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 neutron star?
- Pulsars
- Close binary systems



What is a neutron star? A neutron star is a ball of neutrons created by the collapse of the iron core in a massive star supernova. It is just a few kilometers in radius but has more mass than the Sun. Basically, a neutron star is made almost entirely of neutrons held together by overwhelmingly powerful gravity, making it like a giant atomic nucleus.

We learned how white dwarfs resist the crush of gravity with electron degeneracy pressure, which occurs when electrons are closely packed. In a neutron star, closely packed neutrons result in neutron degeneracy pressure that resists the crush of gravity.

While a neutron star could easily fit in your town, its gravity would destroy the planet!



Evidence of neutron stars was first observed in 1967 when a graduate student named Jocelyn Bell discovered a strange source of radio waves, pulsing on and off at precise intervals. It was not until a year later, in 1968, that astronomers discovered that these "pulsars" originated from the centers of two supernova remnants, and thus were originating from neutron stars.

Neutron stars spin rapidly due to the law of conservation of angular momentum - as the iron core of a star collapses into a neutron star, its rotation rate increases until it is rotating many times per second. The magnetic field gets much stronger as the core compacts, and these magnetic fields direct beams of radiation along the magnetic poles, which we detect as pulses of radiation if the beams sweep by Earth.



Just like a white dwarf, a neutron star with a nearby companion star can pull material from the other star with its strong force of gravity. However, since the gravity is so much stronger in a neutron star, the accretion disk that forms will be much hotter and denser than the one that forms around a white dwarf.

The super high temperatures means the accretion disk emits high levels of energy as X-rays. It is due to this intense X-ray emission that close binaries that contain accreting neutron stars are called X-ray binaries.

The emissions from X-ray binaries pulse as the neutron star spins. While pulsars from normal neutron stars slow down with time, pulsars from X-ray binaries tend to accelerate, possibly because matter from the accretion disk adds angular momentum to the neutron star as it accretes on to its surface.

While accreting white dwarfs occasionally erupt as novae (pronounced NO-vee), accreting neutron stars sporadically erupt due to helium fusion on the surface, generating a burst of energy known as an X-ray burst.

If two neutron stars closely orbit each other, they can spiral and merge releasing more energy than even a massive star supernova. These mergers may be the source of rare elements such as gold and platinum.