Question: We are fortunate that the Earth orbits our Sun in the “Habitable Zone” where it’s not too hot, it’s not too cold and liquid water can exist giving life a chance to evolve. As stars vary so much in size and luminosity how do astronomers work out where the “Habitable Zone” is around any given star? — David
Answer: The definition as to what exactly is a “habitable zone” for planets orbiting a star is rather complex, involving many factors, including the type of star, size of the planet, and even the location of the planetary system in a galaxy. A nice explanation of the constraints placed on the physical properties of a star which hosts potentially habitable planets can be found on the Penn State Astronomy Department e-Education description of the habitable zone. Generally speaking, the larger or hotter a star is, the farther away its habitable zone will be (and vice-versa).