Nebular Contraction and Star Formation

Study Notes

A large cloud of interstellar dust and gas, called a nebula, contracts under its own gravity due to external influences such as a collision with another interstellar cloud or a nearby star explosion.
As the nebula contracts, it becomes denser and hotter, eventually forming a star at its center.

In 1796, Pierre Simon de Laplace showed that conservation of angular momentum demands that the nebula must spin faster as it contracts.
The increase in rotation speed causes the nebula's shape to change as it shrinks, with centrifugal forces opposing the contraction in directions perpendicular to the rotation axis.
As a result, the nebula collapses most rapidly along the rotation axis, flattening into a pancake-shaped disk.

The swirling mass destined to become our solar system is referred to as the solar nebula.
The planets form out of this spinning disk, explaining the origin of the architecture observed in our planetary system today, such as:
- Near-circularity of the planets' orbits
- Prograde orbits in almost the same plane

Similar disks have been observed (or inferred) around other stars, such as Beta Pictoris.
The disk around Beta Pictoris is roughly 1000 A.U. across, about 10 times the diameter of Pluto's orbit.
Astronomers believe that Beta Pictoris is a very young star, perhaps only 100 million years old, and that we are witnessing its passage through an evolutionary stage similar to that experienced by our own Sun 4.6 billion years ago.

Laplace's description of the collapse and flattening of the solar nebula was essentially correct, but a disk of gas would not form clumps of matter that would subsequently evolve into planets.
Computer calculations predict that any clumps in the gas would tend to disperse, not contract further.
The currently favored model, the condensation theory, rests on the old nebular theory, combining its basic physical reasoning with new information about interstellar chemistry to avoid most of the original theory's problems.

Podcast