Back To CourseBasics of Astronomy
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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.
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 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.
I'd like to end this lesson on what I think is a very interesting note. As this and other lessons have pointed out, when one star dies, another is born, thanks in part to the remnants of the old star. Originally, there was only hydrogen and helium for the first stars in our galaxy.
As these original stars lived and as they were in their death throes, they produced elements like carbon, nitrogen, oxygen, iron, gold, and so forth. All of this was incorporated into further generations of stars, and many of these atoms make up the bulk of who you are in a molecular and atomic sense. Therefore, your very distant relatives are stars. You exist, literally, thanks to these stars.
For example, the iodine in your thyroid gland, the iron in your blood, the calcium in your bones, and the selenium in your nerve cells are there all thanks to massive supernova explosions that occurred millions and billions of years ago. As the saying goes, you should thank your lucky stars.
Monolithic collapse, also known as the top-down hypothesis, states that large galaxies, like the Milky Way galaxy, formed by way of gravitational collapse from a single, turbulent, large gas cloud. Basically, this hypothesis says that our galaxy started with one big cloud that later fragmented, and this is what helped form the different parts of our galaxy. The problem is that it doesn't explain some discrepancies, like the ages of clusters found in our galaxy's halo.
So in came the bottom-up hypothesis, which states that large galaxies, like the Milky Way galaxy, formed from the combinations of small galaxies and star clusters. This hypothesis posits that our galaxy swallowed up other smaller galaxies as it formed, which helps explain why we have differently aged clusters in the halo of the Milky Way.
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Back To CourseBasics of Astronomy
28 chapters | 325 lessons