I've been thinking about what highly technologically advanced civilisations would do once their energy requirements become comparable to the total output of a star, and how such activity could be detected from Earth.
Of course, one possibility is to build a Dyson sphere of one kind or another, which would be detectable as a star with an unusual spectrum, heavy in the infra-red, with emission lines corresponding to the heavy elements from which the structure is composed.
However, if we're talking about a civilisation that's really cracked fusion power in a big way, it strikes me that another possibility would be to extract hydrogen and/or helium nuclei from the star itself and fuse them directly in reactors. In principle this has a significant advantage over a Dyson sphere in that it creates heavy elements as a waste product; with careful control over the nuclear chemistry, it could be used to create not only energy but also the material needed to build more reactors. Thus, if such technology is possible then an initial investment can pay off exponentially; perhaps the rate of power output from such activity could eventually exceed the rate of power output of the original star.
I have two closely related questions in relation to this idea.
Is there a practical way to extract hydrogen from the surface of a star? This is assuming a civilisation that has access to literally astronomical amounts of energy, but which nevertheless doesn't want to waste any of it unnecessarily. For example, if one were able to produce a sufficiently strong electromagnetic field, is there some configuration that could be used to funnel charged hydrogen ions away from the star's surface?
If this were to be done on a sufficiently large scale, would it be detectable astronomically, and what would the resulting signature be? This signature might come from the act of extracting the material itself, or from its effect on the star, whose distribution of elements would (presumably) eventually be changed by the process.
Note that this is in many ways the opposite question to "What is the easiest way to stop a star?". In that case I'm assuming a forward-thinking civilisation that wants to prevent the star from wasting hydrogen so that it can burn it more slowly in the future; in this case I'm assuming the goal is simply to produce energy and heavy elements as rapidly as possible, perhaps even faster than the star would do by itself.