The Formation of the Milky Way Galaxy

An error occurred trying to load this video.

Try refreshing the page, or contact customer support.

Coming up next: How Our Galaxy Was Discovered

You're on a roll. Keep up the good work!

Take Quiz Watch Next Lesson
Your next lesson will play in 10 seconds
  • 0:02 How Our Galaxy Formed
  • 0:30 Monolithic Collapse
  • 2:39 The Bottom-Up Hypothesis
  • 5:02 The Galaxy's Stars & You
  • 6:10 Lesson Summary
Add to Add to Add to

Want to watch this again later?

Log in or sign up to add this lesson to a Custom Course.

Login or Sign up

Create an account to start this course today
Try it free for 5 days!
Create An Account
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.

As they live and die in violent explosions called supernovae, the process of nucleosynthesis produces metals that are thrown off into the gas cloud, only for them to be incorporated into a new generation of stars forming in the galaxy. This helps explain why the oldest Population II stars have traces of metals. At this stage, a central bulge, halo, and thick low-density disk would have formed as well. Eventually, the disk would've flattened out with time.

What's most interesting is that this hypothesis proposes that smaller galaxies were captured as our galaxy grew. When such smaller, partially evolved, galaxies were swallowed up, they added their own unique clusters to our galaxy. Therefore, it would explain why there are age discrepancies for globular clusters and different types of metallicities found within them that can't be explained by a monolithic collapse.

To unlock this lesson you must be a Study.com Member.
Create your account

Register for a free trial

Are you a student or a teacher?
I am a teacher

Unlock Your Education

See for yourself why 30 million people use Study.com

Become a Study.com member and start learning now.
Become a Member  Back

Earning College Credit

Did you know… We have over 95 college courses that prepare you to earn credit by exam that is accepted by over 2,000 colleges and universities. You can test out of the first two years of college and save thousands off your degree. Anyone can earn credit-by-exam regardless of age or education level.

To learn more, visit our Earning Credit Page

Transferring credit to the school of your choice

Not sure what college you want to attend yet? Study.com has thousands of articles about every imaginable degree, area of study and career path that can help you find the school that's right for you.

Create an account to start this course today
Try it free for 5 days!
Create An Account