Although Type Ia supernovae (SNe Ia) have been extensively studied, their progenitor systems and explosion mechanisms remain subjects of debate. The conventional model suggests that a white dwarf explodes in a thermonuclear runaway as it accretes mass from its companion and approaches the Chandrasekhar mass limit. However, decades ago, an alternative theory proposed that a white dwarf could explode by accumulating enough helium onto a sub-Chandrasekhar mass, triggering a full explosion in the core—a process known as double detonation. Initially, this scenario was not widely accepted because the predicted line blanketing in the blue regime did not match observations of SNe Ia. Here, I present two exceptionally rare supernovae that exhibit characteristics consistent with double detonation theories. According to the theory, the size of the helium shell around a sub-Chandrasekhar mass white dwarf significantly influences its observational characteristics. Models with thicker shells predict an early suppression in flux due to radioactive material in the remnants of the helium shell, leading to a pronounced reddish color as these remnants obscure spectral lines in the UV and blue parts of the spectrum. In contrast, thinner shell models replicate several conventional features typically associated with Type Ia supernovae around maximum light, although their early-time observables can differ significantly. My findings, which represent less than 1% of SNe Ia, have nearly doubled the sample size of these rare events in the literature, marking substantial progress in the field. Thin helium shell detonations might be the primary triggering mechanism for the majority of SNe Ia, making it even more critical to obtain early data for these supernovae. The use of artificial intelligence (AI) and upcoming surveys will be instrumental in quickly classifying and facilitating fast follow-up observations of these events. Future research is essential to determine whether this is an exotic triggering mechanism for around 1% of SNe Ia or if these events are simply the most evident examples of a mechanism that could be common to most, if not all, SNe Ia.

We present the first high-redshift Hubble diagram for Type II-P supernovae (SNe II-P) based upon five events at redshift up toz~;0.3. This diagram was constructed using photometry from the Canada-France-Hawaii Telescope Supernova Legacy Survey and absorption line spectroscopy from the Keck observatory. The method used to measure distances to these supernovae is based on recent work by Hamuy& Pinto (2002) and exploits a correlation between the absolute brightness of SNe II-P and the expansion velocities derived from the minimum of the Fe II 516.9 nm P-Cygni feature observed during the plateau phases. We present three refinements to this method which significantly improve the practicality of measuring the distances ofSNe II-P at cosmologically interesting redshifts. These are an extinction correction measurement based on the V-I colors at day 50, across-correlation measurement for the expansion velocity and theability to extrapolate such velocities accurately over almost the entire plateau phase. We apply this revised method to our dataset of high-redshift SNe II-P and find that the resulting Hubble diagram hasa scatter of only 0.26 magnitudes, thus demonstrating the feasibility of measuring the expansion history, with present facilities, using amethod independent of that based upon supernovae of Type Ia.