The Pair Instability Mass Gap

Welcome to Three Alpha! The newsletter is slightly late this month, because of delays caused by the recent heatwave in Europe. Thanks, climate change!

In this edition of the newsletter we’re focusing on the upper limit to stellar-mass black holes formed in supernovae. Read on for more…

Last month we talked about how core-collapse supernovae can create black holes, but usually not below a certain mass (about five solar masses), leaving a so-called mass gap between the heaviest neutron stars and the lightest black holes. This gap, you may remember, is caused by the timing of the supernova explosion. When the explosion is delayed, that results in a black hole rather than a neutron star being formed, but also gives the black hole a chance to grow to over five solar masses.

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That’s not the whole story. There also seems to be an upper mass gap, limiting how big stellar-mass black holes can be when they form in supernovae. Based on observations and theory, the gap seems to be, very roughly, from about 50 to 130 solar masses. (Black holes can form above 130 solar masses, and we’ll talk about that next month!)

The upper mass gap has a nice sort of symmetry with the lower gap. There are a few different models, but the gist is that the lower gap is caused by the timing of the explosion after the neutron star forms, and the upper gap is caused by an explosion happening before it can form at all. If the supernova happens before the neutron star or black hole can form, then the star will be completely destroyed. No remnant will be left behind.

But, you may also remember from last month, that the supernova explosion in core-collapse supernovae is caused by heating from the neutrinos created when the neutron star forms. So how can the explosion happen before the neutron star forms? It turns out, there is another mechanism which can trigger a supernova without needing neutrinos at all. This type of supernova is called a pair-instability supernova, and they only happen in very massive stars (130 to 250 solar masses).

This all involves a quantum process called pair production. If a photon has enough energy (it requires a lot of energy for this to happen, so it needs to be a gamma-ray photon), it can create a new pair of particles: a normal particle and its antiparticle. In very large stars, before the fuel runs out and before core-collapse can even start, the conditions can be just right for pair production to occur. In such stars, the photons created in the core have enough energy to create an electron-positron pair.

With the right conditions, this can happen rapidly in the core of the star, in a feedback loop that rapidly escalates into an explosion. First, a few gamma-ray photons create particle pairs, reducing outward pressure in the core. That makes it collapse further, heating it up and producing more photons. That creates even more particle pairs, further reducing pressure, and speeding up the collapse. This cycle repeats and accelerates. The positrons do annihilate with electrons, releasing some energy, but it’s not enough to stop the collapse. Eventually, this triggers something called thermonuclear runaway, igniting oxygen and other elements in one final burst of nuclear fusion.

This last, massive burst of fusion is powerful enough to completely break the gravitational binding holding the star together. The result is a truly huge supernova explosion. The entire star, including the core, is disrupted.

Nothing is left behind.

The explosion is called a pair-instability supernova, and while none has been definitively detected, a few candidates have been seen. Such explosions could explain why stellar-mass black holes don’t seem to form above about 50 solar masses. We’re probably getting closer to definitively detecting the first confirmed pair-instability supernova, and then we’ll have a much better idea of the physics of what happens in these colossal explosions.

What is Three Alpha? Other than being the name of the newsletter you’re reading now, the name “three alpha” comes from the triple-alpha process, a nuclear chain reaction in stars which turns helium into carbon. Read more here.

Who writes this? My name is Dr. Adam McMaster. I’m an astronomer in the UK, where I mainly work on finding black holes. You can find me on BlueSky, @adammc.space.

Let me know what you think! You can send comments and feedback by hitting reply or by emailing [email protected].