Chapter 24. A Universe of Galaxies

24.1 Introduction

A spiral nebula in Pegasus

The goals of this module: After completing this exercise, you should be able to:

1. Summarize the controversy debated by Shapley and Curtis in 1920.
2. Explain how Edwin Hubble resolved the problem of spiral nebulae.
3. Describe Hubble's galaxy classification system.

In this module you will explore:

1. How astronomers determined that the Milky Way Galaxy is one of many galaxies in the universe.
2. How astronomers classify galaxies based on appearance.

Why you are doing it: Less than 100 years ago, most astronomers thought that our Milky Way Galaxy was pretty much all there was in space. They had no idea of the size and extent of the universe. Observers noticed and classified objects like this spiral shaped object shown in the picture above as "spiral nebulae". Look carefully and you will see several smaller spirals and football-shaped fuzzy objects as well. Gradually, observations made with bigger and better telescopes and new instruments began to reveal that we live in a universe dominated not by the Milky Way's stars, but by other galaxies.

24.2 Background

Two "nebulae" from Messier's catalog

During the 1700s-1800s, astronomers began to view objects at ever increasing distances with bigger and better telescopes. Objects with similar characteristics were grouped together in an attempt to understand their true natures. Between 1758-1782, the French comet hunter Charles Messier assembled a catalog of fuzzy appearing objects, so that he wouldn't confuse them with a real comet. These objects today are still referred to by their number in his catalog, but are classified as star forming regions like the Orion Nebula, Messier 42 (M42); planetary nebulae like the Ring Nebula, M57; star clusters, like the globular cluster in Hercules, M13; and the Crab Nebula supernova remnant, M1. Some of Messier's objects displayed a spiral or oval shape, like M81 and M105 shown to the right.

Astronomers simply drew what they saw through a telescope, since they had no cameras to take a picture or instruments to spread faint starlight into a spectrum. Lord Rosse used the large telescope shown below to look at M51, the Whirlpool Nebula. Note that his sketch is accurate when compared to a modern photograph. (Fortunately, artistic ability isn't a requirement for modern astronomers!)

But in the mid-1800s, Lord Rosse and other astronomers had no way of measuring the distances to the objects in Messier’s catalog. Actually, the distances to stars were just starting to be measured with the first parallax determined in 1831.

24.3 A Universal Debate

By the early 1900s American astronomer Harlow Shapley had earned respect by estimating the size of the Milky Way Galaxy using the distribution of globular star clusters. In Shapley's opinion, the spiral nebulae were small and members of our galaxy. An opposing view offered by another American, Heber Curtis, was based on an earlier idea by the German philosopher Immanuel Kant. In this "island universe" model, the Milky Way was only one huge collection of stars; there were others. Curtis proposed that these spiral nebulae were each a huge rotating system of stars, similar to our Milky Way Galaxy.

The Shapley-Curtis debate took place in 1920 at the National Academy of Sciences in Washington, D.C. Shapley argued that the Milky Way was the main entity in the universe, the Sun was not located at the center of the Milky Way, and that the spiral nebulae were nearby clouds of gas. Curtis took the position that the Milky Way was a small galaxy, the Sun was located near the center of our galaxy, and the spiral nebulae were other galaxies like our Milky Way, but far away.

24.4 The Mt. Wilson Telescope

The next big breakthrough in resolving the Shapley-Curtis debate involved a new telescope located in the mountains near Los Angeles and the astronomer Edwin Hubble, both shown to the right. In 1917, the new Mt. Wilson telescope saw its first light. Its huge mirror, 100-inches across, allowed astronomers to collect light more efficiently than any other telescope for the next three decades. Hubble, for whom the Hubble Space Telescope is named, was one of the first astronomers to regularly use the Mt. Wilson 100-inch Telescope. He saw things that no one had ever seen before!

Hubble photographed the spiral nebula shown below, located in the constellation Andromeda. In 1923, he identified a Cepheid variable star embedded in the nebula. This was exciting because the distance to these stars can be calculated. Hubble found more Cepheids, and he started observing these stars regularly to determine the period of variation for each one.

Henrietta Leavitt had developed a period-luminosity relation based on Cepheid variables she had studied in the Magellanic Clouds. The period-luminosity relation is shown below.

The Period - Luminosity Relation for Cepheid Variables

24.5 A Universe of Galaxies

Hubble's determination of the distance to the Andromeda Galaxy supported the "island universe" hypothesis. A few Cepheid variable stars in other galaxies were studied by Hubble, and gradually it was accepted that the other spiral and football shaped objects in Messier’s catalog were indeed also galaxies. As scientists often do when something new is discovered, Hubble started classifying galaxies based on their appearance.

Observe the characteristics of these three galaxy types.

24.6 Hubble's Tuning Fork Diagram

Hubble arranged his galaxy types in the "Tuning Fork Diagram," which is shown below. When he first proposed this arrangement, Hubble thought that one type of galaxy naturally evolved into another type over time. Today we know that galaxy evolution is more complicated.

Hubble's Tuning Fork Diagram

24.7 Quick Check Quiz

Indepth Activity: A Universe of Galaxies