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Type Ia supernovae (SNe Ia), thermonuclear explosions of carbon-oxygen whitedwarfs (CO-WDs), are currently the best cosmological "standard candles", butthe triggering mechanism of the explosion is unknown. It was recently shownthat the rate of head-on collisions of typical field CO-WDs in triple systemsmay be comparable to the SNe Ia rate. Here we provide evidence supporting ascenario in which the majority of SNe Ia are the result of such head-oncollisions of CO-WDs. In this case, the nuclear detonation is due to a wellunderstood shock ignition, devoid of commonly introduced free parameters suchas the deflagration velocity or transition to detonation criteria. By usingtwo-dimensional hydrodynamical simulations with a fully resolved ignitionprocess, we show that zero-impact-parameter collisions of typical CO-WDs withmasses $0.5-1\,M_{\odot}$ result in explosions that synthesize $^{56}$Ni massesin the range of $\sim0.1-1\,M_{\odot}$, spanning the wide distribution ofyields observed for the majority of SNe Ia. All collision models yield the samelate-time ($>60$ days since explosion) bolometric light curve when normalizedby $^{56}$Ni masses (to better than $30\%$), in agreement with observations.The calculated widths of the $^{56}$Ni-mass-weighted-line-of-sight velocitydistributions are correlated with the calculated $^{56}$Ni yield, agreeing withthe observed correlation. The strong correlation, shown here for the firsttime, between $^{56}$Ni yield and total mass of the colliding CO-WDs(insensitive to their mass ratio), is suggestive as the source for thecontinuous distribution of observed SN Ia features, possibly including thePhilips relation.

Head-on collisions of white dwarfs in triple systems could explain type Ia supernova