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:

- The cause of solar activity and sunspots
- Solar prominences and solar flares
- The sunspot cycle



Solar weather, or solar activity, is caused by strong magnetic fields which are constantly forming and changing due to the convecting plasma in the outer layers of the Sun.

The most striking form of solar activity -- sunspots -- appear dark in photographs because they are cooler than the surrounding photosphere. Sunspots are about 4000 Kelvin while the photosphere has an average temperature of 5,800 Kelvin.

How are they so cool relative to their surroundings? The answer is that sunspots are regions that are isolated from the plasma around them by strong magnetic fields. These tight magnetic fields suppress convection within the sunspot and prevent surrounding plasma from entering the sunspot.

During the few weeks that an individual sunspot lasts, the plasma in that sunspot cools. Eventually, the magnetic field weakens or changes, hotter plasma flows in, and the sunspot disappears.



Solar prominences occur where two sunspots are connected by a loop of magnetic field lines. Gas in the Sun's chromosphere and corona becomes trapped in these loops, sometimes rising more than 100,000 kilometers above the Sun's surface.

Some scientists believe that these magnetic field lines sometimes become twisted and eventually snap, and that is what causes intense storms on the surface of the Sun. When these lines snap, they release energy that heats the nearby plasma to 100 million Kelvin, generating the intense radiation that we see from solar flares.

As we have learned before, the Corona is nearly 1 million Kelvin and the Chromosphere is roughly 10,000 Kelvin. Both are much hotter than the photosphere, which is 5800 Kelvin. This happens because the same magnetic fields that keep sunspots cool also carry energy upwards, depositing heat in the upper layers of the Sun's atmosphere.

Coronal mass ejections, which occur when solar flares eject highly energetic charged particles from the Sun's corona, can reach Earth if they happen to be aimed in our direction. They have strong magnetic fields and can disrupt electricity and radio communications, and damage orbiting satellites.



The sunspot cycle is a notable pattern in sunspot activity. At the time of highest average solar activity, called the solar maximum, there are dozens of Sunspots on the Sun, which results in many prominences, flares, and coronal mass ejections. At solar minimum, sunspots are rare and there is little solar activity.

The sunspot cycle has an average period of 11 years, which means usually about eleven years pass from one solar maximum to the next. Scientists tie the sunspot cycle and other solar activity to the Sun's magnetic field which is constantly changing. Its magnetic field is constantly changing due to convection in its interior and the Sun's rotation.

The Sun's pattern of rotation is different from the planets -- it rotates faster near its equator than at its poles, causing magnetic field lines to twist. It may be that sunspots form as these magnetic field lines get more and more twisted with time, until they eventually snap back and reset.