Understanding Science lessons Galaxy classification

Modified version of a galaxy classification activity for introducing students to peer review.

Author: Lisé Whitfield

Overview: In this activity, students view NASA images of galaxies and develop their own galaxy classification scheme. To mimic the community analysis and feedback (peer review) used in scientific communities, students then compare and contrast their classification schemes with each other to come to a stronger classification scheme. Finally, they compare the groups' final scheme with that developed by Edwin Hubble. This activity allows them to learn about galaxy classification while also experiencing a simple simulation of peer review and community analysis.

Lesson concepts: As a result of this lesson, students will:

Grade span: 8-12


Advance preparation: Print out the handouts. If you decide to use the envelope idea, cut out the galaxy pictures and place them in the envelopes.

Time: One class period

Grouping: Individual, small groups, and whole class

Teacher background: There is a great teacher background section with color images available in the original lesson.

Student prerequisites: Students should be familiar with the definition of a galaxy as collections of billions of stars.


  1. Define classification and ask students to brainstorm in which areas of science classification is used. Ask for specific examples. Discuss why classification is useful in science. Ask students to explain why it is useful or necessary to have one agreed-upon classification scheme as opposed to each scientist having their own personal scheme.
  2. Review the definition of a galaxy with students, and then pass out the handout or envelopes with galaxy photos, depending on which you decide to use. Task students with individually developing a classification scheme for galaxies and placing each galaxy in one of their categories. They can decide to classify them based on whatever characteristics they desire, and they can have as many categories as they need, but they must record, in their lab notebooks, the defining characteristics for each category in detail such that other scientists could sort their own galaxy images using these descriptions. Give students 10-15 minutes to do this.
  3. When everyone has a well-defined classification scheme, ask the class "How will we, as a scientific community, come to agreement on which scheme to use?" Explain that scientists put their work through a process called peer review in which they share their conclusions with one another, try to replicate and verify results, and eventually publish their results. This process ensures that published results meet good scientific standards (e.g., acknowledge and build upon other work in the field, rely on logical reasoning and well-designed studies, back up claims with evidence, etc.). Following peer review and publication, scientists often continue to refine and build upon their work through discussions at meetings and conferences. If desired, you can have students read about peer review using the Understanding Science page or you could project a copy of the cartoon on this page to guide your discussion of peer review with your students.
  4. Tell students that they will now break into small group in which they will evaluate each person's individual scheme to make sure they meet good scientific standards. This will involve testing each idea and then revising it as needed. Depending on your class size, break students into groups of 4-5. Communicate that you expect each student to explain their classification schemes to the rest of the group, one by one, and that after each presentation, the rest of the group will sort the galaxy photos using the defining characteristics presented. If all the other members of the group sort the photos into the same categories, then the scheme was replicable. If not, then the group must revise the defining characteristics to make sure it can be replicated.
  5. After everyone has presented and refined their classification schemes, the group should engage in a conversation about which of their schemes is the strongest. Remind them that scientists often have to be willing to give up their own ideas if others prove to be more effective. They can choose one or they can take pieces of each and combine to form the strongest, most logical classification scheme. They will then choose one group member to present this to the class.
  6. One representative from each group will present the group's classification scheme and defining characteristics. After each group has presented, the whole class will discuss the similarities and differences of each possible scheme. They should additionally discuss wording of the defining characteristics so that the final choice is very clearly described.
  7. After they've decided upon a final scheme as a class, take a moment to debrief the process they went through. Ask them which parts of the process mimicked peer review. Which parts mimicked the community feedback and analysis that helps to advance science? How did it work for them? What are the benefits of this process?
  8. Pass out Hubble's classification scheme and explain that the tuning fork style diagram represents the evolution of galaxies as they age. Ask them to classify their collection of galaxies using Hubble's scheme.
  9. When they are done, discuss the similarities and differences between their scheme and Hubble's scheme. Which of the two does the class prefer? Why? If they prefer Hubble's, ask them if it is okay to adopt that instead of the previously agreed upon model. Get them to think about how science continually advances through new ideas like this coming in, going through peer review, and further analysis and discussion. Science is not done in isolation and is a dynamic process.

An Understanding Science lesson
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