  Stars form from large clouds of gas and dust that contract under their own
gravity. The birth of a star occurred when a fusion reaction of hydrogen into
helium has ignited in its core. The key variable that determines the formation
of a star is mass. If the mass of the contracting cloud is below certain
minimum value, instead of a star, a substelar object -- known as a brown dwarf
-- will form. How much mass is required for a star to form? This article aims
to answer this question by means of a simple heuristic argument. The found
value is 0.016 solar masses, which is of the same order of magnitude as the
accepted value 0.08 solar masses. This article may be useful as pedagogical
material in an introductory undergraduate astronomy course.
