Some of the most powerful explosions in the universe just got a bit more varied. Astronomers have already categorized two broad groups of supernovae: type Ia supernovae presumed to result from the complete disruption of a white dwarf star, and the type II, Ib, and Ic supernovae thought to explode when the core of giant star collapses. Now, they say that a type of exploding star first thought to be an unusual sort of type Ia supernova is actually a different class of supernova altogether. Explosions of these stars, dubbed type Iax, release somewhere between 1% and 50% the energy of a type Ia supernova, and there are hints that in many cases a remnant of the star may survive the initial outburst. Like type Ia supernovae (but unlike the smaller, garden-variety exploding stars called novae), spectra of type Iax stars don't include any signs of hydrogen. Most of the 25 stars share more than 25 different characteristics—a sign that the stars probably not only look alike but are physically similar, the researchers report online and in a forthcoming issue of The Astrophysical Journal. Type Iax supernovae most likely form in binary star systems when a superdense, carbon- and oxygen-rich white dwarf star (center of disk at left) robs material from its helium-rich partner, eventually accumulating enough mass on its surface to trigger an explosion. Astronomers likely have discovered so few type Iax supernovae only because they are faint, not because they are rare: The team estimates that for every 100 type Ia supernovae explosions that occur, there are about 31 type Iax supernovae.
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