  Observations have been carried out with SCUBA at the JCMT of 52 molecular
cloud cores that do not contain any sign of protostellar activity. These are
all therefore candidate prestellar cores, which are believed to represent the
stage of star formation that precedes the formation of a protostar. 29 of the
52 cores were detected at 850 microns at varying levels of signal-to-noise
ratio greater than 3 sigma at peak. The detected cores were split into 'bright'
cores and `intermediate' cores, depending on their peak flux density at 850
microns. Cores with peak 850 microns flux densities greater than 170 mJy/beam
were designated 'bright' (13 cores), while those flux densities below this
value were designated 'intermediate' (16 cores). This dividing line corresponds
to A_v~50 under typical assumptions. The data are combined with our previously
published ISO data, and the physical parameters of the cores, such as density
and temperature, are calculated. Detailed fitting of the bright core radial
profiles shows that they are not critical Bonnor-Ebert spheres, in agreement
with previous findings. However, we find that intermediate cores, such as B68
(which has previously been claimed to be a Bonnor-Ebert sphere), may in fact be
consistent with the Bonnor-Ebert criterion, suggesting perhaps that cores pass
through such a phase during their evolution. We make rough estimates of core
lifetimes based on the statistics of detections and find that the lifetime of a
prestellar core is roughly ~3x10^5 years, while that of a bright core is
\~1.5x10^5 years. Comparisons with some magnetic and turbulence regulated
collapse models show that no model can match all of the data. Models that are
tuned to fit the total prestellar core lifetime, do not predict the relative
numbers of cores seen at each stage.
