Are stars truly powered by internal nuclear fusion, or is the energy coming externally from the galaxy in the form of electromagnetic energy and plasma?

It is much more the latter than the former. The energy is coming from elsewhere. This is true of everything in the universe. Everything observable through the commonly understood and detectable forms of energy and seeming matter are perceptible because they are in a certain energetic state that will be noticed visually or indirectly though scientific instrumentation to detect their presence. All of that substance, no matter what form is coming from a deeper level of the universe, and that is true of all the energy exhibited by solar bodies and other heavenly bodies with varying natures that are observable in the heavens, so to speak. This is a scientific blind spot as is true of many, many, many other fields of scientific inquiry. There is a reason for this and that is that the interlopers constrain the growth of human knowledge and awareness and depth across the board. Everything science uncovers that has a practical utility for humanity can serve them to improve their lot. The interlopers want to see the opposite happen. They want to see humanity struggling, stumbling, even as they believe they are moving forward. It is a process of fits and starts, and through misdirection through co-opting information, through manipulating scientists directly to disfavor certain precepts in their thinking. They can influence the opinion leaders in a field of inquiry and hold back progress for decades to centuries. This has been going on all through history. The divine realm can counter this, up to a point, and that is the reason you do have scientific inquiry at all. It is the reason you have modern conveniences and ways of thinking to create a large-scale civilization, a large-scale society made up of component parts that can work together after a fashion. These edifices or institutions are heavily monitored, heavily manipulated and heavily corrupted. There is no getting around this. It is the way things are. When you look to science to solve your problems, you are likely to get a constrained and a filtered answer of some kind. It will likely be partial, it will likely do relatively little of benefit, other than to satisfy curiosity and give a new generation of would-be scientists something to pursue further, but it will not unearth the deepest truths, and will not solve the largest of problems, and oftentimes will bring more difficulty in the doing, as a side effect and unintended consequence, particularly with the imposition of technological processes. There is always a poison pill added in. So, we digress in this way to make the point that there has been relatively little progress in understanding solar mechanics and the energetics because it is larger than your physics in the first instance, in understanding the orchestration and the energetics, but beyond that there is a built-in constraint in pursuing things in a way that will be as effective as needed to fully answer this question in scientific terms. So, the best we can do is give you the divine clue that science has not arrived at meaningful interpretations of what they see with solar output and its origins. There are many, many things that can be learned and many practical potential advantages in unearthing the truth here. This is both why it has been constrained and why it is unlikely to happen, still, in the world as it is. It is up to you to change that.