The x-rays we detect are not being emitted by the black hole itself, but rather, by the intensely hot gas in the accretion disc plummeting into the black hole. An ordinary star is just a huge ball of hot gas. The combined effect of its own gravity is to pull the whole star toward the center. This causes a tendency for the star to contract or collapse unless it is held up by an opposing force. The star has to maintain a high interior temperature or gravity would make it collapse.
Nuclear fusion produces energy by combining light atomic nuclei together to make heavier nuclei.
If the ball of gas will ever get hot enough to undergo nuclear fusion of hydrogen to helium, it is a star. It may surprise you, but we can determine this accurately by making computer models of how stars should behave. Astronomers do not yet agree on the dividing point between brown dwarfs and gas giant planets. I would say that brown dwarfs are in the range 15 to 80 M J . Below 15 M J , the objects are very similar to planets like Jupiter and Saturn in appearance.