In directions parallel to the rotation axis, the matter contracts toward a plane until the thermal pressure inside the disk roughly equals the gravitational force.
The outward force of the light escaping the core of a star, working with thermal pressure, acts to balance the inward gravitational forces on the outer layers of the star.
If the thermal pressure, which resists compression, is small compared with gravity and rotation, which cause the disk to contract vertically, the disk will be geometrically thin, and its thickness will be much smaller than its radial extent.
Perhaps most important, the equilibrium conditions of stellar structure are determined largely by the opposing forces of gravitational attraction on the one hand and radiation pressure and thermal pressure on the other.
Good arguments exist that the central regions should collapse first, producing a condensed protostar whose contraction is halted by the large buildup of thermal pressure when radiation can no longer escape from the interior to keep the (now opaque) body relatively cool.
Top panels: rest-mass density (left) and thermal pressure (right).
The thermal pressure p then follows from the particular EOS in use (Section 2.4).
Panel (a) displays the thermal pressure of ions, the magnetic pressure, and their sum.
In order to excite the MRI inside the torus, the thermal pressure is perturbed by 4% white noise.
The ionosphere hinders the solar wind when the thermal pressure of the ionosphere exceeds the solar wind dynamic pressure.
The solar wind dynamic pressure and the thermal pressure of the ionospheric plasma balance each other at the ionopause surface.
