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The title of this lesson is a bit strange: 'The Collision and Assembly of Galaxies.' You'd think a collision would destroy stuff, not assemble it. Since when does a car collision assemble a new car?
Another strange thing about this lesson is that it has to do with something you've heard about - Starburst. But, it's not the candy you might be thinking of. And, in addition to that, another odd thing you'll learn is you're actually inside of a cannibal as I speak!
I hope all that caught your attention as we move into discussing how galaxies assemble and what, if anything, collisions have to do with it.
On occasion, galaxies can collide with one another. Galaxies are moving through space just like cars move on a road. Accidents are bound to happen! Actually, galaxy M31 (aka the Andromeda galaxy) will eventually collide with the Milky Way galaxy, our very own galaxy, about 4 billion years from now. Upon the collision of the spiral Andromeda galaxy with the barred spiral Milky Way, a large elliptical galaxy will eventually form.
Elliptical galaxies cannot be young galaxies because they contain almost no gas and dust from which new stars can form. The ingredients for new star formation are simply absent. Appropriately, the disappearance of this youth-giving gas and dust can be explained by an event like the merger of two galaxies.
This is because when two spiral galaxies, which have comparatively lots more gas for new star formation than an elliptical galaxy, when they combine, the interstellar gas and dust is either stripped away or used up right away for new star formation. Therefore, a recent collision can result in a sudden burst of new star formation, leading to a starburst galaxy. You can define a starburst galaxy as follows: a galaxy that shines brightly as a result of a rapid and sudden burst of star formation.
When two galaxies collide, it's actually very unlikely the individual stars in either galaxy collide with one another. That's because the distances that separate these stars from one another are humongous! So they kind of just pass by and wave hello. What actually collides is the interstellar (or 'between stars') gas and dust in each galaxy. And such a collision can obviously trigger a starburst. With that one simple example, you should realize that one type of galaxy can assemble from other types as a result of a collision between galaxies.
Collisions between galaxies can logically also rip pieces of one galaxy or another completely off, resulting in a smaller galaxy floating about in space. It's like a car collision, where pieces of the cars just fly off in every direction. Just as well, it should be pretty evident that spiral galaxies could not have undergone such a gigantic collision during their assembly, otherwise their delicate disks would've been destroyed and they would be devoid of gas.
However, that doesn't mean that spiral galaxies are free from intergalactic interactions of any kind. Not at all. In fact, our very own Milky Way galaxy is a galactic cannibal. Galactic cannibalism is the absorption of small galaxies by larger ones.
The Bottom-Up Hypothesis states that large galaxies, like the Milky Way galaxy, formed from the combinations of small galaxies and star clusters. So the Milky Way assembled thanks in part to the capture and absorption of smaller galaxies as it formed, such as the Magellanic Clouds.
The reason small galaxies can be munched on by a larger galaxy without distorting the latter is because the small galaxies don't generate forces large enough to deform the much larger galaxy. It's sort of like adding a tiny snowball to a very large snowman. It won't change the shape of the snowman very much as the snowball, metaphorically speaking, merges with him.
In other instances, galaxies don't full-out collide, as I explained before, nor do they cannibalize another galaxy. Sometimes there are near misses where they come close enough to each other to influence one another through tidal forces.
For example, if two uniform disk galaxies were to come near one another, but not collide, then the tidal forces could deform the galaxies enough to cause the formation of spiral arms. Astronomers believe such forces are what cause the formation of the bars in barred spiral galaxies as well.
Galaxies can assemble from the collision of other galaxies. When two galaxies collide, they can form a starburst galaxy, a galaxy that shines brightly as a result of a rapid and sudden bursts of star formation.
Elliptical galaxies can form when two spiral galaxies merge, use up all of their interstellar dust and gas, and combine into one galaxy. During this process, little bits and pieces of the colliding galaxies may break away to form smaller galaxies.
In some cases, very large galaxies cannibalize smaller ones and grow. Galactic cannibalism is thus the absorption of small galaxies by larger ones. This is how we believe our Milky Way came to be, in what's known as the Bottom-Up Hypothesis, which states that large galaxies, like the Milky Way galaxy, formed from the combinations of small galaxies and star clusters.
Other times, the way galaxies assemble has nothing to do with a full-out collision or a merger. Sometimes, there's a near miss between two galaxies that can trigger the formation of spiral arms and bars in spiral galaxies.
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Back To CourseBasics of Astronomy
28 chapters | 325 lessons