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:

- Halo and disk stars
- How did our galaxy form?
- The black hole at the center of our galaxy



The stars that are in the halo of the galaxy are known as the halo population and have some key differences from the disk population of stars.

As we learned previously, stars in the halo orbit the center of the galaxy with random orientations, while stars in the disk all orbit in the same direction in roughly the same plane.

Halo stars are all old, which means they are all long-lived, low-mass stars. Their spectra show that they contain very little of the heavy elements, as little as 0.02% (point-zero-two percent), indicating they formed early in the galaxy's history.

The cold, dense molecular clouds required for star formation are all in the disk. That is why the disk stars have new stars as well as old stars, and contain a higher proportion of heavy elements.



There are various models of how our galaxy might have formed, but in the simplest model, it began with a protogalactic cloud of hydrogen and helium gas. The earliest stars that formed in this blob of gas were the halo stars. Since they were in random localized regions within the cloud, they would not have had any organized rotation, resulting in the randomly oriented orbits of halo stars. Over time, as this protogalactic cloud continued to collapse, conservation of angular momentum caused the gas to flatten into a spinning disk, and the stars that formed in this disk were the disk stars that shared the organized motion of the disk.

The one problem with this simple model is that if it were true, the oldest halo stars should be the farthest away, and as the cloud compacted, the stars closer to the disk and bulge should be newer and have a higher proportion of heavy elements. Since this has not matched up with observations, another model expands on this simplistic model to propose that several small protogalactic clouds may have merged to form the large protogalactic cloud that became the Milky Way galaxy.

In both models, the interstellar gas settled into the Milky Way's disk, which is why stars no longer form in the halo.



The interstellar medium obscures visible light, keeping us from seeing the center of the galaxy in the visible light spectrum, but radio, infrared, and X-ray telescopes have allowed astronomers to see through those clouds. A variety of evidence points to the existence of a black hole about 4 million times as massive as the Sun at the center of our galaxy.

This evidence includes a unique source of radio emissions coming from the exact center called Sagittarius A-star. Radio emissions also trace out massive magnetic field lines near this center. Orbits of hundreds of stars within a light year of the center point have orbits that indicate a super massive object packed into a relatively small space. A black hole is the only known object that fits the description.