  One of the most fundamental topics of exobiology concerns the identification
of stars with environments consistent with life. Although it is believed that
most types of main-sequence stars might be able to support life, particularly
extremophiles, special requirements appear to be necessary for the development
and sustainability of advanced life forms. From our study, orange main-sequence
stars, ranging from spectral type late-G to mid-K (with a maximum at early-K),
are most promising. Our analysis considers a variety of aspects, including (1)
the frequency of the various types of stars, (2) the speed of stellar evolution
their lifetimes, (3) the size of the stellar climatological habitable zones
(CLI-HZs), (4) the strengths and persistence of their magnetic dynamo generated
X-ray - UV emissions, and (5) the frequency and severity of flares, including
superflares; both (4) and (5) greatly reduce the suitability of red dwarfs to
host life-bearing planets. The various phenomena show pronounced dependencies
on the stellar key parameters such as effective temperature and mass,
permitting the assessment of the astrobiological significance of various types
of stars. Thus, we developed a "Habitable-Planetary-Real-Estate Parameter"
(HabPREP) that provides a measure for stars that are most suitable for planets
with life. Early K stars are found to have the highest HabPREP values,
indicating they may be "Goldilocks" stars for life-hosting planets. Red dwarfs
are numerous, having long lifetimes, but their narrow CLI-HZs and hazards from
magnetic activity make them less suitable for hosting exolife. Moreover, we
provide X-ray - FUV irradiances for G0 V - M5 V stars over a wide range of
ages.
