← Back to course
Earth and Space 6-8 / Week 06 / Tuesday
2/6
Week 06 · A Disk of Dust and Gas

Tuesday

Why a spinning cloud flattens
// How a spinning cloud became a solar system
⏱ about 20 min

Tuesday: Why a Spinning Cloud Flattens

Rocket stands in the middle of the yard with his arms out and turns slowly. "I am the cloud," he says.

"Now pull your arms in," Raven says. He does, and he spins noticeably faster. "What do you notice?"

"Whoa. I sped up without trying." Rocket stops, wobbling. "The cloud did that too?"

Nova hovers at a safe distance. "As the cloud shrank, it spun faster," she says. "Think about what a fast spin does to a shape."

Raven holds up her Sky Log and spins it flat on one finger. "It spreads out. Like a disk."

Rocket grins. "And everything in the disk goes around the same way. That is my answer from yesterday!" Nova glows. "Check it against the book."

Shrinking makes it spin faster

OpenStax Astronomy calls the original cloud the solar nebula. As it collapsed under its own gravity, material fell toward the center and grew hotter.

At the same time, the collapsing nebula began to rotate faster. OpenStax compares it to a figure skater pulling her arms in to spin faster.

Near the poles of the spinning cloud, material fell straight into the center.

Faster-moving material collapsed into a flat disk revolving around the central object. The cloud became a disk with a bright, hot middle.

Why the planets all go the same way

This disk explains the main motions in the solar system. The planets all revolve around the Sun in the same direction.

They orbit in approximately the same plane. OpenStax compares them to cars on concentric tracks on a giant, flat racecourse.

Most planets also rotate in the same direction they revolve. Venus is an exception. It spins backward, very slowly.

Since the planets formed from a rotating disk, they all orbit the same way. The disk is the reason.

ObservationWhat the disk model says
All planets orbit in the same directionThey formed inside one spinning disk
All planets orbit in nearly the same planeThe disk was flat
The Sun holds almost all the massMost material fell to the hot center
Rock near the Sun, ice and gas far outThe disk was hot inside and cooler farther out
THE SPINNING DISK
  • Read the question.
  • Tap your answer.
What happened to the cloud's spin as it shrank?
Why do the planets all orbit the Sun in the same direction?
Which planet rotates backward, very slowly?
FROM CLOUD TO DISK
  • ?A cloud of gas and dust begins to collapse under its own gravity
  • ?Faster-moving material flattens into a disk around the center
  • ?The shrinking cloud spins faster
  • ?The planets form inside the disk and all orbit the same way
  • ?Material falls toward the center and heats up
WHY THIS EXERCISEEach step causes the next. That chain is the model the standard asks you to use.
The planets all orbit the Sun in the same ____.
OpenStax compares the planets to cars on a flat ____.
Try it
In a clear space, turn slowly with your arms out. Pull them in. Notice the speed change. Stop before you get dizzy.
Write what happened. Then write what the cloud did that matched it.
Safety first
Spin slowly, in an open space away from furniture, and stop as soon as you feel dizzy.
Do this on a soft floor or grass, never near stairs or a table edge.
Try this
Hold a coin or a flat lid and give it a spin on a table. Watch how it stays flat while it spins fast.
Write how that is like the disk and how it is different.
Draw the Sun as a dot. Draw four orbit circles around it, all with arrows going the same way. Add one small backward arrow on Venus for its spin.

Strong reasoning. Tomorrow is Sky Deck Lab: you stir up a disk of your own and watch where the grains go.

← Monday