The physics of accretion: how the universe pulled itself together
Each star grows on its own schedule. The protostar stage is like the unstable adolescence of a star. When its accretion disk stabilizes and matter stops falling into the core, it becomes a main-sequence star. There may still be a disk of debris and the planets around could still determine where they orbit, but accretion has largely stopped. That doesn’t mean there won’t be more accretion in the star’s future, though. Depending on its mass, when the fusion stops, it will then transform into a white dwarf, a neutron star or a black hole, all of which can form their own accretion disks.
The supply of this new disc can come from various sources. Compact objects, like white dwarfs and black holes, can siphon gas from a companion star. A white dwarf can also shoot material that it blew during the previous red giant phase. And when black holes grow and merge to become the supermassive black holes (SMBH) at the center of galaxies, they draw material from the vast, errant stars, clouds and nebulae within the galaxy itself.
When the disc material falls into the central object – whether it is a star, planet or singularity – it releases energy in the form of radiation. The disc itself also radiates as it swirls around the gravity well and heats up, with different factors such as viscosity, friction and speed making some parts hotter than others. The stronger the attraction of the central object, the more powerful the radiation emitted, because the gas can turn into plasma. The groundbreaking 2019 image of the supermassive black hole at the center of the M87 galaxy 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 whatever it accumulates over time. But while we understand how Sun-sized black holes form, we don’t know how SMBHs got as big as them. 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 matter to grow so large at the current age of the universe.
“It’s one of the biggest mysteries in black hole research,” says Joanna Piotrowska, a graduate student at the University of Cambridge. The laws of physics limit the rate at which an object can accumulate matter, called the Eddington limit. Above this limit, the radiation from the accretion disk is so intense that it evacuates matter, preventing further accretion from occurring. “The mass of [SMBHs] exceeds what is expected of continued accretion at the Eddington limit over the lifetime of our universe,” says Piotrowska.
One proposed solution is that SMBHs were large to begin with. Perhaps in the early universe, even before the first stars, there were molecular clouds with just the right conditions to collapse right away into singularities. The James Webb Space Telescope may be able to shed some light on this dark subject when it comes online this year. It was designed specifically to see the first galaxies and stars, and these primordial formations could help us understand the initial distribution of collapsing matter.