This chapter presents results for certain specific stellar types, divided into pre-main-sequence stars, mainsequence stars, X-ray sources in general, and post-mainsequence evolutionary phases. Additionally, it covers the use of the Hipparcos and Tycho stars as probes of the local interstellar medium.

Pre-main-sequence stars

Introduction

According to the theory of Hayashi (1961), a star begins its pre-main-sequence life with a very large radius, well in excess of the value that it finally assumes when it reaches the main sequence. The large surface area implies a high luminosity, more than can be transported through radiation alone. Thus, all sufficiently massive pre-main-sequence objects were predicted to be highly convective, spanning a broad region of the HR diagram above the main sequence, and evolving to the main sequence over the Kelvin–Helmholtz time scale. Subsequent refinements regarding the ignition of light elements and final approach to the main sequence were added by Iben (1965) and others. Detailed models of pre-main-sequence evolution have subsequently been developed by, e.g. D'Antona & Mazzitelli (1994, 1997), Palla & Stahler (1993, 1999), amongst others.

The starting radius is now considered to follow a certain locus, or ‘birthline’, in the HR diagram, where an object first becomes visible. Subsequently, detailed evolution depends on mass, protostellar mass accretion rate, mass loss due to winds, internal rotation, magnetic field, etc. Typical models, however, consider the pre-main-sequence star to be a non-rotating sphere of constant mass, contracting from the birthline to the ZAMS, with fusion dominated by hydrogen with contributions from deuterium and, at ages close to 107 yr, pre-main-sequence lithium burning (for further details see, e.g. D'Antona & Mazzitelli 1997 and Palla & Stahler 1999).