 To get a star, we simply have to add more mass. Here's Ogil-TR-122B. It's one of the smallest known stars. It's around 96 Jupiter masses, but with only a 16% larger radius. The conditions in the core are so extreme that hydrogen fusion begins. That's what makes it a true star. Hydrogen fusion creates so much energy that it breaks out of the electron pressure quantum state and goes back into the thermodynamic realm. More mass will make it bigger and burn hotter, like our sun. One measurable effect is that early on in a star's hydrogen-burning life, it burns away all of its lithium. This gives us a way to distinguish brown dwarfs from low mass stars. Brown dwarfs have lithium in their atmospheres and stars don't. So we see that distinguishing brown dwarfs from stars is easier than distinguishing them from planets.