Galaxies and gravity | Earth in space | Middle school Earth and space science | Khan Academy

Hello everyone! Today we're going to be talking about galaxies and gravity. We know the Earth is a planet that is in orbit around the Sun. This is called the heliocentric model, and the solar system is an enormous space for us, encompassing every place that humans or our robots have ever been.

But it's actually quite small, cosmically speaking. Even our furthest spacecraft, Voyager 1, has only traveled less than 0.1 percent of the distance to the nearest star. Our solar system is just a small component of a much, much larger set of structures. The Sun is just one star of hundreds of billions of stars in our galaxy, which is called the Milky Way.

Our galaxy is one of several dozen in our galaxy group, which is called the Local Group. Other galaxies might belong to larger collections that are called galaxy clusters. Now, the same thing that holds Earth in its orbit around the Sun is what holds together massive groups and clusters of galaxies: gravity.

This is the same as the gravity that makes a ball fall back to the ground when you toss it up into the air. Gravity is a force that operates between two objects that have mass and tries to pull them closer together, whether those objects are a ball and the Earth or two galaxies.

Now, the strength of this pull depends on two things. First is the mass of both objects. So, as either object gets more massive, the strength of gravity increases. For example, you have a lot more mass than a ball, so the force of gravity between you and the Earth is pulling harder than the force of gravity between a ball and the Earth.

That's why it's a lot harder to throw you into the air than a ball. Gravity also depends on the distance between the centers of the objects. As the objects get farther apart, the strength of gravity decreases. For example, if there is a ball on top of Mount Everest, the pull of Earth's gravity on it is actually slightly less than the pull of gravity on that same ball at sea level because the top of the mountain is farther away from the center of the Earth.

Now, this is a very small effect, less than a percent difference, because the difference between sea level and mountaintop is still very small compared to the distance to the center of the Earth. But that's just on Earth. We know that things in space are really far apart. The Earth is almost 93 million miles— that's 150 million kilometers—from the Sun.

So if gravity is weaker when objects are farther apart, then in order for gravity to be keeping these structures bound together, they must be really massive. And they are. The Sun has a mass of about 2 million kilograms—that's 10 to the power of 30. The Milky Way galaxy, which is about a quintillion kilometers in diameter, has a mass of about 1.5 trillion times that of the Sun.

So thanks to their high masses, objects like our Milky Way galaxy and our nearest neighbor, the Andromeda galaxy, which are the two largest galaxies in the Local Group, can be pulled together by gravity even across millions of light-years. In fact, gravity between our two galaxies is so strong that it's actually pulling the Milky Way and Andromeda galaxies together into an eventual collision.

But eventually, in this case, means almost 5 billion years from now, so no need to worry! To review, in this video we covered how the Earth is part of the solar system, and our solar system is part of a galaxy, and our galaxy is part of a group. Galaxy groups, galaxies, and solar systems are all bound together by the same force: that of gravity.

Gravity is a force which tries to pull objects with mass closer together, and it is weaker for less massive objects or for objects that are farther apart. Thanks for watching, and I hope you learned a little bit of something!