Astronomy

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Previous Lessons
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?



As we learned previously, rocky planetesimals are called asteroids, and ice-rich planetesimals are comets.

Just as Ceres is a large asteroid, Pluto and other large objects of the Kuiper belt are essentially large comets.

The vast majority of comets are at the outer reaches of our solar system, far from the Sun, and on the surface, they look similar to asteroids. Instead of being made out of rock, however, they are basically chunks of ice mixed with rocky dust and some other chemicals. That is why comets can be compared to "dirty snowballs."



Far out in space, a comet is basically just an ice ball. It is only if a comet comes in closer to the Sun that it develops the tail that we normally associate with comets. When the comet starts accelerating towards the inner solar system, closer to the Sun, then this ice ball becomes the 'nucleus' of the comet.

As its surface temperature increases, ices begin to vaporize into gas and dust is also released, forming the dusty atmosphere around the nucleus called a 'coma'.

There are two tails on a comet. The plasma tail is made up of ionized gas that is pushed outward by the solar wind. That is why the plasma tail extends directly away from the Sun. The dust tail consists of dust-size particles that the comet leaves behind and curve back in the direction the comet came from.

Larger particles, from the size of a grain of sand to a pebble, may also be ejected from comets. When Earth crosses a comet's orbit, these particles enter our atmosphere causing meteor showers.



There are two reservoirs of comets in the distant outer solar system. They are the Kuiper belt and the Oort cloud.

We cannot see these reservoirs of comets since they are so far away, but their existence has been determined by tracing the orbits of observed comets that have reached our inner solar system and figuring out which direction they came from.

The comets in the Kuiper belt are the leftover planetesimals that originated on the outer edges of where the planets formed, beyond the orbit of Neptune. Since they formed so far away, they were most unaffected by the gravity of the jovian planets and were able to maintain their original ecliptic plane and orbital direction as the planets.

The comets of the Oort cloud, however, may have actually formed much closer -- in between the orbits of Jupiter, Saturn, Uranus, and Neptune. Due to collisions or close gravitational encounters with the jovian planets, they were flung off at high speed to end up at the outer limits of the solar system.