13 Mj is the current mass at which people think deuterium fusion can start. Above 80 Mj, hydrogen fusion will start. These numbers depend on a lot of details o

13 Mj is the current mass at which people think deuterium fusion can start. Above 80 Mj, hydrogen fusion will start. These numbers depend on a lot of details of stellar atmospheres. I took a graduate-level Stellar Atmospheres class from S. Kumar, who first theorized about this class of not-quite-star.