Astronomy

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Open Chapter Ch. 1: A Modern View of the Universe
Lesson #1 The Scale of the Universe
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Lesson #2 The History of the Universe
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Lesson #3 Spaceship Earth
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Open Chapter Ch. 2: Discovering the Universe for Yourself
Lesson #4 Patterns in the Night Sky
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Lesson #5 The Reason for Seasons
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Lesson #6 The Moon, our Constant Companion
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Lesson #7 Ancient Mystery of the Planets
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Open Chapter Ch. 3: The Science of Astronomy
Lesson #8 The Ancient Roots of Science
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Lesson #9 Ancient Greek Science
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Lesson #10 The Copernican Revolution
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Lesson #11 The Nature of Science
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Open Chapter Ch. 4: Understanding Motion, Energy, and Gravity
Lesson #12 Describing Motion
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Lesson #13 Newton's Laws of Motion
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Lesson #14 Conservation Laws in Astronomy
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Lesson #15 The Force of Gravity
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Open Chapter Ch. 5: Light: The Cosmic Messenger
Lesson #16 Basic Properties of Light and Matter
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Lesson #17 Learning from Light
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Lesson #18 Collecting Light with Telescopes
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Exam Exam 1
Open Chapter Ch. 6: Formation of the Solar System
Lesson #19 A Brief Tour of the Solar System
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Lesson #20 The Nebular Theory of Solar System Formation
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Lesson #21 Explaining the Major Features of the Solar System
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Lesson #22 The Age of the Solar System
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Open Chapter Ch. 7: Earth and the Terrestrial Worlds
Lesson #23 Earth as a Planet
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Lesson #24 The Moon and Mercury: Geologically Dead
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Lesson #25 Mars, a Victim of Planetary Freeze Drying
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Lesson #26 Venus, a Hothouse World
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Lesson #27 Earth as a living planet
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Open Chapter Ch. 8: Jovian Planet Systems
Lesson #28 A Different Kind of Planet
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Lesson #29 A Wealth of Worlds: Satellites of Ice and Rock
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Open Chapter Ch. 9: Asteroids, Comets, and Dwarf Planets
Lesson #30 Classifying Small Bodies
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Lesson #31 Asteroids
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Lesson #32 Comets
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Lesson #33 Pluto and the Kuiper Belt
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Lesson #34 Cosmic Collisions - Small Bodies vs Planets
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Open Chapter Ch. 10: Other Planetary Systems
Lesson #35 Detecting Planets Around Other Stars
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Lesson #36 The Nature of Planets Around Other Stars
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Lesson #37 The Formation of Other Planetary Systems
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Exam Midterm Exam
Open Chapter Ch. 11: Our Star
Lesson #38 The Sun, Our Star
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Lesson #39 Nuclear Fusion in the Sun
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Lesson #40 Sun-Earth Connection
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Open Chapter Ch. 12: Surveying the Stars
Lesson #41 Properties of Stars
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Lesson #42 Patterns in the Stars
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Lesson #43 Star Clusters
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Open Chapter Ch. 13: Star Stuff
Lesson #44 Star Birth
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Lesson #45 Life as a Low Mass Star
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Lesson #46 Life as a High Mass Star
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Open Chapter Ch. 14: The Bizarre Stellar Graveyard
Lesson #47 White Dwarfs
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Lesson #48 Neutron Stars
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Lesson #49 Black Holes: Gravity’s Ultimate Victory
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Exam Exam 3
Open Chapter Ch. 15: Our Galaxy
Lesson #50 The Milky Way Revealed
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Lesson #51 Galactic Recycling
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Lesson #52 The History of the Milky Way
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Open Chapter Ch. 16: A Universe of Galaxies
Lesson #53 Islands of Stars
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Lesson #54 Distances of Galaxies
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Lesson #55 Galaxy Evolution
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Lesson #56 The Role of Supermassive Black Holes
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Open Chapter Ch. 17: The Birth of the Universe
Lesson #57 The Big Bang Theory
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Lesson #58 Evidence for the Big Bang
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Lesson #59 The Big Bang and Inflation
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Open Chapter Ch. 18: Dark Matter, Dark Energy, and the Fate of the Universe
Lesson #60 Unseen Influences in the Cosmos
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Lesson #61 Structure Formation
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Open Chapter Ch. 19: Life in the Universe
Lesson #62 Life on Earth
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Lesson #63 Life in the Solar System
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Lesson #64 The Search for Extraterrestrial Intelligence
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Lesson #65 Interstellar Travel and Implications for Civilizations
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Exam Final Exam

Assignments:

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Lesson Objectives:

- An overview of comets
- Why do comets have tails?
- Where do comets come from?



The nebular theory states that the planets in our solar system formed as a natural part of the same process that formed our Sun. If the theory is correct, then planets should just as easily form around other stars.

The discovery of many extrasolar planets means one of the most basic predictions of nebular theory has come true.

There are other key elements of nebular theory that appear to be correct. For example, based on nebular theory, jovian planets form when a seed of rock and ice condenses and reaches a size that allows it to capture nebular gas. Stars richer in the elements that make up jovian planets appear to have more large planets, supporting this aspect of the theory.



There are two major challenges to nebular theory based on what we have observed in extrasolar planets.

The first challenge involves explaining planetary orbits. Why are there "Hot Jupiters" -- massive, Jupiter-size planets orbiting so close to their star? Based on the nebular theory, jovian planets should form in the cold, outer regions of star systems where it is cold enough for ice to condense.

Also, why do many planets have such eccentric orbits?

One possibility is that the planets formed with circular orbits far from their stars, but then underwent some sort of "planetary migration." One recently discovered planet appears to be on a slow death spiral into its star, and some stars have been found with an unusual mix of elements in their outer layers, maybe from swallowing planets that spiraled in too close.

Planetary migration could also explain eccentric orbits in that planets migrating inward could affect the orbits of other planets. This would be similar to the elliptical orbits for Jupiter's moons Io, Europa, and Ganymede, caused by their gravitational effects on each other.



The second challenge to the nebular theory is the fact that extrasolar planets do not always fall neatly into the terrestrial and jovian categories identified in our solar system.

For example, there is a huge range in the densities of extrasolar jovian planets. One planet is twice the size of Jupiter but has the density of Styrofoam while another planet is a little larger than Jupiter but several times as massive, being as dense as iron.

Scientists speculate that the first case is caused by the planet's proximity to its star; the extreme temperatures cause the planet's atmosphere to puff up to a large size and low density. In the case of the dense planet, they believe that once a planet reaches a certain size, its gravity grows so strong that it begins to shrink and become more compact and dense.

As for the possible water worlds detected in other star systems, that may simply be a case of where an ice-rich planetesimal that was accreting to form a jovian planet did not get a chance to capture nebular gases -- the nebular gases were cleared away before the planet could start absorbing them.



In conclusion, it appears the nebular theory is not incorrect; it is just incomplete. It needs to be updated to account for situations that cause planetary migration as well as variations in the basic planetary types.

The validation of the nebulary theory does lead us to question whether there are other star systems more similar to ours, and with the possibility of life. Current evidence indicates that planetary systems are common, but there is not yet enough data to know whether similar star systems with Earth-like planets might exist.