## Angular Momentum Explained

Most importantly, Laplace introduced conservation of angular momentum to the discussion of planetary formation. He demonstrated mathematically that the solar nebula—the gaseous mass that would become the solar system—would spin faster as it contracted. Anyone who has watched an ice skater spinning knows this is true. As a skater pulls in his arms, bringing his mass closer to his axis of rotation, he will spin faster. If he were to put his arms out at his side, his rotation would slow. Newton described how all objects with mass were mutually attracted. As the cloud of gas that eventually formed the solar system started to collapse, it would have to rotate faster and faster to conserve angular momentum. And, as the speed of rotation increased, the shape of the solar nebula would change, becoming the pancake-like disk Kant had first pictured. Think of that the next time you watch the local pizza maker throw dough in the air, making it spin, flatten, and strech all at once.

Laplace theorized that as the spinning disk contracted, it would form concentric rings, each of which would clump together into a "protoplanet"

### Star Words

Angular momentum can be thought of as the rotating version of linear momentum. Any object that rotates possesses this property. If an object gets smaller, it must rotate faster to conserve angular momentum.

### Star Words

Angular momentum can be thought of as the rotating version of linear momentum. Any object that rotates possesses this property. If an object gets smaller, it must rotate faster to conserve angular momentum.

(a sort of embryonic planet), which ultimately developed into a mature planet. The center of the disk (in this picture) would coalesce into a hot, gaseous "protosun," which ultimately became the sun (we'll talk about that in Chapter 18, "Stellar Careers").

## Telescopes Mastery

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