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:

- What is a white dwarf?
- Composition, density, and size
- Close binary systems



As you recall, a white dwarf is the exposed, inert carbon core that is left after the low-mass star expels its outer layers in a planetary nebula.

A white dwarf is very dense -- it usually has a mass similar to that of our Sun but a size no larger than our Earth. Since it has so much mass in a small space, gravity is very strong near its surface. What keeps it from contracting further under this powerful force of gravity is called electron degeneracy pressure.

Basically, the idea of electron degeneracy pressure is that the subatomic particles in atoms can only be compressed into so small of a space before they resist. More specifically, the laws of quantum mechanics limit how closely electrons can be packed together. Once they reach this threshold, they exert an outward pressure called degeneracy pressure. In white dwarfs, this degeneracy pressure is enough to counter the inward crush of gravity and maintain the white dwarf's size even though its core is dead and it is not generating any energy through nuclear fusion.



The white dwarf left behind by a low-mass star is mostly made of carbon, since that is what is left over when low-mass stars fuse helium in the latter stages of their lives.

A typical white dwarf contains the mass of our Sun but is the size of Earth. More massive dwarfs are compressed into smaller sizes because greater mass means greater gravity which compresses matter further. For example, a white dwarf with 1.3 times the mass of our Sun would be half the size of our Earth. As they get more compressed, electrons get more excited, moving around faster.

Theoretical calculations indicate that the limit for a white dwarf is about 1.4 times the mass of our Sun since that is when the compressed electrons would reach the speed of light. Since nothing can travel faster than the speed of light, if a white dwarf's mass exceeds this limit, it will explode as a white dwarf supernova.



If a white dwarf has a main-sequence or giant star close enough to it, it can pull away some of that companion star's material, leading to a rapidly rotating disk of gas around the white dwarf called an accretion disk.

This hydrogen gas from the companion star accumulates on the surface of the white dwarf, pressure and temperatures rise from as this layer of hydrogen contracts, and eventually, it reaches 10 million Kelvin where nuclear fusion can occur on the surface, causing a nova. A nova is not nearly as bright as a supernova, but can still shine as brightly as 100,000 Suns.

If accretion causes a white dwarf to exceed 1.4 times the mass of the Sun or if two white dwarfs orbiting each other merge together and exceed that mass limit, the result is a white dwarf supernova.