Accretion is a fundamental physical process in astronomy, describing the gradual accumulation of matter due to gravitational attraction, which leads to the growth of a larger body. This universal phenomenon involves physical mechanisms that govern how diffuse material, such as gas and dust, is drawn in and added to a central object. Accretion operates across immense scales, from microscopic dust grains coalescing in a nebula to supermassive objects gathering material at the center of galaxies, shaping the cosmos and determining the characteristics of observed structures.
Fundamental Physics of Accumulation
The driving force behind accretion is gravity, which causes any particle of mass to exert an attractive pull on surrounding matter. In a cloud of gas and dust, this gravitational pull initiates a process where denser regions collect surrounding material, causing them to grow larger and increase their gravitational influence. This initial clumping results from gravitational instability, where slight over-densities collapse under their own weight.
For matter to successfully accrete, it must shed the angular momentum it possesses from its initial orbit or motion. If the infalling material retains too much angular momentum, it will simply orbit the central object without ever settling onto it. The removal of this rotational energy is often accomplished through friction, or viscosity, within the accreting material.
Viscosity acts as a braking mechanism, transferring angular momentum outward through the disk of material, which allows matter closer to the center to spiral inward and accrete. Molecular viscosity is too weak to account for observed accretion rates; therefore, turbulence and magnetic fields provide the necessary effective viscosity. The Magnetorotational Instability (MRI) is a leading theory for how magnetic fields generate the turbulence required to efficiently transport angular momentum outward in astrophysical disks.
In environments where solid particles are accreting, the speed of collisions determines the outcome of the accumulation process. For growth to occur, collisions must be gentle, or at low velocity, allowing dust grains to stick together through van der Waals forces or electrostatic attraction. High-velocity impacts result in fragmentation, where colliding bodies shatter or erode one another, preventing net growth. This distinction is important in the early stages of planetary formation, where the balance between sticking and shattering dictates the initial growth rate of solid bodies.
Building Stars and Planets
Accretion is the primary mechanism for forming stars, beginning when a dense core within a molecular cloud collapses under its own gravity. As the central region contracts, gas and dust from the surrounding cloud fall onto the forming protostar, increasing its mass and temperature. This infalling material often forms a flattened, rotating protoplanetary disk, which continues to feed the central stellar core.
Within the protoplanetary disk, planet formation relies on accretion, following the core accretion model. The initial stage involves dust grains settling into the midplane and colliding at low speeds, gradually growing from micron-sized particles to centimeter-sized clumps. These clumps aggregate to form kilometer-sized bodies called planetesimals, which are massive enough for their own gravity to influence their surroundings.
As planetesimals continue to collide and merge, they form larger planetary embryos, a process known as oligarchic growth, where the largest bodies in a region grow fastest. This growth is efficient because their increasing mass gravitationally focuses smaller planetesimals toward them, increasing the effective collision cross-section. The final size and composition of a planet depend on the amount and type of material available in its specific orbit.
For gas giants, a third phase of accretion occurs once the solid core reaches a critical mass, typically estimated between 5 and 20 Earth masses. At this point, the core’s gravity becomes powerful enough to rapidly capture vast amounts of surrounding hydrogen and helium gas from the protoplanetary disk. This phase is termed “runaway accretion” because the rate of gas inflow accelerates dramatically, quickly building a massive gaseous envelope until the available gas is depleted.
Accretion Around Black Holes and Neutron Stars
Accretion onto compact objects like black holes and neutron stars takes place in an environment of extreme gravity and high energy. Matter from a companion star or the interstellar medium spirals inward, forming a distinct accretion disk. Due to intense gravitational forces, the material in this disk orbits at immense velocities, close to the speed of light near the central object.
The friction, or viscous stress, between the rapidly orbiting layers of gas in the disk is efficient at converting the material’s kinetic energy into heat. This heating raises the temperature of the inner disk to millions of degrees, causing it to emit copious amounts of high-energy radiation, primarily X-rays and Gamma rays. This release of energy makes accreting compact objects the brightest sources in the universe, often outshining the companion stars that feed them.
The dynamics differ between the two types of compact objects. Gas falling toward a black hole crosses the event horizon without emitting further radiation, contributing directly to the black hole’s mass. Conversely, material accreting onto a neutron star impacts its solid surface, releasing a final burst of energy upon collision. In both cases, the intense conditions near the inner edge of the disk can lead to the formation of powerful, collimated outflows called astrophysical jets.
These jets are narrow beams of plasma ejected at relativistic speeds, perpendicular to the plane of the accretion disk. Strong magnetic fields threading the disk and the central object’s rotation are thought to funnel and accelerate the plasma away from the core. These energetic jets carry away a fraction of the accreting material’s energy, demonstrating that accretion in these environments is often more about the conversion and release of energy than the growth of the central mass.