Galaxies

A galaxy is a massive, gravitationally bound system that consists of stars, stellar remnants, interstellar gas, dust, dark matter, and an important but poorly understood component tentatively dubbed dark energy. Galaxies are categorized according to their visual morphology as elliptical, spiral, or irregular. Around the Milky Way alone, there are billions of galaxies, each containing billions of stars.

Types of Galaxies

There are three main types of galaxies:

1. Elliptical Galaxies: These galaxies have an ellipsoidal shape and contain mostly older stars. They have very little interstellar matter and show little to no ongoing star formation.
2. Spiral Galaxies: These galaxies have a flat, rotating disk with a central bulge and spiral arms. They have ongoing star formation and are often rich in interstellar matter.
3. Irregular Galaxies: These galaxies have no distinct shape and often have high levels of interstellar matter. They usually have ongoing star formation and are considered to be the most chaotic of the galaxy types.

Study Guide

1. What is a galaxy?

A galaxy is a massive system that consists of stars, stellar remnants, interstellar gas, dust, dark matter, and dark energy, all gravitationally bound together.

2. How are galaxies categorized?

Galaxies are categorized based on their visual morphology as elliptical, spiral, or irregular.

3. Describe the three main types of galaxies.

Elliptical galaxies have an ellipsoidal shape, spiral galaxies have a flat, rotating disk with spiral arms, and irregular galaxies have no distinct shape.

4. What are some characteristics of each type of galaxy?

Elliptical galaxies contain mostly older stars and little interstellar matter, spiral galaxies have ongoing star formation and are rich in interstellar matter, and irregular galaxies have high levels of interstellar matter and ongoing star formation.

5. How many galaxies are estimated to be around the Milky Way?

There are billions of galaxies around the Milky Way, each containing billions of stars.