Galaxies Lesson

Galileo discovered that the light in it is actually stars.

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!

• 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.

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.

• Called this because they appear to form a spiral shape

• Stars are still forming in these types of galaxies

• One of the more common types of 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 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.

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.

• Seem to be older

• Stars formation is no longer occurring

• Lack dust and gas

• Might be the result of galaxies colliding

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.

• Usually smaller

• Believed to form for 2 reasons - too small to form into a structure, or a massive collision caused them to lose their structure

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

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!

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