Astronomy
The Physics of Accretion Disks around Young Stars
Quick fact
A typical young star can accrete about 10^{-8} solar masses per year, and the disk's temperature ranges from a few thousand Kelvin near the star to tens of Kelvin at large distances, emitting across infrared to radio wavelengths.
Why this is interesting
You've probably seen stunning images of swirling disks around newborn stars, but what makes them glow and why are they so flat? The answer lies in a balance between gravity and a surprising spin effect.
Read the full explanation
Understanding The Physics of Accretion Disks around Young Stars
Imagine a cloud of gas and dust collapsing under its own gravity. As it collapses, it spins faster (like an ice skater pulling in arms), and the material flattens into a disk because the spin resists collapse in the plane of rotation. This disk is called an accretion disk. The young star sits at the center, and gas slowly spirals inward, eventually falling onto the star. This inward flow transfers mass and energy, and the disk shines brightly as gravitational potential energy is converted into heat. The star grows by eating from the disk.
A deeper explanation
The key physics is conservation of angular momentum. As gas moves inward, it must lose angular momentum to be accreted. Viscous friction and turbulence within the disk transport angular momentum outward, allowing material to spiral inward while some gas moves outward, carrying the angular momentum. As gas loses gravitational potential energy, half is radiated as heat, and the other half is converted into kinetic energy of the higher-speed inner orbits. This heats the disk, making it emit light. The disk's temperature profile (T ∝ r^{-3/4}) and luminosity are directly tied to the accretion rate and the star's mass. In young stellar systems, this accretion process also provides the raw material for planet formation: dust grains collide and grow into planetesimals and eventually planets.