Nebular Contraction and Star Formation

What is the name of the theory that proposes planets form from a spinning disk?

What is the main force that causes the nebula to contract?

What is the shape of the nebula after it has shrunk to about 100 A.U.?

What is the result of the increase in rotation speed as the nebula contracts?

Who showed mathematically that conservation of angular momentum demands that the nebula must spin faster as it contracts?

What is the name of the star that is about 50 light-years from the Sun and has a visible-light image of a disk surrounding it?

What is the result of the conservation of angular momentum on the nebula's shape?

What is the name of the cloud of interstellar dust and gas that forms the solar system?

What is the approximate diameter of the disk of warm matter observed around Beta Pictoris?

What is the estimated age of the star Beta Pictoris?

What was the main issue with Laplace's description of the solar nebula?

What is the main prediction of computer calculations regarding the formation of clumps in a disk of gas?

What is the name of the currently favored model of planetary formation?

What is the main advantage of the condensation theory over the old nebular theory?

Nebular Contraction and Star Formation

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.

Nebular Theory by Pierre Simon de Laplace

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.

Solar Nebula and Planetary Formation

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

Observational Evidence

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.

Limitations and Evolution of the Theory

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.

Learn about the process of star formation and how it leads to the creation of planets. Understand the contraction of nebulae and its effects on density and temperature.