Artificial explosion of the Sun: a new x-risk?

I suppose that would mean it would inevitably burn up in less than billions of years so that makes sense.

Yeah. The sun is just so hot due to its huge mass to surface area ratio - so much volume per square meter of emitting area. The total heat energy inside the sun at any given moment is also equal to several hundred thousand years worth of its fusion output. You could somehow shut down the fusion and never have anyone else in the solar system notice a thing for kiloyears unless they had a neutrino detector, until it starts to slooooooooowly shrink and redden, taking millions of years...

The only way to increase the fusion rate is to increase the temperature or pressure (or both at the same time which is more likely). But the heat contributed to a volume by fusion is so ridiculously tiny over any timescale that anything could inject heat into it over that once the heat being used to increase the fusion rate went away the fusion would never be able to sustain the higher temperature that was causing it. The gas would expand under the increased temperature and pressure, and undo whatever was being done to it.

There are variable stars that pulsate on human timescales, but those don't happen due to pulsations in the fusion rate. Instead they are mostly old red giants with puffed up cool outer atmospheres where some ions can actually hold onto a few electrons. The outer atmosphere expands outwards and cools, the ions grab a few electrons and become more transparent due to fewer charged particles to intercept photons, the atmosphere then loses energy it cant intercept and shrinks, it heats up from adiabatic contraction, re-ionizes, becomes more opaque, heats up more and expands, rinse, repeat. Only the outer layers, a tiny fraction of the mass, are affected. Other stars pulsate via other mechanisms too but thats the most common.

The only times that fusion changes on human timescales are in red giants that have a kernel of degenerate matter at the center, held up only by electron degeneracy (AKA the pauli exclusion principle). That is because in that case, pressure is a constant rather than being a function of temperature and the negative feedback I mentioned cannot operate because as the core heats up it fails to expand. This is what gives you 'helium flashes' in red giants where they suddenly put out a few hundred thousand years worth of output in ten seconds... at the center of a solar mass or more of gas that already contained nearly that much heat energy to start with, such that it takes hundreds of thousands of years for the doubled heat energy content to work its way out to the surface and bleed out into space.

He did author some bullshit, now that you mention it.

I can't wait until 2060 (if I survive that long). We can have the "Newton was WRONG?!" party, and then start planning the April 5th, 2063 "Vulcans Welcome" party.

The absence of 747s spontaneously assembling is evidence for their impossibility. Its just that evidence is completely overwhelmed by all the additional evidence we have indicating that it is possible - evidence which appears lacking from this particular case.

Or nuclear bombs. Why could we apply the same argument here? "We never see any nuclear explosions on earth, either during human history or in the form of radioactive craters, therefore it is very implausible that if we combine specially crafted and refined substances we will get something like that."

If Sun would be hit by a comet with 100 km diameter [...] the energy of the impact would be 1000 times more than Sun's output in a second, which means that for a few seconds the Sun will became hundreds times brighter and this could cause fires everywhere. And also it could provoke strong magnetic event and some nuclear reactions in the moment of the impact, the main risk from which is not energy boost but radioactive contamination of space and Earth.

That all sounds very very wrong. The impact introduces kinetic energy, which presumably manifests as some sort of temporary turbulence at the impact site in the outer layer of the sun. It's not going to turn into radiative energy. Similarly, there is no reason for the impact to produce nuclear reactions - the spread-out substance of the vaporized comet would just become a minor contaminant of the mostly-hydrogen sun, spectroscopically detectable for some years. Maybe the impact would make a small coronal hole, but giant ones happen naturally anyway.

I recall reading (but cannot find at the moment) a different study looking at sunlike stars with known exoplanets that did not involve the kepler data. It found that there was a preponderance of super-flare prone stars amongst sunlike stars with hot-jupiter-style large close in exoplanets, adding fuel to the idea that magnetic interactions cause a subset of them. Might not account for all of course even though the Kepler data would only actually see a tiny fraction of the hot jupiters that exist in its field of view.

Stars as gravitational sinks that allow strong energy release just from objects falling onto them is an interesting concept. I presume that what makes such an event extremely rare is the very small amount of angular momentum you need to actually hit one and the fact that light pressure and sublimation will tend to destroy or deflect most objects long before they come close to anything like an impact?

Barring changes of type, such as a successful orbital elevator or a similar change to a technique where fuel is not part of the payload, pretty likely. The technical costs of space flight have decreased, but the costs of fuel and materials haven't changed that dramatically.

And that understates the energy costs involved when you're talking about the sun, specifically. There's a huge amount of delta-v involved in a straight shot -- the Earth orbits the sun at 30 kilometers per second, compared to the 8-10 km/s that a space ship needs to orbit the Earth, where the cost of delta-v increases exponentially -- and not many good targets for gravitational slingshotting to reduce that.

it depends: Manufacturing and space delivery will become cheaper and easier but surveillance and data gathering will also. I don't know what will prove more important but I feel comfortable not planning for this disaster 50 years from now when we'll definitely have more knowledge about it in 15 years or whatever.