The physics of accretion: How the universe pulled itself together


Gathering dust

Every star grows up on its own schedule. The protostar stage is like a star’s volatile teen years. When its accretion disk stabilizes and material stops falling into the core, it becomes a main sequence star. There may still be a debris disk and the planets around might still be figuring out where they orbit, but accretion has largely stopped. That doesn’t mean there won’t be any more accretion in the star’s future, though. Depending on its mass, when fusion ceases, it will then transition into either a white dwarf, a neutron star, or a black hole, all of which can form accretion disks of their own.

The supply for this new disk can come from a variety of sources. Compact objects, like white dwarfs and black holes, may siphon gas from a companion star. A white dwarf may also pull in material that it puffed off in the earlier red giant phase. And when black holes grow and merge to become the supermassive black holes (SMBHs) at the centers of galaxies, they draw material from the vast roaming stars, clouds, and nebulae within the galaxy itself.

As material from the disk falls into the central object — whether a star, planet, or singularity — it releases energy in the form of radiation. The disk itself also radiates as it swirls around the gravity well and heats up, with different factors like viscosity, friction, and speed making some parts hotter than others. The stronger the draw of the central object, the more powerful the radiation emitted, as gas can be transformed into plasma. The groundbreaking 2019 image of the supermassive black hole at the center of the galaxy M87 is not of the hole itself, but of the black hole’s shadow on the charged plasma swirling around it.

A black hole gains mass from everything it accretes over time. But while we understand how Sun-sized black holes form, we don’t know how SMBHs got as big as they are. For example, the SMBH at the center of the Whirlpool Galaxy (M51) in Canes Venatici has a mass equivalent to 1 million Suns. There is no way for a single small, stellar-mass black hole to accrete enough material to grow this large at the universe’s current age.

“It’s one of the biggest mysteries of black hole research,” says Joanna Piotrowska, a graduate student at Cambridge University. The laws of physics limit how quickly an object can accrete matter, called the Eddington limit. Above that limit, the radiation from the accretion disk is so intense, it blows material away — preventing more accretion from happening. “The mass of [SMBHs] exceeds what is expected from continuous accretion at the Eddington limit over the lifetime of our universe,” says Piotrowska.

One proposed solution is that SMBHs were big to start with. Perhaps in the early universe, even before the first stars, there were molecular clouds with just the right conditions to collapse straight away into singularities. The James Webb Space Telescope might be able to shed some light on this dark topic when it comes online this year. It was designed especially to see the first galaxies and stars, and those primordial formations could help us to understand the initial distribution of potential collapsible matter.

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