Question

A white dwarf has a greater mass than either a red dwarf or a brown dwarf. Yet a white dwarf has a smaller radius than either a red dwarf or a brown dwarf. Explain why, in terms of the types of pressure that keep the different kinds of dwarfs from collapsing under their own gravity.

Answer 1

Explanation:

Red dwarf and brown dwarf masses are less than a typical white dwarf mass measuring around 1.2 solar masses. But it's only a few kilometers of the radius. This is precisely because there is no force to overcome the contraction due to gravity. There is a constant  battle between the external force of fusion (who wants to expand the star) and inward pressure because of gravity (who wants to compact the star) of regular stars on the main sequence. There remains a balance between these two forces as long as the star remains on the celestial equator.

Red dwarfs are helped by the nuclear fusion force, but brown dwarfs were not large enough to cause the fusion of hydrogen, they are massive enough to generate sufficient energy in the core by fusing deuterium to sustain their volume.  However as soon as the star runs out of hydrogen to burn it weakens the force of the external fusion and gravity starts to compact the center of the star. The contraction heats up the core into more massive stars and helium fusion begins, rendering the star once again stable. However this helium fusion does not occur in stars with masses below 1.44Mo. Tightness persists for such stars until the star's gasses degenerate.