Listed below are the names, spectral types (in parentheses), and approximate masses of several nearby main-sequence stars. Rank the stars based on the distances to their habitable zones (from the central star), from shortest to longest. Alpha Centauri A (G2)Sirius (A1)Barnard's Star (M4)
61 Cygnia A (K5)Spica (B1)

Rank of the stars from shortest to longest distances:

Barnard's Star (M4)

61 Cygnia A (K5)

Alpha Centauri A (G2)

Sirius (A1)

Spica (B1)

Explanation:

The habitable zone, also known as the circumstellar habitable zone, is the range of distances from a star where it is not too hot and not too cold for liquid water to exist on the surface of surrounding planets.

The location of a star’s habitable zone is dependent upon its luminosity, which is the amount of light emitted by an object in a unit of time, because a star’s luminosity increases with time; and also the star's mass.

The inverse square law of light brightness can be used to determine the extent of the habitable zones for different luminosity stars with the formula:

star boundary = Sun boundary × squareroot[(star luminosity)/(Sun luminosity].