Galaxies

Galaxies

This fuzzy band of light in the night skys is the Milky Way, our Galaxy

Galileo discovered that the light in it is actually stars.

What is a Galaxy?

What Is a Galaxy?

Galaxies are huge clusters of stars, gas, and dust bound by the force of gravity. Galaxies can contain billions of stars and are quite massive in size! For example, Andromeda, a nearby galaxy similar to the Milky Way, measures an astounding 1,290,000,000,000,000,000 miles across!

Light Years

• Distances in space are so large that it makes calculations using ordinary units of measurement, such as miles or kilometers, impractical. As a result, scientists defined a unit of measure, called the light year, to determine distances in space. A  light year is the distance light travels through space in one earth year. Light travels through space at a speed of 186,000 miles per second.

• Something that is one light year away is about 5,866,000,000,000 miles away. Alpha Centauri is one of the closest stars to Earth. It is about four light years away.

The Shapes of Galaxies

Galaxies are huge, yet they are very unique, and there are billions of them in the universe. Galaxies are different ages, sizes, and distances from Earth. Astronomers classify galaxies by shape: spiral, elliptical, and irregular.

Spiral Galaxies

get their name for their arms of gas, dust, and stars that spiral out from a densely packed cluster of stars in the center. Spiral galaxies typically have a mix of stars, some young and some old.

The Milky Way

The Milky Way is a special type of spiral galaxy, called a barred-spiral galaxy. Barred-spiral galaxies have a thick bar of stars and gas passing through their centers. From Earth, the fuzzy white band of the Milky Way doesn’t look like a spiral; it looks like a stripe. That is because you are looking at the Milky Way sideways, from inside one of its spiral arms.

Elliptical Galaxies

An elliptical galaxy has a shape like a stretched circle, or oval. Elliptical galaxies usually include only old stars. This is because they contain little gas or dust.

Irregular Galaxies

As their name suggests, irregular galaxies have no distinct or orderly shape. Many new stars are born in irregular galaxies. This is because these galaxies contain far more gas and dust than elliptical or spiral galaxies.

Galaxy Clusters

• A group of close together galaxies is known as a galaxy cluster.

• There are many of these clusters throughout space.

• They seem to occur randomly, sometimes there are lots of clusters in one area, then almost none in others.

Spinning Galaxies

• Since the big bang, galaxies have been moving away from each other. Galaxies move in other ways, too. For example, spiral galaxies spin fast in the center, while their arms spin slower. Many scientists think that several, or perhaps most, galaxies rotate around the enormous gravitational pull of a massive black hole at their center.

Black Hole

a region in space where it is theorized that no mass or light can escape

The speed of a galaxy

The sun circles the center of the Milky Way at a speed of about a half a million miles an hour. Even at that rate, it takes the sun more than 200 million years to make one complete orbit Opens in modal popup windowaround the center of the galaxy!

Holding it all together

How do each of these gigantic galaxies stay clumped together? What keeps everything in the universe from spinning out into space? The law of universal gravitation holds each galaxy together. This same force causes you to return to the ground after you jump into the air. In the next lesson, you will learn more about the force of gravity

Open Ended

What are the different types of galaxies?

Open Ended

What is a light year?

Open Ended

How far apart are galaxies?

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