Science & Technology

How Big (or Small) Can a Black Hole Get?

The biggest astronomy story of 2019 arguably was the first-ever image of a black hole, captured by a world-spanning observatory made up of dozens of telescopes. One big reason this achievement was so astounding is because black holes are relatively tiny compared to their mass: this black hole is 6.5 billion times the mass of our sun, but in overall size, it's comparable to the size of the solar system. So what sets the size of a black hole, and how big — or small — can they get? And what does the size of a black hole even mean?

Beyond the Blue Event Horizon

Black holes are objects of pure gravity: they don't have chemical composition or any of the defining characteristics of stars, planets, and other more ordinary inhabitants of the universe. That means they don't have a surface, atmosphere, or any of the usual things that indicate size.

Instead, a black hole's size is defined by its event horizon, which is the boundary past which nothing can escape the gravitational pull, not even light. So even though no material is actually at the horizon, it's what matters. We literally can't study what's inside it.

Unlike stars, which change size a lot over their lifetimes, a black hole's size is entirely determined by two factors: its mass and how fast it spins. (Technically it could also carry an electric charge, but realistic black holes probably don't have enough charge buildup to make a measurable difference.) If it's not spinning, the diameter of a black hole is approximately 6 kilometers (3.7 miles) for each solar mass — the mass of one sun — it packs in. In other words, a one-solar-mass black hole would be 6 kilometers across, while a 10 solar mass black hole is 60 kilometers (37.3 miles) across. To be very clear, that's very tiny compared to its mass: the sun is 1.4 million kilometers (865,000 miles) in diameter, while a black hole of equivalent mass has a diameter less than many foot races.

Realistic black holes spin, though, based on astronomical observations. This rotation shrinks the event horizon diameter by as much as half, making realistic black holes even tinier in comparison to their masses. The rapidly-spinning 6.5 billion solar mass black hole in M87 is the size of a solar system, but 6.5 billion stars are enough for a small galaxy.

Absolute Units

Even M87's black hole isn't as massive as they get. The record-holder is a 40 billion solar mass giant in the galaxy Holm 15A, and it's possible even bigger black holes lurk elsewhere. That's because the only upper limit on a black hole's size is a practical one.

In fact, how supermassive black holes get that big is still mysterious. They seem to have formed to be that massive, based on observations of galaxies from early times. However, they can also get bigger by eating matter — though like Cookie Monster, they're messy eaters — and by merging with other black holes. We haven't seen that happen for supermassive black holes yet, so between that and the messiness of black hole eating habits, it's unlikely black holes can get too much bulkier than Holm 15A over the 13.8 billion year history of the universe so far.

How Low Can You Go?

Supermassive black holes are outnumbered by their "stellar mass" cousins, which are no bigger than a few dozen solar masses. These are formed from the supernova explosions of very massive stars, which sets a lower limit on stellar-mass black holes: they can't be any smaller than three solar masses (give or take) because smaller stars leave behind neutron stars or white dwarfs rather than black holes.

Since stars only grow so large before they're too unstable, scientists predict the maximum is about 20 solar masses. However, the gravitational wave observatories LIGO and Virgo have identified multiple stellar-mass black holes bigger than that, and astronomers detected what might be a 70-solar-mass black hole in the Milky Way, so we still have some mysteries to solve.

Some theories also predict smaller black holes that formed in the very early universe. These primordial black holes could range in size from smaller than atoms to very large, with the smaller variety being more likely. However, we've never convincingly seen primordial black holes, and various studies have limited how many of them there might be in the universe. Rare or common, truly tiny black holes weighing in at less than a gram would be very difficult to detect, since the way we find their astronomical relatives is by their influence on nearby stars or gas.

As a result, the universe could conceivably contain really low-mass black holes and we'd never know it without a lucky break. However, the monsters like that of M87 are the ones that will continue to give us the best shot at seeing the way black holes twist and bend spacetime, as small (relatively) as they are.

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Written by Matthew R. Francis December 21, 2019

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