[2] 2H fusion allows further accretion of mass by acting as a thermostat that temporarily stops the central temperature from rising above about one million degrees, a temperature not high enough for hydrogen fusion, but allowing time for the accumulation of more mass.
Hydrogen fusion will begin at 107 K. The rate of energy generation is proportional to the product of deuterium concentration, density and temperature.
The generation of nuclear energy in these low-density outer regions causes the protostar to swell, delaying the gravitational contraction of the object and postponing its arrival on the main sequence.
[9][10] In this scenario a low-mass star or brown dwarf that is fully convective will become pulsationally unstable due to the nuclear reaction being sensitive to temperature.
[10] This pulsation is hard to observe because the onset of deuterium burning is thought to begin at <0.5 Myrs for >0.1 M☉ stars.
Brown dwarfs with masses between 20 and 80 MJ should be easier targets because the onset of deuterium burning does occur at an older age of 1 to 10 Myrs.
[10][11] Observations of very low-mass stars failed to detect variability that could be connected to deuterium-burning instability, despite these predictions.