Kardashev Level 2+ civilizations could manage their stars
The list of nearest stars contains all known stars and brown dwarfs at a distance of up to five parsecs (16.308 light-years) from the Solar System. In addition to the Solar System, there are another 51 stellar systems currently known lying within this distance. These systems contain a total of 61 hydrogen-fusing stars and 9 brown dwarfs
Giant stars can produce 100 to 1000 times the energy output of the Sun.
Another assumption made by Tom Murphy is that the Kardashev 2 civilization would not start heading out and managing a stable of solar systems before the time when they would start to max out on the energy demands of one star. They have dozens of stars within 17 light years and hundreds to thousands within 100 light years. Some already are producing at higher rates and other stars could be easily (for a type II civilization have production increased.)
What happens when a main sequence star runs out of hydrogen in its core? Stars such as our Sun move off the main sequence and up the red giant branch (RGB), fusing hydrogen into helium in hydrogen shell burning. A very short helium flash sees the start of helium core fusion and the star moves along the horizontal branch (HB). Once shell temperature is sufficient, helium shell burning starts and the star moves up into the asymptotic giant branch (AGB).
A type 2 civilization would only need to go a hundred light years to have many stars under their control and many dozens would be red giants or manipulatable into Red Giants. Within 100,000 light years are all of the roughly one trillion stars of the Milky Way.
As the gas expands it cools, just as a spray can feels colder after use as the gas has been released. This expansion and cooling causes the effective temperature to drop. Convection transports the energy to the outer layers of the star from the shell-burning region. The star’s luminosity eventually increases by a factor of 1000 × or so. During this stage of expansion, the star will move up and to the right on the HR diagram along the Red Giant Branch (RGB). A G (V)-class star may end up as a high-K or low-M luminosity class III giant.
A red giant displays extremes of density. The outer envelope is grossly extended and thus at a density below that of a vacuum on Earth. It is only weakly held by gravitational force to the rest of the star and easily ejected. Mass loss from a giant is typically about 10^-7 solar masses per year, compared with only 10^-17 solar masses per year currently for the Sun.
Post-main sequence evolutionary tracks for 1, 5 and 10 solar mass stars
There is also the possibility that a Type II or Type III civilization might be able to reach the fabled Planck energy with their machines (10^19 billion electron volts). This is energy is a quadrillion times larger than our most powerful atom smasher. This energy, as fantastic as it may seem, is (by definition) within the range of a Type II or III civilization.
The Planck energy only occurs at the center of black holes and the instant of the Big Bang. But with recent advances in quantum gravity and superstring theory, there is renewed interest among physicists about energies so vast that quantum effects rip apart the fabric of space and time. Although it is by no means certain that quantum physics allows for stable wormholes, this raises the remote possibility that a sufficiently advanced civilizations may be able to move via holes in space, like Alice’s Looking Glass.
And if these civilizations can successfully navigate through stable wormholes, then attaining a specific impulse of a million seconds is no longer a problem. They merely take a short-cut through the galaxy. This would greatly cut down the transition between a Type II and Type III civilization.
Second, the ability to tear holes in space and time may come in handy one day. Astronomers, analyzing light from distant supernovas, have concluded recently that the universe may be accelerating, rather than slowing down. If this is true, there may be an anti-gravity force (perhaps Einstein’s cosmological constant) which is counteracting the gravitational attraction of distant galaxies.
Physicist Alan Guth of MIT, one of the originators of the inflationary universe theory, has even computed the energy necessary to create a baby universe in the laboratory (the temperature is 1,000 trillion degrees, which is within the range of these hypothetical civilizations).