Just as an aside, these barriers do evolve, and depending on the mass of the star, you can see processed materials later on in the development of the star. This is very apparent in the class of objects called Wolf-Rayet stars, which appear to be extremely high mass stars. High mass stars are easily the most luminous stars in the galaxy and they are all surrounded by a nebulae of gas. The class of stars is defined by anomolous spectral lines of doubly ionized Helium or triply ionized lines of Carbon. They have effective temperatures of about 150,000 degrees Kelvin (the Sun's is about 5800). The gas and dust of the inner nebulae has spectral line widths indicative of a large radial velocity of 1000 to 3000 km/sec. All of this evidence has led to the conclusion that Wolf-Rayet stars are high mass stars that are so bright that the radiation pressure of the photons created by the fusion reactions in the core are overcoming the infall due to gravity in the atmosphere. This pushes the atmospheric material away from the star (at a rate of 1 solar mass every 10,000 years or so). Over time, the processed core material begins to be exposed, and you begin to see the spectral lines indicative of a Wolf-Rayet star.
There are only 150 Wolf-Rayet stars known in the galaxy and about another 30 in the Large Magellanic Cloud. They are very rare.
Thanx for the summary of W-R stars. I assume my earlier comments are valid with respect to Main Sequence stars...