is to be developed on an alien planet using terrestrial plants, it is important to have both red and blue light. I made some experiments on my own, using different plants and enclosing them into large opaque boxes, with limited windows, allowing plants to receive only limited amount of light. The result is that, with 1% of light (similar to the orbit of Jupiter), plants (including grain) could survive. At 0.1% luminosity (similar to what we would find around Neptune), only some plants managed to survive. However, if you give a plant only red or only blue light, it will die. For example, if a theoretical planet exists around a red dwarf and that planet has 1% of solar red light and only 0.01% of solar blue light, settlers will have to provide their gardens with artificial blue light.
Tidal forces would be great around red dwarfs, tremendous around brown dwarfs and white dwarfs, but extremely faint around O, B main sequence stars and red giants.
Temperature control is possible with some technology. Greenhouse gasses can be used to increase temperature, while some mirrors can be used to decrease temperature, as proposed for Venus. There is a major problem when we talk about bodies that emit most of their light in infrared, like red and brown dwarfs. Greenhouse gasses will also reflect infrared light back into space, so heating a planet orbiting a brown dwarf is a difficult task.