| Summary: | In this thesis I will present optical and near-infrared photometric and spectroscopic observations of an evolved field brown dwarf binary pair and of populations of low-mass stars in high-mass young stellar clusters and will compare them to stellar and substellar theoretical model predictions. Epsilon Indi Ba and Bb are the closest known brown dwarfs to the Earth, and, as such, make possible a concerted observational campaign to obtain a complete characterisation of two intermediate-age T dwarfs. Although some recent observations suggest substellar atmospheric and evolutionary models may be inconsistent with observations, there have been few conclusive tests to date. I will present high angular resolution optical, near-infrared, and thermal-infrared imaging and medium-resolution (up to R~5000) spectroscopy of these two T dwarfs. Using these data I have derived luminosities of log L/Lsun = -4.699 +/- 0.017 and -5.232 +/- 0.020 for Epsilon Indi Ba, Bb, respectively, and I will show that the predictions of substellar evolutionary models using luminosity and mass constraints are inconsistent with the effective temperatures and surface gravities derived from fitting atmospheric models to observed spectra. Furthermore, I will show that, even where estimates of the effective temperature, surface gravity, and luminosity are available, estimates of the mass of cool brown dwarfs can be up to a factor of two lower than the measured dynamical mass. Considering the difficulty in assigning accurate ages to any system and the mass-luminosity-age degeneracy of brown dwarfs, I would caution against the over-analysis of predicted brown dwarf masses at this time. I have also used Chandra X-ray observations to identify near-complete and relatively unbiased samples of pre-main-sequence stars in the young stellar clusters NGC 2244 and Trumpler 14. Using optical photometric and spectroscopic observations, I will characterise the apparent age spreads seen in the cluster pre-main-sequences. Mass estimates will then be derived for their stellar populations and used to construct the initial mass function for each cluster. It is found that NGC 2244 appears to have a Salpeter-like IMF for stars in the mass range 0.5--2.0 Msun if a likely age of 2 Myr is adopted, while Tr 14 may have a top-heavy IMF at a similar age. However, I will show that because the observed slopes of the cluster pre-main-sequences are not well-matched by the predicted slope in colour-magnitude space, such determinations are heavily dependent on the assumed age of the cluster and complicated due to the large spreads in isochronal ages.
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