Accretion: Definition & Theory

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  • 0:00 Definition of Accretion
  • 1:48 Accretion Process & Examples
  • 2:58 Accretion & Nebular Theory
  • 4:04 Lesson Summary
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Lesson Transcript
Instructor: Josh Corbat

Josh has taught Earth Science and Physical Science at the High School level and holds a Master of Education degree from UNC-Chapel Hill.

We live on an incredible planet. It's hard to believe that relatively simple processes helped shape our planet (and our entire solar system) into what it is today. In this lesson, we'll discuss one of those simple-yet-important processes: accretion.

Definition of Accretion

Have you ever built a snowman? Whether you have, or simply seen it done on television or in a movie, you probably know the basic idea. You take a small ball of snow in your hands, shape it into as perfect a sphere as you can, then put it back on the ground and start to roll it. What happens when you roll it? Since snow tends to stick to itself, the ball will get bigger as more and more layers of snow are added. Eventually, the ball that once fit in your hands becomes a massive snowball worthy of being part of a snowman.

This same idea is at work in the formation of a planet. Early in the formation of a solar system, the entire system of a star and its planets, all that exists is a giant cloud of gas and dust called a nebula. Instead of just sitting still in space, the bits of gas and dust will eventually start colliding with each other. Some of the dust will stick to other bits of dust and form larger chunks of matter. This sticking together is the beginning of the process of accretion.

Accretion is the gradual increase in the size of an object by the buildup of matter due to gravity. In other words, as small bits of matter are attracted toward an object due to gravity and stick to the object, it will get bigger and bigger. Eventually, as in the case of a solar system forming, the objects will get big enough to become planets and stars. As the objects grow larger, they will attract more bits of matter because their gravitational pull will be stronger.

Accretion Process & Examples

Since accretion is the gradual increase in size by the buildup of matter, it is easy to demonstrate using common, everyday examples. If you have ever had to clean a house or apartment, you've probably found what are known as dust bunnies in the corners or under furniture. Dust bunnies are collections of dust and hair that form into loose clumps. This is a great example of accretion, as dust and hair are attracted to each other and cause the dust bunny to increase in size.

Another great example that you may have seen is the formation of cotton candy. If you've been to a fair or carnival, you may have had the pleasure of watching someone spin sugar into cotton candy and make it stick to a paper cone. The strung sugar sticks to itself and becomes larger in an example of accretion at work.

It is important to keep in mind, though, that the physical forces causing accretion in these examples differ from those that drive accretion in astronomy. In these examples, static and cohesive forces are at work. In astronomy, the force responsible for accretion is gravity.

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