Discovery of Galaxies

It took astronomers many centuries to understand the true nature of galaxies. For many years, astronomers believed that they were just clouds or nebulas within our own galaxy. In fact, astronomers believed that our Milky Way represented the entire universe - that there was nothing else beyond it. It wasn’t until astronomers were able to measure the distances to galaxies that they really understood just how unbelievably enormous our universe really is. (These measurements were first made by Hubble in the 1920’s.)
Explore 26.1 The Discovery of Galaxies to learn about the historical development of our understanding of galaxies, including the role of Hubble in establishing the existence of galaxies beyond the Milky Way.
Galaxies come in two major varieties: spirals and ellipticals. Spiral galaxies have the most pronounced structure, with clear spiral arms in a disk and a bright bulging core in the center. Often we can see clear streaks of dust clouds within a galaxy’s spiral arms. Check out this gallery of spiral galaxy images. Try to find an example of a spiral galaxy viewed face-on, one viewed edge-on, and one viewed somewhat at an angle.
The structure of elliptical galaxies is much less pronounced. These galaxies tend to just look like a glowing ball of light. Check out this gallery of elliptical galaxies to see how they look very different from spirals.
Read 26.2 Types of Galaxies to understand the classification of galaxies based on their shapes and compositions, including spirals, ellipticals, and irregulars, as well as the factors influencing their appearances over time.
Astronomers who study galaxies continue to classify them using the system that Hubble invented. Today, though, astronomers have so many pictures of galaxies that they cannot possibly classify them all by hand (and computers aren’t very good at making the classifications automatically). For this reason, a group of astronomers have created a website where people can help them classify galaxies. It is called, “Galaxy Zoo.”
Spend a few minutes exploring the Galaxy Zoo Tutorial, which shows you how the Galaxy Zoo website asks you a series of questions about a picture of a galaxy. By answering the questions, you can ultimately classify a galaxy like an astronomer does.