1. Gravity, Stars and Planets

2. Interstellar cloud where star & solar system formation occurs.

3. Galaxy containing BILLIONS of stars! ONE giant star exploding! Supernova example

4. The interstellar cloud pulls together due to gravity, and forms a spinning disc with a young star forming at its center and planets forming in the disc. Young Star Planet-forming disc

5. Young stars surrounded by planet-forming discs in Orion Nebula (Photos taken using Hubble Space Telescope) Young Star Planet-forming disc

6. Early Solar System: The Sun contracts under gravity and starts burning nuclear fuel; planets grow in size as their gravity sweeps up dust, ice, and gases while they orbit the Sun. Outer Solar System has more gases and ice, which makes planets more massive.

7. The Solar System today: Formed and held together by gravity.

8. Io (large and round) Amalthea (small and boulder-like) Two Moons of Jupiter

9. Round shape guarantees that force of rock pushing up from below exactly balances inward force of gravity.

10. Too Small to Be Round: Asteroid Gaspra and the Moons of Mars 951 Gaspra Deimos Phobos

11. Phases of the Moon: Same side of the Moon always faces Earth.

12. Far Side of the Moon

13. (A) Bulge lines up with Earth due to Earth’s gravity. (B) Moon tries to rotate bulge away, but Earth pulls it back, causing friction between the bulge and the Moon. (C) Over time, friction slows down the Moon’s rotation, until the bulge always faces Earth.

14. Volcanic eruptions on Io

15. Orbits of Neptune, Pluto, and Orcus (a plutino) Orcus’s period = Pluto’s period = 248 yrs. = 50% longer than Neptune’s period

16. Neptune’s gravity helps keep the Kuiper Belt in place, while KBOs may have pulled Neptune further away from the Sun.