1. Chapter 20 – The Formation of the Solar System
Nebula – A large, interstellar cloud of gas and dust from which many bodies in space are formed. To hold this cloud together, gravity and pressure must be in balance.
Solar Nebula – The nebula from which our solar system formed.
Planetesimals – Pieces of matter that collide and accrete to form planets.

2. Structure of the Sun Corona – The outer atmosphere.
Chromosphere – Below the corona where the color of the sun originates.
Photosphere – the visible part we can see.
Convection Zone – The region where gases circulate.
Radiative Zone – A dense region towards the core where light and energy are scattered.
Core – the center of the sun where energy is produced.

3. Nuclear fusion – The process where the sun produces energy by fusing two hydrogen nuclei together to form a helium nucleus + energy.
Solar Activity – The photosphere is ever-changing. The convection zone circulates gases and the photosphere gases boil and churn along with the sun’s rotation creates magnetic fields that reach out into space.
Sunspots – Cooler, dark regions of the photosphere that change on a regular cycle of about 11 years. Sunspots have very strong magnetic fields that can affect the Earth’s climate & temperature.
Solar Flares – Giant eruptions on the sun that send electrically charged solar material into space disrupting communications and creating the Auroras.

4. The Formation of the Earth
The Earth started out as planetesimals collided and built up heat. As more and more material collected, the temperature continued to rise eventually melting the rocky material that composed the early Earth. Stratification of the Earth’s layers occurred when the denser materials sank to the core and the lighter materials rose to form the crust. The material of intermediate density settled in between these layers forming the mantle. The Earth’s original atmosphere was very different than today's being composed primarily of CO2 and H2O. Volcanoes and comets also contributed other gases along with early life forms adding oxygen through photosynthesis. The oceans formed as the Earth cooled enough for water vapor to condense and heavy rains to fall for millions of years. The continents thickened and rose above the oceans.

5. Planetary Motions Rotation – The spinning of the Earth on it’s axis.
Revolution – A complete trip of the Earth around the sun along it’s orbit.
Orbit – The path a body takes around another body.
Kepler – A famous mathematician who calculated many discoveries on planetary motion. He calculated the elliptical orbits of planets (Fig. 2, p. 631) around a star. He also explained why a planet closer to the sun must orbit faster.
Newton – Developed the Law of Universal Gravitation which explains:
More mass = more gravity
Closer objects = more gravity
Planets closer to the sun have to orbit faster.
Gravity can change the path of a straight moving object to curve into an orbit.