We May Have Found The Origin Of The Most Massive Black Hole Merger We Can Currently Detect

Artist's conception shows two merging black holes similar to those detected by LIGO and Virgo. LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet)

Gravitational-wave observatories have detected many collisions between black holes over the last five years. These objects are the product of stars going supernova and given that stars cannot be arbitrarily big, the question remains just how massive can these objects we detect be? A new study answers this important question.

As reported in The Astrophysical Journal, simulations suggest that the largest black hole that can be formed by known stars can't be much larger than 50 times the mass of the Sun. And that is only in particular types of supernovae, where the progenitor star has a mass between 80 and 140 times the mass of the Sun.

These are known as Pulsating Pair Instability Supernovae. They are so massive that before they reach the fateful kaboom moment, they experience a series of dramatic collapses and expansions. The reason behind this is how hot these stars get inside. They are hot enough that the photons, the particle of light, can spontaneously turn into a matter-antimatter pair.

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Schematic diagram of the binary black hole formation path for GW170729. Shing-Chi Leung et al./Kavli IPMU

Any doctor would suggest that having antimatter inside you is not good, given its tendency to turn matter into pure energy. This is why the core of these stars, made of carbon and oxygen, becomes unstable and collapses on itself. The compressed oxygen is ignited explosively, the outer shell of the star expands dramatically, and the inner part instead cools and collapses again. This repeats until the oxygen has been exhausted, the star forms an iron core, and finally collapsing into a black hole, the star goes supernova.

Reaching the iron stage is key. For elements lighter than the metal, nuclear fusion releases energy. For iron and heavier elements, instead, it requires energy. Once a star has an iron core, it can’t produce enough radiation to balance out the gravitational pull. Such stellar cores collapse under their own weight.

This final stage also happens in smaller stars going supernova: the core-collapse supernova. These smaller stars, with a mass between 40 and 80 times that of our Sun, will explode and produce a black hole less than 38 solar masses. Larger ones are produced by Pulsating Pair Instability Supernovae.

Black holes larger than these (but not too large, like supermassive black holes) can only be formed by mergers. Stars bigger than 140 solar masses explode so violently that nothing is left behind, not even a black hole.

This work will need more observational backing, which the researchers are excited about. The LIGO and Virgo detectors have spotted a collision between a 50 solar masses black hole and a 34 solar masses black holes (with uncertainties), known as GW170729. This study suggests that the possible products of Pulsating Pair Instability Supernovae are there waiting to be found.

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