= Solution
In a massive star undergoing <core-collapse supernova> formation, an iron-rich core cannot gain further support from exothermic fusion. <Electron captures> and nuclear <photodisintegration> promote collapse. The inner core stiffens near nuclear <density> and bounces, launching a <shock> into the infalling outer core. Dissociation and <neutrino> losses drain its <energy>, so the prompt <shock> generally stalls while matter continues to accrete. <Neutrino> absorption behind the <shock>, aided by multidimensional flow, can revive it and eject the envelope; sufficiently rapid rotation and strong magnetic fields offer additional possibilities. The remnant may be a <neutron star> or <black hole>. Major uncertainties include progenitor structure, nuclear <equation of state>, <neutrino> transport and interactions, turbulent flow, rotation and magnetic fields.
The <standing accretion shock instability> is a nonradial instability of this stalled <shock> and accretion flow. Feedback between advected disturbances and acoustic response can amplify large-scale sloshing or spiral motions, producing an asymmetric <shock> even without an initially unstable convective gradient.<Stability of Standing Accretion Shocks, With an Eye Toward Core Collapse Supernovae by John M. Blondin and others>[Abstract and nonradial simulations: growth of turbulence from a distorted standing shock]{ref}
An asymmetric explosion can impart an opposite recoil to the remnant by <momentum conservation>. In neutrino-driven calculations, long-lasting gravitational attraction by asymmetric, slower inner ejecta can dominate the acceleration: the gravitational tug-boat mechanism. <SASI> and convection can seed the asymmetries, but the kick is not necessarily an instantaneous hydrodynamic push or uniquely attributable to <SASI>.<Neutron star kicks by the gravitational tug-boat mechanism in asymmetric supernova explosions by Hans-Thomas Janka>[Abstract and mechanism discussion: anisotropic ejecta, gravitational acceleration and subdominant hydrodynamic forces]{ref}
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