Which massive stars explode and which collapse quietly to black holes?
In plain words
Some massive stars may vanish into black holes without an explosion; which ones do is unknown.
Precise statement
Map the outcome (explosion leaving a neutron star, fallback black hole, or failed supernova with direct black hole formation) against pre-collapse core structure (compactness parameter xi_2.5, iron-core mass, Ertl two-parameter criterion) for initial masses $8 \text{ to } 120\,M_{\mathrm{sun}}$. Answer: an outcome map from 3D simulations validated against the absence of observed red supergiant progenitors above $\sim 17-18\,M_{\mathrm{sun}}$ and against disappearing-star searches.
What would settle it
Agreement between simulated outcome maps and a statistical sample of failed-supernova candidates and measured progenitor masses.
Status in the literature
Unverified note
2026: the disappearance of the massive hydrogen-depleted star M31-2014-DS1 in the Andromeda galaxy was published as a failed supernova forming a black hole.