Supernova, or Type Ia Supernova, are cosmologists favourite "standard candle" in the universe. Theory tells us exactly how bright they are (in absolute terms), so we can tell exactly how far away they, and the galaxy that contains them are. And yes they are that bright at -19.8 absolute magnitude they can outshine a galaxy for days, the light curve decay, follows the decay of Nickel-56, half life six days, the most common radioactive element in the cloud of elements the supernova shoots out, and of Cobalt-56, half life 77 days.
The theory of Type Ia Supernova is that they consist of a White Drawf that slow gains enough extra matter to get with %1 of a white drawf massive mass 1.38 Suns, at which point the pressure from the electrons in white drawf can't support the Drawf against gravity. The interior of the White drawf collapses to a neutron star, while the exterior burns carbon and oxygen into iron and nickel which are blown into space.
News today is of a paper by Richard Scalzo, look a supernova from 2007. SN2007if, weighted the supernova by calculating amount of Nickel and Cobalt produced by fraction of its mass. The heavier that star, they move Nickel produced. And he achieves a unbelievable massive result, 2.4+/-0.2 Solar masses. A whole Solar mass above the Chandrasekhar limit, 1.38 suns, for a white drawf.
The Physics World article. wonders if they result undermines the whole measurement of dark energy, which is determined part by measurement of the speed and distance of galaxies. Most of the distances of the furthest galaxies are measured by single Type 1a Supernova, the brightest standard candle there is. If the supernova are brighter than we think they are, then they are further away, and the universe is not expanding as fast as previously measured. However one or a few rare super heavy supernova, would not upset the calculation, and would show up as outlines on the graph of speed versus distance.
What then is this super heavy supernova. One theory is that its two colliding white drawf each under the 1.38 Chasdrasekhar limit, but totaling 2.1 suns, together. This would be a very rare process plus it would likely be such a catastrophic event that the fusion process would not be very regular or complete. Another explanation is of white drawf made extra heavy by a particular type of dark matter, Mirror Matter, since mirror matter is a second copy of matter, with the same interaction in mirror form, its only mirror light and electromagnetism. A combination of ordinary and mirror matter in a white drawf, might have masses up to 2.8 Suns in a white drawf, and the neutron star mass is also increase in its equation of state, moving up, but not doubling the Tolman–Oppenheimer–Volkoff limit. Indeed in the Year 2000 Paper in which Robert Foot et al predicted such objects he stated: Also note that this type of fake white dwarf could have a mass exceeding the Chandrasekhar limit of 1.4 solar masses: such an object would be a smoking gun for some sort of invisible clumped matter that happens to have acquired an ordinary matter core. Indeed This superheavy supernova might be the Smoking gun he was looking for. ">Richard Scalzo's Paper see a whole four such smoking guns, SN2003 fg, SN 2006gz, SN 2007if and SN 2009dc.
It does however look like SN 2007 if, as predicted by Scalzo's paper, is the result of two colliding white dwarf, as pointed to by, evidence of an existing envelope of carbon around the stars, ejected by tidal distribution of both prior to the final collapse into a supernova. This envelope is evidence by the slowing of the explosion and also by a shell of increased density in the explosion, where the in falling or blocking envelope of material encounters the out going material.