SN 2006gy and pair-instability supernovas
This image might be a bit tame and fuzzy compared to the usual things I write about here, but it is one of the most violent and novel phenomenon we know about. Usually supernovas leave behind a remnant - a neutron star if it is large, or a black hole if it is even larger. These are what we call as Type II or core-collapse supernovas. There’s another type of common supernova where a white dwarf accretes matter from a campanion and disintegrates in a cataclysmic thermonuclear reaction when its mass reaches the Chandresekhar Limit, known as a type Ia supernova. As opposed to core collapse supernova, a pair-instability supernova doesn’t leave behind a remnant - the star is completely disintegrated by the cataclysmic explosion.
Pair instability supernovas can only happen in a very select cases - it needs stars of roughly 130 to 250 M☉ (solar masses), and with very low metallicity, and possibly lower rotational speed. We have only seen a few cases so far possibly because all these massive stars with low metallicity are mostly older Population III stars which formed when the universe was relatively metal-poor and also because these massive stars tend to evolve and die quickly in stellar time scales. The mechanism (simplified) is as follows - in the core of these massive stars where the temperature is more than 3 x 108 K, the photons coming from the core are high-energy, mostly gamma rays. Gamma rays with sufficiently high energy can interact with the nuclei, electrons, or one another to produce electron and positron (anti-electron) pairs which annihilate each other and produce more gamma rays, albeit with weaker energy. This leads to lesser and lesser outward radiation pressure and core is compressed (and heated) to produce more and more high energy gamma rays which are more likely to form electron-positron pairs and thus lose energy. This leads to a runaway loss of pressure and contraction of the core and the completely disintegrates in the ensuing supernova.
Pair-instability supernova are novel in many ways, but the most distinct, in my opinion, is its light signature. Their light curves are highly extended as compared to other supernova, and the curve reaches its peak luminosity typically months after the explosion. 1 A large portion of the core is converted into radioactive nickel-56 (half-life: 6.1 days), which subsequently decays into cobalt-56 (half-life: 77 days), which finally decays into the stable iron-56. Apart from the initial explosion, a large part of the energy comes from the radioactive decay, as well as the highly energetic core ejecta colliding with the previously ejected stellar gas during its lifetime, explaining the extended light curve.
Since the entired star is disrupted and it leaves no remnant behind, this can potentially contribute to the upper mass gap in the mass distribution of stellar black holes. We do not find a lot of black holes in the 50 to 150 M☉ range, which is what a star of the sizes resulting in pair-instability supernova would have formed, if it had followed the core-collapse model. Although, we can possibly still find a few black holes in the gap due to non-single stellar situations like a black hole merger, or neutron star merger, or both.