Structure of the Milky Way

Learning Objectives

  • Students will be able to explain the geometry of the Milky Way and its separation into two components, a flat disk, and a spherical halo
  • Students will quantitatively compare the distances to bright stars, open clusters, and globular clusters and will use these to define the size scale of the Milky Way

Standards Addressed

  • From SAS:  3.3.10.B2 – Scale and Measurement – Explain the scaled used to measure the sizes of stars and galaxies and the distances between them

Supplies/Materials/Articles Needed

Preparation Time Needed

For teachers familiar with the databases used and the quantities included in the tables, <1 hour.

Class Time Required

2 hours.

Activity Description

This is meant to be an introduction to CER using real data on Milky Way star clusters.  It can be expanded to cover many more concepts, but in this version it is meant to reveal the contrast between the shape of the Milky Way’s disk and halo components. 

Using data from professional catalogues, students are going to look for patterns in the positions in space of two different types of star clusters in the Milky Way – globular clusters and open clusters.

Essential Question(s)

  • How do astronomers know the size and shape of the Milky Way if we are embedded in it and can’t see it from any other vantage point than our Earth-bound view?


  • Students need time to investigate the data tables and to compare the columns to the key provided in order to become familiar with the quantities that are contained.
  • A short lecture or discussion is necessary to review the important quantities and their units.  For this activity, students need to be familiar with using latitude and longitude to describe locations on Earth as well as astronomical units of distance, including light years, parsecs, and kiloparsecs.  Here is a resource that describes Galactic latitude and longitude, which is important for this activity: -- However, when doing this presentation or discussion, you have to be careful not to explain Galactic coordinates in so much detail that you reveal all of the patterns you want students to look for in the activity! 
  • After this introduction, you should divide students into groups of 3 – 4 and task them to study the data and look for patterns that answer the following question:  “How are the globular clusters, open clusters, and bright stars distributed on the sky, and what does that tell us about the size and shape of the Milky Way?”
  • Each group should be asked to write on their white board or chart paper a claim that directly answers this question and to provide at least one piece of evidence from the data tables to support their claim.


  • To assess the student work, each group should post their white boards / chart paper with their claims and evidence so that all students can see them.
  • You should ask the class to look to compare and their claims to decide on which are most likely to be correct
  • The claims / evidence that students are likely to find are:
    • Claim:  The Milky Way is flat  --  Evidence:  Open Clusters are mostly found in a small range of latitudes on either side of the equator
    • Claim:  The Milky Way is spherical  -- Evidence:  The Globular Clusters are found at all latitudes
    • Claim:  The Sun is offset from the center of the Milky Way – Evidence:  The Globular Clusters are found in a restricted range of longitudes 
    • Claim:  The Milky Way is probably less than 20 or so kiloparsecs in radius – Evidence:  most of the globular clusters and open clusters are found at distances less than 20,000 parsecs, although several are as far as > 100,000 parsecs!
    • Claim:  The bright stars are only a very small part of the Milky Way – Evidence:  most bright stars are tens to hundreds of light years away, far closer than any of the clusters.

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