Is there a plausible causal mechanism to explain why the Sun would be heating up faster/ sooner than we thought? Question for a novel I am writing a novel about what it will be like on earth 500 or so years before the death of our Sun. The Sun is heating up gradually and one day that will negatively affect life on Earth. I know that we are a long ways away from the heat being to much to permit life though. That being said, the novel is more about the social/political/religious ramifications of living in such a time. I still am very interested in making it as relatable and understandable as possible.

Is there any causal mechanism (hypothetically or imaginatively possible) that could cause the sun to heat up faster than it is at present? This could be internal (something collapses and the increase in core pressure speeds up the reaction rate of hydrogen fusion) or external (something impacts the sun with enough extra mass to increase net g, thus increasing pressure in the core).

If not a causal mechanism, maybe something you can think of that current science/ scientists could have missed/ aren't taking into account in their models that, if true, would mean the sun is going to be much hotter, much sooner than we thought.

Thank you so much for your help! I am pretty passionate about writing and I am only an amateur in the subject of astronomy. I'm hoping some brilliant minds can help me flesh out this idea to make it more concrete.
The Solar Neutrino Problem


  The solar neutrino problem concerned a large discrepancy between the
  flux of solar neutrinos as predicted from the Sun's luminosity and
  measured directly. The discrepancy was first observed in the mid-1960s
  and finally resolved around 2002.
  
  The flux of neutrinos at Earth is several tens of billions per square
  centimetre per second, mostly from the Sun's core. They are
  nevertheless hard to detect, because they interact very weakly with
  matter, traversing the whole Earth as light does thin air. Of the
  three types (flavors) of neutrinos known in the Standard Model of
  particle physics, the Sun produces only electron neutrinos. When
  neutrino detectors became sensitive enough to measure the flow of
  electron neutrinos from the Sun, the number detected was much lower
  than predicted. In various experiments, the number deficit was between
  one half and two thirds.


The problem is practically solved today. It wasn't so until $\approx$ 2002. However, a little chance that we know something not enough well, still exists, but it is little.

The currently accepted solution of the problem is this: yes, we know the Sun enough well, but the neutrinos can change flavor on the way they reach us. Thus, for example, if an electron-neutrino becomes muon-neutrino, then the detectors capable to see only electron-neutrinos, won't detect it.

It is today a hot research topic in the particle physics, thus some surprising result in the future isn't yet closed out. The main problem is that the neutrino practically don't interact with anything (a light year of lead would stop half of them), so there are only very little possibilities to measure their properties.

The KATRIN neutrino experiment


  KATRIN is a German acronym (Karlsruhe Tritium Neutrino Experiment) for
  an undertaking to measure the mass of the electron antineutrino with
  sub-eV precision by examining the spectrum of electrons emitted from
  the beta decay of tritium. The core of the apparatus is a 200-ton
  spectrometer. In 2015, the commissioning measurements on this
  spectrometer were completed, successfully verifying its basic vacuum,
  transmission and background properties. The experiment began
  running tests in October 2016. Regular measurements started
  2018-06-11, with a projected duration of 5 years.


As far I know, too much surprising results aren't expected from this - most likely, the electron neutrino mass of some tenths of eV will be found, or possible the result will be that the experiment is not enough precise to find anything. (In this case, we will still get an "upper limit").

However, there is a little chance that some surprising result will happen.

For example, such a neutrino mass could be found, which can be explained only by some hyphotetical effect (like sterile neutrinos, which might also contribute to the solution of the dark matter problem).

The idea

Go back to the 80ties, where a possible solution to the solar neutrino problem was a different working and internal structure of the Sun, which looks the same for us, but generates lesser neutrinos. By analogy, a similarly different Sun should be expected, which goes into a red giant phase soon. (Or, at least its luminosity will increase enough in the near future to make the Earth inhabitable).

Note, we also know a lot of other stars today, there are very good models running on big computers, and these are enough sophisticated to close this possibility out with a good probability. Thus, the Sun should be somehow exceptional on these results.