Ways to make a star go supernova? Asking for a fictional storyline. 
Say a type iii civilisation on kardashev scale want to make a star go supernova. Then what are the possible ways to achieve this?
The said star being massive enough to go supernova on its own in a few million years. 
Would collapsing a Dyson sphere made of iron into the star do it?
What would be the relation between weight of iron and stars mass that would kill it?
 A: One obvious way of making stars go supernova is to drop a black hole inside it. Unfortunately, the accretion rate is surprisingly slow for small black holes. Assuming Bondi accretion, $M'\approx \pi\rho G^2M^2/c_s^3$ where $c_s$ is the local speed of sound. If we assume $c_s\approx 0.1c$ and $\rho\approx 10^{12}$ kg per cubic meter I get $M'(1M_\odot)\approx 1M_\odot$ per second (instant boom), but for a Jupiter mass hole $M'(1M_J)\approx 10^{-6}M_\odot$ per second (about 11 days) and for an Earth mass hole hole about 3,400 years. Same thing for dropping a neutron star or lump of strangelets inside.
This may take a while, especially since there is also an upper limit on the accretion rate due to the Eddington luminosity where the radiation from accretion produces a light pressure that counters the gravity. This limit is around $\dot{M}_{Edd}=4\pi GM/\epsilon c \kappa$ where $0<\epsilon<1$ is the efficiency of converting rest mass into energy ($\approx 0.1$ is standard), and $\kappa$ the opacity ($\approx 3\cdot 10^{-15}$ m$^2$/kg). For a Jupiter mass black hole this is about $8\cdot 10^{-6} M_\odot$ per second so the flow does not choke, but for a solar mass hole this is $9\cdot 10^{-3} M_\odot$ per second - the inflow chokes (this is avoided in "proper" gravitational collapses).
While iron seeding sounds good, you need to add astrophysically significant amounts of iron. I suspect this would be on the order of solar masses, also troublesome.
My best bet would be if one could set up radial oscillations to induce a proper gravitational collapse since the star is by assumption close to the end anyway - maybe a bit of a push is enough. Helioseismology is not my forte, but I can imagine some stars could be pushed over (this at least may happen in white dwarf Type Ia supernovas). You would "just" need to drive the oscillation somehow, perhaps by coupling other modes using your K3 tech. 
A: Since it is a fiction. Maybe write a plot where the star is small and was not going to go supernova but still its close towards the end of its life but people are prepared for that timeline, then there is a Dyson sphere that you just collided which increased the mass so much that it is now gonna go supernova really fast and they don't have enough time to do anything about it. To be a red supergiant, the star has to be like 10 times bigger than our sun.
Because it is fiction... let the creativity flow!
