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Black holes, neutron stars, and white dwarfs also can pull matter into disks. The physical processes that make disks are important for understanding how planets form and how various atoms are distributed through space.

From Disks to Planets

The force of gravity tends to make things into spheres, which is why planets and stars are round. However, spinning flattens objects out: gas clouds, stars in spiral galaxies, and other systems form disks under rapid rotation.

Stars form out of clouds of gas, which slowly collapse under the force of gravity. The atoms and molecules in the gas impart a small amount of rotation as they fall toward the protostar. That rotation gives the star its spin and creates a rotating platter of gas and dust called a protoplanetary disk. The composition of that disk, including how much mass it has, determines how many planets and what kind are formed.

We are still puzzling out exactly how planets form from these beginnings, since the other known planetary systems don’t resemble our Solar System. Astronomers study disks to understand how they turn into planets, and how those newborn planets move around in the infant system. They also want to know how the various atoms and molecules distribute themselves within the disk, to understand where important chemicals like water and organic molecules end up. That has implications for life, both in the Solar System and elsewhere.

The young star HL Tau is surrounded by a protoplanetary disk, as seen in this image from ALMA. The rings and gaps in the disk mark the likely presence of newborn planets as they gather up dust and gas.

Credit: ALMA(ESO/NAOJ/NRAO); C. Brogan, B. Saxton (NRAO/AUI/NSF)

Accretion Disks

Compact objects have far more intense gravity than stars. These objects are white dwarfs, neutron stars, or stellar-mass black holes — those formed from the deaths of massive stars. When these are in binary systems with ordinary stars, the gravitational tidal force from the compact object can pull the star out of shape to the point where gas flows from the star to its companion. This matter forms an accretion disk, which is much hotter and spins faster than a protoplanetary disk.   

As with protoplanetary disks, astronomers study accretion disks to understand the physical processes that make them. Accretion is the precursor to powerful cosmic phenomena like type Ia supernovas, when white dwarfs attract enough mass that they explode. Neutron stars and black holes in binary systems with ordinary stars can be very bright X-ray sources, as the gas they strip from their companion heats up to incandescent temperatures.

Accretion disks around supermassive black holes are made from gas pulled in by the black hole. These are the fuel for active galactic nuclei, including the quasars visible from billions of light-years away. Understanding accretion disks is essential for knowing how supermassive black holes feed and scatter atoms around the galaxy.