The Impossible Black Holes

Welcome to Three Alpha! In this edition of the newsletter we’re focusing on the why black holes can’t be too small, and why we’re detecting small ones anyway. Read on for more…

You may know that stellar-mass black holes form from stars in supernova explosions, through a process called core collapse. But did you know in recent years astronomers have found candidate stellar-mass black holes which are unlikely to have formed that way? These are black hole candidates whose masses may fall in the mass gap, a range of masses thought to be too low to result from a supernova explosion. So where did these black holes come from?

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Black holes range in mass from the ultra-tiny (and hypothetical) primordial black holes, to stellar-mass black holes, to supermassive black holes. But that doesn’t mean black holes can form with any mass you like. Far from it. Actually, black holes form via a handful of physical processes that usually result in black holes in particular mass ranges.

Stellar-mass black holes are so called because they have roughly the same mass as stars, and they also form from stars. They are one type of stellar remnant, a class of object which also includes white dwarfs and neutron stars. All stars, more or less, will leave behind one of these three types of object when they die, and which one is created depends on the mass of the star. Very roughly, it works like this: eight solar masses or less, and it will create a white dwarf, eight to 20 solar masses, and it will form a neutron star, and more than 20 solar masses, and it will form a black hole.

Not all the mass of the star ends up being part of the remnant. A lot of it is expelled into space, being either thrown off to form a planetary nebula after the star becomes a red giant, or ejected as part of a supernova explosion. That means white dwarfs form between about 0.5 and 1.4 solar masses, while neutron stars can be between 1.2 and 2–3 solar masses. Surprisingly, there appears to be a lack of black holes just above the limit for neutron stars. Observations suggest black holes probably form above about 5 solar masses, possibly leaving a gap between neutron stars and black holes. Why might such a gap exist? (There is possibly also an upper limit for the initial mass of stellar-mass black holes. I’ll come back to that in a future newsletter.)

One reason the lower gap may exist has a lot to do with how the supernova explosion is actually triggered, what is often referred to as the supernova engine. In stars more massive than eight solar masses, when the star’s fuel runs out, this will trigger a process called core collapse. That’s precisely what it sounds like. With no nuclear fusion to hold up the crushing weight of the star, the core of the star begins to contract.

Rapidly, within a couple of hundred milliseconds, the matter in the core becomes dense enough to form a neutron star. Neutron stars are so-named because they are largely made of neutrons. Before the collapse, the core of the star is made of the nuclei of iron and other elements (which contain protons), plus free electrons. Many of the neutrons that make up the neutron star are produced by the protons and electrons combining under the immense pressure, in a process called electron capture.

As well as creating neutrons, electron capture also produces neutrinos. Those are the tiny, almost massless particles which are also produced by the Sun, and usually pass through other matter without interacting with it. The neutrinos produced during core collapse are extremely energetic and there are a lot of them. They carry away most of the gravitational energy of the collapsing star, with most of them heading out into space unimpeded. A relatively small fraction will interact with the dense, collapsing stellar matter and deposit a significant amount of energy into the star. Depending on the mass of the star and the structure of the layers around the core, that energy can heat the surrounding matter and trigger the supernova explosion.

A supernova explosion triggered right after the neutron star forms will leave the neutron star behind, unscathed. But what happens if the neutrinos’ energy is not enough to trigger the explosion that quickly? What if it takes a bit longer to trigger the explosion? In that case, the collapse continues to rain matter onto the newly formed neutron star. Very rapidly, the mass exceeds the maximum mass for a neutron star, and it collapses into a black hole. Still, the star’s collapse continues.

The now-black-hole, previously-neutron-star continues to acquire mass, as more matter from the collapsing star accretes onto it. Sometimes the explosion never happens, resulting in what’s called direct collapse, creating an even larger black hole. If the explosion does happen, that short delay after the neutron star forms can be enough time for the black hole mass to shoot up to over 5 solar masses.

So, in this scenario, the reason for the gap is timing: physically a black hole could have a mass of, say, 3 solar masses. But what could stop the collapse at that point? Either the collapse stops before the black hole forms, or the delay allows the black hole to grow beyond that mass.

So it sounds like it is impossible for black holes to form in a supernova and end up with a mass of less than five solar masses. But have a look at this list of recent black hole candidates:

• 2018: 2MASS J05215658+4359220, 3.3 solar masses

• 2018: NGC 3201, 4.36 solar masses

• 2022: Swift J1728.9–3613, 4.6 solar masses

• 2025: Swift J1727.8−1613, 3.12 solar masses

Every one of these is possibly a black hole in the mass gap (mass estimates always come with some uncertainty, so some of them may be neutron stars and some may be more massive black holes; some of them are more certain than others, and some of these mass estimates are just lower limits). If a core collapse supernova is unlikely to make black holes with such low masses, how did these come to exist?

There are a few other ways black holes can form, which can result in masses inside the gap. In such cases, the black hole forms not directly from a supernova, but from the white dwarfs and neutron stars left behind by less massive stars. Neutron stars and white dwarfs can merge with other objects (with each other, or with stars). They can steal matter from stellar companions. In rare cases, neutron stars can even end up forming over their mass limit but rotating so rapidly that the centrifugal force stops them collapsing, until their rotation eventually slows down. These scenarios can sometimes result in the creation of a black hole which is less massive than the lower limit for supernovae.

It’s also possible that some unknown processes could interrupt a supernova early. Finding more examples of mass gap black holes could allow us to work out what processes could be involved there.

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].