Description:
<jats:p>For a simplified model of a gaseous nebula, that is spherical, isothermal, homogeneous and composed of pure hydrogen, the amount of Lyman-<jats:italic>α</jats:italic> radiation emitted from the surface of the nebula is evaluated for various physical conditions. The problem of observability of the Lyman-<jats:italic>α</jats:italic> emission line is examined, taking into account absorption by dust and by interstellar neutral hydrogen.</jats:p>