Lesson 10: The Hubble Classification of Galaxies
Overview
We give an overview of the many different shapes and sizes that galaxies have, starting with the broad categories of spiral, barred spiral, elliptical, and irregular, and relating each kind to terrestrial objects. Given prototypical shapes of these categories, students will then categorize shapes of real galaxies.
Learning Outcomes
- Explain what the Hubble Tuning Fork for classifying galaxies is and how astronomers classify galaxies.
- Briefly describe the main characteristics of the three main types of galaxies as well as irregular galaxies.
- List the types of stars found in each type of galaxy, whether star formation is happening, and if there is abundant gas and dust present.
- Classify shapes of galaxies and identify the difficulties in classifying strictly by how galaxies look.
Materials
Pieces of stiff paper or light card board cut into the shapes of various different types of galaxies. Keep the shapes basic; the goal is for students to categorize the galaxies by the basic groups of elliptical spiral, barred spiral, and irregular without consideration of sub-types. See the last page of this lesson for the patterns (Figure 5.10.1).
 Elliptical galaxies that are spherical are classified as E0. |
 Depending on how oval a galaxy is, it may be classified as E1, E2 down to an E7 or E8. |
 Spiral galaxies may be viewed from face on to edge on and every angle in between. This is the shape of an edge on spiral galaxy. |
 If a galaxy cannot be definitely classified as a spiral or an elliptical galaxy, it may be called an irregular galaxy |
 Spiral galaxies are classified as Sa/SBa, Sb/SBb or Sc/SBc (classic/barred) according to the tightness of their spiral, the clumpiness of their spiral arms, and the size of their central bulge This is the shape of an Sa spiral galaxy. |
 Spiral galaxies are classified as Sa/SBa, Sb/SBb or Sc/SBc (classic/barred) according to the tightness of their spiral, the clumpiness of their spiral arms, and the size of their central bulge. These differences can be traced back to the relative amounts of gas and dust contained within the galaxies. This image represents an SBb galaxy. |
Figure 5.10.1 Examples http://spaceplace.nasa.gov/galactic-mobile/). Make a Galactic Mobile available at this website includes galaxy patterns covering all classifications and ideas for making a galaxy mobile.
Pre-assessment Questions and Discussion
Q. What type of galaxy is the Milky Way?
A. The Milky way is a barred spiral galaxy
Q. Do all Galaxies look the same? If not what can they look like?
A. No, there are spiral galaxies, elliptical galaxies and lenticular galaxies. The look like our galaxy, footballs and basket balls.
Q. How do you think astronomers might classify different galaxies?
A. By their different shapes; location; number of stars; colors; ages.
Text
Galaxies come in many different shapes and sizes, not just barred spirals like our Milky Way galaxy. Astronomers find it useful to divide galaxies into groups based on how they look in visible light, much like one would see if looking through a telescope. The Hubble sequence is a way of classifying galaxies that was invented by Edwin Hubble in 1936. It is often referred to as the “Hubble tuning-fork” because the different galaxies are laid out in a shape much like a tuning-fork. The galaxies are classified based on their shapes. We are going to consider elliptical galaxies, spiral galaxies, barred spiral galaxies, and irregular galaxies in this lesson. As you might guess, an irregular galaxy is one that doesn’t fit any where else in the classification.
Elliptical galaxies have shapes that resemble basketballs or footballs. Unlike spiral galaxies that we learned about during our Milky Way lesson, ellipticals do not have spiral arms. They are basically made up of one gigantic bulge. They are represented by the letter E and a number representing how round or flattened they look. For example, an elliptical galaxy that was perfectly round like a basketball would be called E0, while the one that looked like a football that had been badly squished would be called E7. Elliptical galaxies tend to made up of only very old stars and have far less dust and gas than spirals. There are exceptions to these rules, as it appears some ellipticals are the result of the mergers of one or more spiral galaxies.
Spiral galaxies have a flattened disk with two or more arms spiraling out from it, and a bulge in the center. Spiral galaxies are broken up into two types, those having a bar-like structure as part of their bulge and those that do not. The ones with the bar-like structure are referred to as barred spirals and are given the capital letters SB. The ones without a bar get just the letter S. Both of the classifications are divided into even more groups, depending on how tightly the spiral arms are wrapped up and how large the central bulge is. Of the three types of galaxies, spirals have the most dust and gas, and also are still forming new stars. Our Milky Way is a barred spiral galaxy and is classified as an SB galaxy.
Irregular galaxies come in all shapes and sizes, and usually look distorted and basically like something crazy has happened or is happening to them. Usually these galaxies are undergoing active star formation and can contain lots of dust. Astronomers observe irregular galaxies that have long “wispy” tails, look like two big eyes, have rings of stars around them, have a “path” of gas going to a nearby galaxy, just about anything imaginable. Observations have led astronomers to guess that these irregular galaxies have had close encounters with other galaxies.
The Hubble tuning-fork is called that because, well, the original design looked just like a tuning fork. Starting on the left edge, we find elliptical galaxies, starting with those that are almost perfectly spherical to those that look flattened. Then, the fork splits, with the regular spirals forming the top prong and the barred spirals forming a parallel bottom prong. Irregular galaxies do not get included under this classification, as they just do not seem to fit anywhere in it.
Follow-Up Questions on the Reading
- How do astronomers divide up the different types of galaxies?
- Briefly describe each type of galaxy.
- Which types of galaxies have the youngest stars, as well as abundant gas and dust?
- Which type of galaxy has mainly very old stars, no new star formation, and little if any gas and dust?
Reinforcing Hands-On Activity
Classification:
Students are given containers with small card board cut-outs of different 2-D representations of galaxies and label them by using the Hubble tuning-fork classifications. During the activity, one will be able to point out how hard it is to classify a galaxy if you do not have a good face-on image of it.
Students discuss which galaxies they had the hardest time classifying and why. Discuss what is learned by classifying objects, and why humans seem to do this for lots of things.
Summary and Post-Assessment Questions
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- Discuss with students what they learned. Have them list at least three things they didn’t know before doing this study.
- How close did your classification of the galaxies compare to Hubble’s tuning fork?
- On what basis do astronomers classify galaxies? Choose among shape, size, content, distance, structure, and age.
- Ask, “If you could choose to be a galaxy, what kind of a galaxy would you be and why?” What are your main characteristics as a(n) — spiral / barred spiral / elliptical / irregular — galaxy?
- Sort the following contents of galaxies into whether they are found in spiral galaxies (regular and barred), irregular galaxies, or elliptical galaxies. Some may belong to more than one type of galaxy.
a. Massive, hot stars(spirals, irregulars)
b. Star forming regions (spirals, irregulars)
c. Lots of gas and dust (spirals, irregulars)
d. Low-mass stars like the Sun (all types)
e. No star forming regions or gas and dust (elliptical)