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Lesson Transcript
Instructor: Artem Cheprasov
This lesson will explore two hypotheses as to how the Milky Way galaxy formed. These are the top-down and bottom-up hypotheses. We'll also go into what special connection you personally have to all of this.
How Our Galaxy Formed
When products are made, they're either made whole with the option of being taken apart later, or they're made in pieces and assembled later. For example, bread is baked as one piece of bread dough. But afterwards, you can slice it apart into different sizes and shapes. On the flip side, you can buy something like a bookshelf that comes in pieces, which you then need to assemble at home. These two examples are reminiscent of the general understanding of how our Milky Way galaxy may have originated.
Monolithic Collapse
The Milky Way galaxy has three basic components: the disk, which contains the spiral arms; the halo; and the nucleus, or central bulge. Older stars, a.k.a. Population II stars, are poor in metal and are found in the halo and the central bulge. Younger, metal-rich stars, a.k.a. Population I stars, are found in the disk.
These facts helped astronomers develop an older theory called monolithic collapse, or the top-down hypothesis, which basically says that large galaxies, like the Milky Way galaxy, formed by way of gravitational collapse from a single, turbulent, large gas cloud.
The monolithic collapse hypothesis states that one big gas cloud originated over 13 billion years ago, which is what we estimate the age of the Milky Way galaxy to be. But the large gas cloud was not strong enough to resist the force of gravity pulling gas inward. As the gas was pulled inwards, the cloud fragmented into smaller clouds. You can equate it to a big cloud in the sky that splits apart into smaller ones due to the wind ripping it apart. Except in space, it was gravity, not wind, that ripped the big cloud apart.
Some of the smaller gas clouds wound up colliding with one another and combining, sort of like you would plop pieces of bread dough together to make a big pizza dough ball. This bigger, low-density gas cloud rotated, and like a rotating piece of pizza dough, could not resist being flattened out into a disk due to its low density. This helped form the disk of our galaxy.
As the cloud flattened out, the abundance of metal created by a process called nucleosynthesis increased, explaining why newer stars in the disk were metal-rich in comparison to the older stars. And when the cloud flattened out into a disk, it left behind the older, metal-poor stars, globular clusters, and the halo, in general, in its wake.
The Bottom-Up Hypothesis
The top-down hypothesis makes it pretty clear that because the halo formed first, it should contain stars of roughly the same age. The ones farthest away should be the oldest, and they should be metal free. But recent evidence and observations have thrown some kinks in the armor of the top-down hypothesis. Some younger star clusters are apparently in the outer halo, and even the oldest stars are metal-poor but not necessarily completely free of metal.
So what's going on then? A new hypothesis has been developed to account for these new discoveries. It's called the bottom-up hypothesis. In essence, it states that large galaxies, like the Milky Way galaxy, formed from the combinations of small galaxies and star clusters. This would be like assembling a bookshelf from smaller parts into one large piece of furniture.
What astronomers believe happened goes something like this. First, there was a large gas cloud that basically had no metal at all. The very first stars that formed from this cloud were truly massive. Massive stars, like massive cars, burn through their energy-producing fuel very quickly. This means they die very quickly since they run out of fuel so fast.
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You are viewing lesson 2 in chapter 23 of the course:
Basics of Astronomy
28 chapters | 325 lessons
Ch 1. Celestial Navigation &...
Ch 2. Characteristics of Our Solar...
Ch 3. Earth's Spheres and...
Ch 4. Galaxies: Properties &...
Ch 5. How Scientists Think and...
Ch 6. Influences on Climate
Ch 7. Life in the Universe
Ch 8. Light in Astronomy
Ch 9. Matter in Astronomy
Ch 10. Measurement of Star...
Ch 11. Momentum, Energy, Pressure,...
Ch 12. Newton's Laws in...
Ch 13. Relativity in Time and...
Ch 14. Rotational Motion in...
Ch 15. Small Celestial Bodies &...
Ch 16. Star Death and Stellar...
Ch 17. Star Types and...
Ch 18. Telescopes
Ch 19. The Atmosphere on Earth and Other...
Ch 20. The Birth and Life of...
Ch 21. The Earth, Sky, and...
Ch 22. The History of...
Ch 23. The Milky Way Galaxy
Galaxy Formation: Spiral, Elliptical & Irregular Galaxies
5:43
The Formation of the Milky Way Galaxy
7:08
7:51
Next Lesson
How Our Galaxy Was Discovered
How Our Galaxy Was Discovered
The Structure of Our Galaxy
6:52
The Nucleus of a Galaxy
5:04
The Spiral Arms of the Milky Way Galaxy
5:34
Density Wave Theory & Spiral Galaxies
5:33
Population I and Population II Stars
5:42
The Mass of Our Galaxy
7:50
Go to
The Milky Way Galaxy
Ch 24. The Moon: Formation &...
Formation of the Moon: Theories
5:14
Lunar Geology: Types of Moon Rocks
5:23
The Moon's Atmosphere
4:46
How Earth & the Sun Affect the Phases of the Moon
6:37
Moon Phases: Names & Sequence
4:55
Motion of the Moon: Sidereal Month vs. Synodic Month
5:11
Predicting When The Moon Will Rise and Set
3:53
Go to
The Moon: Formation & Phases
Ch 25. The Orbits of Celestial...
Ch 26. The Solar System: Layout,...
Ch 27. The Sun's Structure &...
Ch 28. The Universe: Key Concepts &...
Instructor: Artem Cheprasov
This lesson will explore two hypotheses as to how the Milky Way galaxy formed. These are the top-down and bottom-up hypotheses. We'll also go into what special connection you personally have to all of this.
How Our Galaxy Formed
When products are made, they're either made whole with the option of being taken apart later, or they're made in pieces and assembled later. For example, bread is baked as one piece of bread dough. But afterwards, you can slice it apart into different sizes and shapes. On the flip side, you can buy something like a bookshelf that comes in pieces, which you then need to assemble at home. These two examples are reminiscent of the general understanding of how our Milky Way galaxy may have originated.
Monolithic Collapse
The Milky Way galaxy has three basic components: the disk, which contains the spiral arms; the halo; and the nucleus, or central bulge. Older stars, a.k.a. Population II stars, are poor in metal and are found in the halo and the central bulge. Younger, metal-rich stars, a.k.a. Population I stars, are found in the disk.
These facts helped astronomers develop an older theory called monolithic collapse, or the top-down hypothesis, which basically says that large galaxies, like the Milky Way galaxy, formed by way of gravitational collapse from a single, turbulent, large gas cloud.
The monolithic collapse hypothesis states that one big gas cloud originated over 13 billion years ago, which is what we estimate the age of the Milky Way galaxy to be. But the large gas cloud was not strong enough to resist the force of gravity pulling gas inward. As the gas was pulled inwards, the cloud fragmented into smaller clouds. You can equate it to a big cloud in the sky that splits apart into smaller ones due to the wind ripping it apart. Except in space, it was gravity, not wind, that ripped the big cloud apart.
Some of the smaller gas clouds wound up colliding with one another and combining, sort of like you would plop pieces of bread dough together to make a big pizza dough ball. This bigger, low-density gas cloud rotated, and like a rotating piece of pizza dough, could not resist being flattened out into a disk due to its low density. This helped form the disk of our galaxy.
As the cloud flattened out, the abundance of metal created by a process called nucleosynthesis increased, explaining why newer stars in the disk were metal-rich in comparison to the older stars. And when the cloud flattened out into a disk, it left behind the older, metal-poor stars, globular clusters, and the halo, in general, in its wake.
The Bottom-Up Hypothesis
The top-down hypothesis makes it pretty clear that because the halo formed first, it should contain stars of roughly the same age. The ones farthest away should be the oldest, and they should be metal free. But recent evidence and observations have thrown some kinks in the armor of the top-down hypothesis. Some younger star clusters are apparently in the outer halo, and even the oldest stars are metal-poor but not necessarily completely free of metal.
So what's going on then? A new hypothesis has been developed to account for these new discoveries. It's called the bottom-up hypothesis. In essence, it states that large galaxies, like the Milky Way galaxy, formed from the combinations of small galaxies and star clusters. This would be like assembling a bookshelf from smaller parts into one large piece of furniture.
What astronomers believe happened goes something like this. First, there was a large gas cloud that basically had no metal at all. The very first stars that formed from this cloud were truly massive. Massive stars, like massive cars, burn through their energy-producing fuel very quickly. This means they die very quickly since they run out of fuel so fast.
To unlock this lesson you must be a Study.com Member. Create your account
Register to view this lesson
Are you a student or a teacher?
You are viewing lesson 2 in chapter 23 of the course:
Basics of Astronomy
28 chapters | 325 lessons
Ch 1. Celestial Navigation &...
Ch 2. Characteristics of Our Solar...
Ch 3. Earth's Spheres and...
Ch 4. Galaxies: Properties &...
Ch 5. How Scientists Think and...
Ch 6. Influences on Climate
Ch 7. Life in the Universe
Ch 8. Light in Astronomy
Ch 9. Matter in Astronomy
Ch 10. Measurement of Star...
Ch 11. Momentum, Energy, Pressure,...
Ch 12. Newton's Laws in...
Ch 13. Relativity in Time and...
Ch 14. Rotational Motion in...
Ch 15. Small Celestial Bodies &...
Ch 16. Star Death and Stellar...
Ch 17. Star Types and...
Ch 18. Telescopes
Ch 19. The Atmosphere on Earth and Other...
Ch 20. The Birth and Life of...
Ch 21. The Earth, Sky, and...
Ch 22. The History of...
Ch 23. The Milky Way Galaxy
Galaxy Formation: Spiral, Elliptical & Irregular Galaxies
5:43
The Formation of the Milky Way Galaxy
7:08
7:51
Next Lesson
How Our Galaxy Was Discovered
How Our Galaxy Was Discovered
The Structure of Our Galaxy
6:52
The Nucleus of a Galaxy
5:04
The Spiral Arms of the Milky Way Galaxy
5:34
Density Wave Theory & Spiral Galaxies
5:33
Population I and Population II Stars
5:42
The Mass of Our Galaxy
7:50
Go to
The Milky Way Galaxy
Ch 24. The Moon: Formation &...
Formation of the Moon: Theories
5:14
Lunar Geology: Types of Moon Rocks
5:23
The Moon's Atmosphere
4:46
How Earth & the Sun Affect the Phases of the Moon
6:37
Moon Phases: Names & Sequence
4:55
Motion of the Moon: Sidereal Month vs. Synodic Month
5:11
Predicting When The Moon Will Rise and Set
3:53
Go to
The Moon: Formation & Phases
Ch 25. The Orbits of Celestial...
Ch 26. The Solar System: Layout,...
Ch 27. The Sun's Structure &...
Ch 28. The Universe: Key Concepts &...