Galaxies and the Expanding Universe

Active learning works for this topic because galaxies and the expanding universe involve abstract concepts that become concrete through hands-on modeling. Manipulating physical objects and images lets students test ideas about scale, motion, and change, which strengthens their spatial reasoning and evidence-based conclusions about cosmic structure.

• Grade 9
• Science
• 4 activities
• 30 min–50 min

Learning Objectives

1. Classify galaxies into spiral, elliptical, and irregular types based on their visual characteristics.
2. Explain the evidence, including redshift and cosmic microwave background radiation, that supports the expansion of the universe.
3. Analyze how gravitational interactions cause galaxies to merge and evolve over cosmic timescales.
4. Compare the properties of different galaxy types, such as star composition and shape.

Gallery Walk

⏱ 35 min·Small Groups

Gallery Walk: Galaxy Types

Print Hubble images of 20 galaxies labeled A-T. Place at stations around the room. Small groups classify each as spiral, elliptical, or irregular, noting shape and star patterns on worksheets. Debrief with whole-class vote and criteria discussion.

Prepare & details

Differentiate between different types of galaxies (spiral, elliptical, irregular).

Facilitation Tip: During the Gallery Walk, circulate with guiding questions like, 'What patterns help you decide if a galaxy is spiral or elliptical?' to focus observations.

Setup: Wall space or tables arranged around room perimeter

Materials: Large paper/poster boards, Markers, Sticky notes for feedback

• Understand
• Apply
• Analyze
• Create
• Relationship Skills
• Social Awareness

Gallery Walk

⏱ 45 min·Pairs

Redshift Simulation: Balloon Universe

Inflate a balloon with dots as galaxies. Mark one dot as Earth. Students measure dot distances as balloon expands, calculate 'recession speeds,' and plot Hubble's law graph. Compare to real galaxy data.

Prepare & details

Justify what evidence supports the theory that the universe is constantly expanding.

Facilitation Tip: For the Redshift Simulation, have students measure and record distances between marked dots on the balloon before and after inflation to quantify expansion.

Setup: Wall space or tables arranged around room perimeter

Materials: Large paper/poster boards, Markers, Sticky notes for feedback

• Understand
• Apply
• Analyze
• Create
• Relationship Skills
• Social Awareness

Gallery Walk

⏱ 30 min·Pairs

Playdough Collisions: Galaxy Mergers

Pairs sculpt spiral and elliptical galaxies from playdough. Simulate gravitational pull by pushing together slowly, observe deformation into irregular forms. Record changes and link to real mergers like Andromeda-Milky Way.

Prepare & details

Analyze how galaxies interact with one another over billions of years.

Facilitation Tip: In Playdough Collisions, ask students to sketch their models before and after merging to track changes in shape and star density.

Setup: Wall space or tables arranged around room perimeter

Materials: Large paper/poster boards, Markers, Sticky notes for feedback

• Understand
• Apply
• Analyze
• Create
• Relationship Skills
• Social Awareness

Gallery Walk

⏱ 50 min·Small Groups

Spectrum Analysis: Evidence Stations

Set up stations with spectrographs or apps showing galaxy spectra. Groups measure redshift wavelengths, calculate velocities, and justify expansion evidence. Rotate and compile class data.

Prepare & details

Differentiate between different types of galaxies (spiral, elliptical, irregular).

Facilitation Tip: At Spectrum Analysis stations, provide rulers and colored pencils to help students align spectral lines accurately while comparing sources.

Setup: Wall space or tables arranged around room perimeter

Materials: Large paper/poster boards, Markers, Sticky notes for feedback

• Understand
• Apply
• Analyze
• Create
• Relationship Skills
• Social Awareness

Teaching This Topic

Teachers should emphasize evidence over memorization, using real data and models to build conceptual bridges between local observations and cosmic scales. Avoid over-relying on animation; physical models help students internalize abstract ideas like expansion without space. Research shows students grasp metric expansion better when they physically manipulate space (like a balloon) rather than observing it passively.

What to Expect

Students will confidently classify galaxy types, explain expansion using redshift, and describe galaxy interactions through models. They will use evidence from simulations and image analysis to justify their reasoning, demonstrating understanding of how structure and motion reveal the universe's history and future.

Watch Out for These Misconceptions

Common Misconception

During the Redshift Simulation, watch for students interpreting the balloon's edge as the universe's boundary.

What to Teach Instead

Use the Redshift Simulation to redirect their attention to dots moving apart without reaching an edge, emphasizing that space itself stretches uniformly by having students measure increasing distances between same-colored dots.

Common Misconception

During Playdough Collisions, watch for students believing galaxies merge instantly into a single new structure.

What to Teach Instead

In Playdough Collisions, guide students to observe gradual distortions over time by pausing after each 'orbit' to note changes in shape and star distribution, linking these to real merger timescales.

Common Misconception

During the Gallery Walk, watch for students assuming all galaxies fit neatly into only one category.

What to Teach Instead

In the Gallery Walk, have students revisit images where features overlap (e.g., a galaxy with both a bulge and faint arms) and discuss why classification exists as a tool, not a rigid system, using the provided image labels as evidence.

Common Misconception

Assessment Ideas

Exit Ticket

Provide students with images of three different galaxies. Ask them to label each galaxy with its type (spiral, elliptical, irregular) and write one sentence explaining their classification based on observed features.

Discussion Prompt

Pose the question: 'If the universe is expanding, what does that mean for the space between galaxies?' Facilitate a discussion where students explain how redshift and Hubble's Law support this idea and consider the implications for the future of the universe.

Quick Check

Present students with a simplified graph showing distance versus recession velocity for several galaxies. Ask them to identify which galaxies are moving away fastest and explain how this graph illustrates Hubble's Law and the expansion of the universe.

Extensions & Scaffolding

• Challenge students to predict the outcome of a merger between a spiral and elliptical galaxy, then research real examples to compare.
• For students struggling with redshift, provide a pre-labeled spectrum with key absorption lines to match against galaxy images.
• Allow deeper exploration by having students research how dark matter influences galaxy rotation curves using provided datasets.

Key Vocabulary

Galaxy	A vast system of stars, stellar remnants, interstellar gas, dust, and dark matter, bound together by gravity.
Redshift	The phenomenon where light from distant celestial objects shifts towards longer, redder wavelengths, indicating they are moving away from us.
Hubble's Law	The observation that the farther away a galaxy is, the faster it is receding from Earth, providing evidence for the universe's expansion.
Cosmic Microwave Background Radiation	A faint glow of radiation filling the universe, considered a remnant of the Big Bang and strong evidence for an expanding universe.
Dark Matter	A hypothetical form of matter that does not interact with light but exerts gravitational influence, making up a significant portion of a galaxy's mass.