Astronomy

0% Complete
0 out of 65 Lessons
0% Complete
0 out of 4 Exams
Previous Lessons
Open Chapter Ch. 1: A Modern View of the Universe
Lesson #1 The Scale of the Universe
Incomplete Assignment Study Questions for Lesson 1
Lesson #2 The History of the Universe
Incomplete Assignment Study Questions for Lesson 2
Lesson #3 Spaceship Earth
Incomplete Assignment Study Questions for Lesson 3
Open Chapter Ch. 2: Discovering the Universe for Yourself
Lesson #4 Patterns in the Night Sky
Incomplete Assignment Study Questions for Lesson 4
Lesson #5 The Reason for Seasons
Incomplete Assignment Study Questions for Lesson 5
Lesson #6 The Moon, our Constant Companion
Incomplete Assignment Study Questions for Lesson 6
Lesson #7 Ancient Mystery of the Planets
Incomplete Assignment Study Questions for Lesson 7
Open Chapter Ch. 3: The Science of Astronomy
Lesson #8 The Ancient Roots of Science
Incomplete Assignment Study Questions for Lesson 8
Lesson #9 Ancient Greek Science
Incomplete Assignment Study Questions for Lesson 9
Lesson #10 The Copernican Revolution
Incomplete Assignment Study Questions for Lesson 10
Lesson #11 The Nature of Science
Incomplete Assignment Study Questions for Lesson 11
Open Chapter Ch. 4: Understanding Motion, Energy, and Gravity
Lesson #12 Describing Motion
Incomplete Assignment Study Questions for Lesson 12
Lesson #13 Newton's Laws of Motion
Incomplete Assignment Study Questions for Lesson 13
Lesson #14 Conservation Laws in Astronomy
Incomplete Assignment Study Questions for Lesson 14
Lesson #15 The Force of Gravity
Incomplete Assignment Study Questions for Lesson 15
Open Chapter Ch. 5: Light: The Cosmic Messenger
Lesson #16 Basic Properties of Light and Matter
Incomplete Assignment Study Questions for Lesson 16
Lesson #17 Learning from Light
Incomplete Assignment Study Questions for Lesson 17
Lesson #18 Collecting Light with Telescopes
Incomplete Assignment Study Questions for Lesson 18
Exam Exam 1
Open Chapter Ch. 6: Formation of the Solar System
Lesson #19 A Brief Tour of the Solar System
Incomplete Assignment Study Questions for Lesson 19
Lesson #20 The Nebular Theory of Solar System Formation
Incomplete Assignment Study Questions for Lesson 20
Lesson #21 Explaining the Major Features of the Solar System
Incomplete Assignment Study Questions for Lesson 21
Lesson #22 The Age of the Solar System
Incomplete Assignment Study Questions for Lesson 22
Open Chapter Ch. 7: Earth and the Terrestrial Worlds
Lesson #23 Earth as a Planet
Incomplete Assignment Study Questions for Lesson 23
Lesson #24 The Moon and Mercury: Geologically Dead
Incomplete Assignment Study Questions for Lesson 24
Lesson #25 Mars, a Victim of Planetary Freeze Drying
Incomplete Assignment Study Questions for Lesson 25
Lesson #26 Venus, a Hothouse World
Incomplete Assignment Study Questions for Lesson 26
Lesson #27 Earth as a living planet
Incomplete Assignment Study Questions for Lesson 27
Open Chapter Ch. 8: Jovian Planet Systems
Lesson #28 A Different Kind of Planet
Incomplete Assignment Study Questions for Lesson 28
Lesson #29 A Wealth of Worlds: Satellites of Ice and Rock
Incomplete Assignment Study Questions for Lesson 29
Open Chapter Ch. 9: Asteroids, Comets, and Dwarf Planets
Lesson #30 Classifying Small Bodies
Incomplete Assignment Study Questions for Lesson 30
Lesson #31 Asteroids
Incomplete Assignment Study Questions for Lesson 31
Lesson #32 Comets
Incomplete Assignment Study Questions for Lesson 32
Lesson #33 Pluto and the Kuiper Belt
Incomplete Assignment Study Questions for Lesson 33
Lesson #34 Cosmic Collisions - Small Bodies vs Planets
Incomplete Assignment Study Questions for Lesson 34
Open Chapter Ch. 10: Other Planetary Systems
Lesson #35 Detecting Planets Around Other Stars
Incomplete Assignment Study Questions for Lesson 35
Lesson #36 The Nature of Planets Around Other Stars
Incomplete Assignment Study Questions for Lesson 36
Lesson #37 The Formation of Other Planetary Systems
Incomplete Assignment Study Questions for Lesson 37
Exam Midterm Exam
Open Chapter Ch. 11: Our Star
Lesson #38 The Sun, Our Star
Incomplete Assignment Study Questions for Lesson 38
Lesson #39 Nuclear Fusion in the Sun
Incomplete Assignment Study Questions for Lesson 39
Lesson #40 Sun-Earth Connection
Incomplete Assignment Study Questions for Lesson 40
Open Chapter Ch. 12: Surveying the Stars
Lesson #41 Properties of Stars
Incomplete Assignment Study Questions for Lesson 41
Lesson #42 Patterns in the Stars
Incomplete Assignment Study Questions for Lesson 42
Lesson #43 Star Clusters
Incomplete Assignment Study Questions for Lesson 43
Open Chapter Ch. 13: Star Stuff
Lesson #44 Star Birth
Incomplete Assignment Study Questions for Lesson 44
Lesson #45 Life as a Low Mass Star
Incomplete Assignment Study Questions for Lesson 45
Lesson #46 Life as a High Mass Star
Incomplete Assignment Study Questions for Lesson 46
Open Chapter Ch. 14: The Bizarre Stellar Graveyard
Lesson #47 White Dwarfs
Incomplete Assignment Study Questions for Lesson 47
Lesson #48 Neutron Stars
Incomplete Assignment Study Questions for Lesson 48
Lesson #49 Black Holes: Gravity’s Ultimate Victory
Incomplete Assignment Study Questions for Lesson 49
Exam Exam 3
Open Chapter Ch. 15: Our Galaxy
Lesson #50 The Milky Way Revealed
Incomplete Assignment Study Questions for Lesson 50
Lesson #51 Galactic Recycling
Incomplete Assignment Study Questions for Lesson 51
Lesson #52 The History of the Milky Way
Incomplete Assignment Study Questions for Lesson 52
Open Chapter Ch. 16: A Universe of Galaxies
Lesson #53 Islands of Stars
Incomplete Assignment Study Questions for Lesson 53
Lesson #54 Distances of Galaxies
Incomplete Assignment Study Questions for Lesson 54
Lesson #55 Galaxy Evolution
Incomplete Assignment Study Questions for Lesson 55
Lesson #56 The Role of Supermassive Black Holes
Incomplete Assignment Study Questions for Lesson 56
Open Chapter Ch. 17: The Birth of the Universe
Lesson #57 The Big Bang Theory
Incomplete Assignment Study Questions for Lesson 57
Lesson #58 Evidence for the Big Bang
Incomplete Assignment Study Questions for Lesson 58
Lesson #59 The Big Bang and Inflation
Incomplete Assignment Study Questions for Lesson 59
Open Chapter Ch. 18: Dark Matter, Dark Energy, and the Fate of the Universe
Lesson #60 Unseen Influences in the Cosmos
Incomplete Assignment Study Questions for Lesson 60
Lesson #61 Structure Formation
Incomplete Assignment Study Questions for Lesson 61
Open Chapter Ch. 19: Life in the Universe
Lesson #62 Life on Earth
Incomplete Assignment Study Questions for Lesson 62
Lesson #63 Life in the Solar System
Incomplete Assignment Study Questions for Lesson 63
Lesson #64 The Search for Extraterrestrial Intelligence
Incomplete Assignment Study Questions for Lesson 64
Lesson #65 Interstellar Travel and Implications for Civilizations
Incomplete Assignment Study Questions for Lesson 65
Exam Final Exam

Assignments:

Unfinished Assignment Study Questions for Lesson 44

Lesson Objectives:

- Molecular clouds
- From cloud to protostar
- From protostar to main-sequence star
- The mass of stars



Stars are born within the coldest and densest clouds of interstellar gas. These clouds are often called molecular clouds because they are cold and dense enough for atoms to combine and form molecules. The cloud has to be cold and dense because the inward contraction of gravity, gravitational contraction, has to be strong enough to overcome the internal gas pressure.

When the gas molecules are closer together and there are a lot of them, obviously their gravitational attraction will increase. The colder the gas, the less kinetic energy it will have and the lower its pressure. That is why molecular clouds are typically a frosty 10 to 30 Kelvin.

Molecular clouds are also usually very large since the higher total mass helps gravity overcome gas pressure. These large clouds give birth to many stars at a time forming star clusters.



As parts of a molecular cloud collapse and become denser, the densest regions shrink and contract even more, heating up. Initially, most of the heat generated from this contraction is lost, but eventually, the fragments get so dense that the heat generated from gravitational contraction is trapped inside. This dense center of a cloud fragment is now a protostar -- the clump of gas that will become a new star.

As it continues to shrink, a protostar spins more and more quickly, producing a flat, spinning disk of gas around the protostar in which planets may form. Sometimes gravity can pull two protostars close together and due to their high angular momentum, they go into orbit around each other, forming a binary star system.



A protostar does not become a true star until it has contracted enough to reach 10 million Kelvin, which is when hydrogen fusion begins. The star continues to contract, the core temperature continues to rise, and nuclear fusion continues to increase until the star reaches energy balance, which is where the energy it is generating balances out with the energy being radiated from the surface.

At this point, when it achieves energy balance between hydrogen fusion in its core and radiation from its surface, the protostar becomes a main-sequence star.



Low-mass stars greatly outnumber high-mass stars, both because most of the stars formed in a nebula are on the lower end of the scale and also because higher-mass stars die off faster.

The maximum possible mass of a star appears to be around 150 times that of the Sun, while the minimum is calculated to be about 0.08 (point-zero-eight) times that of the Sun. That is because below this mass, the protostar's gravity will be too weak to contract the core enough to reach 10 million Kelvin and begin efficient hydrogen fusion.

If the protostar falls below this mass threshold, then it will stabilize as a brown dwarf, which is sometimes called a "failed star." A concept of quantum mechanics called degeneracy pressure keeps gravity from contracting the core in brown dwarfs to the point where they can sustain nuclear fusion. Since brown dwarfs are not hot enough to maintain steady fusion, they gradually cool with time as they radiate away their internal thermal energy.