The evolution of a star depends on the initial conditions existing during its formation and on the course of the main processes changing its chemical composition and mass. If it is assumed that stars are formed from diffuse matter, the initial conditions are restricted to three factors: 1) mass, 2) chemical composition, 3) angular momentum. The mass of the star can be determined by the conditions of gravitational instability of diffuse matter, which, according to our hypothesis, consisted of solid primary bodies, similar to cometary nuclei, and a gas and dust nebula formed by them. The upper limit for the possible mass of a star is evidently determined by its angular momentum, which cannot exceed a certain critical value.

The present communication contains mainly the discussion of the problem of mass loss by stars of early classes. The investigations show that the loss of mass by the Sun may proceed in different ways. In the atmospheres of O stars different mechanisms of ejection of gases must also exist: thermal dissipation of gases, the ejection of prominences (electromagnetic mechanisms), the ejection of atoms by radiation pressure, the ejection of atoms from active regions which may exist in the atmospheres of O-Stars, a very rapid rotation of stars etc. However our present knowledge on the mechanisms of ejection of gases from the surface of stars of early types is extremely scanty. Therefore it seems that there exists only one way to estimate the rate of mass loss by O-stars. This is the investigation of those effects which can be produced by gases ejected from the star.

Deux mécanismes importants de perte de masse s'accompagnent d'une grande perte de moment angulaire:

1. 1° une étoile en contraction et en rotation peut perdre de la matière le long de son équateur. Sa vitesse de rotation est alors telle qu'à l'équateur la gravité et la force centrifuge s'éuilibrent;

2. 2° l'étoile peut perdre de la matière par des processus éruptifs. La perte de moment angulaire est d'un ordre de grandeur très différent suivant que la matière éjectée est ou non guidée par les lignes de force d'un champ magnétique.