[Widdekind]

In our galaxy, stars form from within Giant Molecular Clouds (GMCs), of cold molecular hydrogen (H₂), characterized by densities of 10^-18km/m³; and, temperatures of 10K. GMCs, which "are not gravitationally bound", may be compressed by the surrounding, hot-but-diffuse, Inter-Stellar Medium (ISM). Collapse/compression continues, until the cloud becomes opaque to its own thermal radiation emissions, at densities of 10^-10-10^-9km/m³; and, temperatures of a few hundred Kelvins. Note, that densities increase by nearly nine orders-of-magnitude; whereas, temperatures increase by only one-and-a-half orders of magnitude, i.e. the clouds contract nearly semi-iso-thermally.

Once the cloud becomes opaque, the newly formed stellar embryo (SE) enters onto the Hayashi track (HyT). The SE contracts fully convectively, until the release of gravitational potential energy (GPE) is sufficient, to heat the core of the collapsing cloud to several tens of thousands of Kelvins; and, to dissociate the molecular hydrogen (H₂ --> 2H); and, to fully ionize the hydrogen & helium atoms; all of which requires ~17eV/m_H. Once the cloud core is both heated & ionized, then the cloud exits the HyT, 'veering to the left' on the HRD; and, enters the Henyey track (HnT); and, has become a proto-star (PS).

Evolving along the HnT, the PS central density increases as the cube of the central temperature, according to the equation of Hydro-static Equilibrium, i.e. P_c/R ~ GM²/R⁵; and, the Virial Theorem, i.e. k_BT ~ GM/R. The resulting "radiative core" is no longer convective, i.e. is evolving supra-adiabatically. And, for stars slightly more massive than our sun, the radiative region of full ionization, eventually expands, from the core, all the way to the surface of the PS, i.e. such stars have no convective outer envelopes, unlike low-mass stars , which retain convective photo-spheres (Prialnik. Theory of Stellar Structure & Evolution, p.114,148,232).

Note, the cloud core temperatures & densities, of infant PS, are comparable to terrestrial lightning:

(focus fusion)

I understand, that the dissociation of H₂; and, ionization of H,He; absorbs tremendous amounts of energy, perhaps permitting the cloud to collapse, via "soaking up" GPE ?? I get the impression, that ionization is akin to "cracking nuts", i.e. at first the atoms are effectively "hard billiard balls" (hard nuts), until the collisions begin to become energetic enough, that the "nut shells start cracking like crumple zones" (and, then, reforming, "like T-1000 terminators"), absorbing the energy, i.e. electron collision excitation; and, until the collisions finally become energetic enough, that the "nuts crack open completely, and the shells are separated from the kernels", i.e. electron collision ionization.