Investigating Star Brightness, Distance, Size, & Temperature

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SWBAT explain what causes stars to be brighter and what causes some stars to appear brighter than others.

Big Idea

In this lesson, students explore the relationships between star brightness, distance, size, and temperature by examining the Hertzsprung-Russel Diagram and investigating with flashlights.

Lesson Overview

Inquiry Based Instructional Model

To intertwine scientific knowledge and practices and to empower students to learn through exploration, it is essential for scientific inquiry to be embedded in science education. While there are many types of inquiry-based models, one model that I've grown to appreciate and use is called the FERA Learning Cycle, developed by the National Science Resources Center (NSRC):

1. Focus

2. Explore

3. Reflect

4. Apply

A framework for implementation can be found here

I absolutely love how the Center for Inquiry Science at the Institute for Systems Biology explains that this is "not a locked-step method" but "rather a cyclical process," meaning that some lessons may start off at the focus phase while others may begin at the explore phase. 

Finally, an amazing article found at, How Inquiry-Based Learning Works with STEM, very clearly outlines how inquiry based learning "paves the way for effective learning in science" and supports College and Career Readiness, particularly in the area of STEM career choices. 

Unit Explanation

In this unit, students begin by studying the location of the Earth in the Universe. Then, students learn about the brightness of stars through investigations and research. At the end of this unit, students explore the patterns on Earth, such as day/night and the length of shadows. 

Summary of Lesson

Today, I open the lesson by introducing the word luminosity and showing students a video that compares star sizes. Students then label, take notes, and make observations on the Hertzsprung-Russel Diagram. Students also explore the relationships between star brightness, distance, and size by conducting a flashlight investigation. At the end of the lesson, students reflect and apply their new understanding of star brightness by writing an explanation. 

Next Generation Science Standards  

This lesson will support the following NGSS Standard(s):

5-ESS1-1. Support an argument that the apparent brightness of the sun and stars is due to their relative distances from Earth.  

Scientific & Engineering Practices

For this lesson, students are engaged in Science & Engineering Practice:

Science & Engineering Practice 2: 

Science & Engineering Practice 2: Developing and Using Models

Students use a video, analogies, diagram, and flashlights to analyze the brightness, distance, size and temperature of stars. 

Crosscutting Concepts

To relate ideas across disciplinary content, during this lesson I focus on the following Crosscutting Concept

Crosscutting Concept 2: Cause and Effect 

Students examine how closer stars can inaccurately appear to be brighter than stars that are located further away from the Earth. 

Disciplinary Core Ideas

In addition, this lesson also aligns with the following Disciplinary Core Ideas

ESS1.A:  The Universe and its Stars

The sun is a star that appears larger and brighter than other stars because it is closer. Stars range greatly in their distance from Earth. (5-ESS1-1)

ELA Integration

To add depth to student understanding, when I can, I'll often integrate ELA standards with science lessons. Today, students will work on meeting CCSS.ELA-LITERACY.W.5.2: Write informative/explanatory texts to examine a topic and convey ideas and information clearly. In this lesson, students will construct an explanation about star brightness.

Choosing Science Teams

With science, it is often difficult to find a balance between providing students with as many hands-on experiences as possible, having plenty of science materials, and offering students a collaborative setting to solve problems. Any time groups have four or more students, the opportunities for individual students to speak and take part in the exploration process decreases. With groups of two, I often struggle to find enough science materials to go around. So this year, I chose to place students in teams of two or three! Picking science teams is always easy as I already have students placed in desk groups based upon behavior, abilities, and communication skills. Each desk group has about six kids, so I simply divide this larger group in half or thirds. 

Gathering Supplies & Assigning Roles

To encourage a smooth running classroom, I ask students to decide who is a 1, 2, or 3 in their groups of three students (without talking). In no time, each student has a number in the air. I'll then ask the "threes" to get certain supplies, "ones" to grab their computers, and "twos" to hand out papers (or whatever is needed for the lesson). This management strategy has proven to be effective when cleaning up and returning supplies as well!  


20 minutes

Partners & Computers

During today's lesson, students will be working in teams of two students (elbow partners). Each team of students will have one laptop computer to share. While we have enough computers for all students, I have found that partners are more successful collaborators when they are sharing one device. 

Envelope Book

For the Sun and Earth's Patterns Unit, students are creating an envelope book to help organize new information and to support an inquiry approach during the learning process. Prior to the unit, I used a plastic comb binding machine (pictured below) to create envelope books using 10 envelopes for each student's book: Envelope Books. During today's lesson, students will be working with the fourth envelope in their books. 

Vocabulary Cards

For each envelope, students are provided with up to 3 vocabulary cards: Vocabulary Cards (I copied these onto green card stock paper & cut each page into 10 cards): Vocabulary Cards. For easy distribution, I placed these cards into ziplock baggies so that each group of students could easily take cards out as needed: Vocabulary Cards in Bags. As an opening to the lesson, I write the following vocabulary word on the board: luminosity.  

Students work together with their partners to discover the meaning of this word, using their computers and/or dictionaries as resources. As students are ready, they share definitions out loud with the rest of the class. We discuss student findings and then I construct a student-friendly definition (using student input) for all students to record on their cards. This process is important for two reasons: (1) sometimes students record definitions that are difficult to understand due to complex language and (2) this also allows students to see how important it is to use multiple sources when conducting research. 

Here's an example of a student's vocabulary card: Student Vocabulary Card.

Developing a Guiding Question 

To support an inquiry-based learning model and Science & Engineering Practice 1 (Asking Questions and Defining Problems), I explain: Today, you will continue learning about the brightness of stars. First, let's talk about the questions you have still have about star brightness. What guiding question do you think we should research today? Student questions include:

  • How are stars similar to flashlights and their distances and sizes? 
  • What is the luminosity of stars from Earth? 
  • Does the brightness of a star go down when they age? 
  • Why are some stars brighter and others not so bright?

As students share their thinking, I'm hoping that with some teacher guidance, students will ask the questions, "What causes stars to be brighter?" and "What causes some stars to appear brighter?"  Sure enough, one student asks, "Why are the stars' luminosity different?" and another student asks, "Why do stars appear brighter than others?" 

Here's a video of this question-development process in action: Students Discussing the Guiding Question.

Lesson Introduction & Goal 

Now that students have helped develop a guiding question, I introduce today's learning goal: I can explain what causes stars to be brighter and what causes some stars to appear brighter than others.

Envelope Cover 

I continue on by passing out an envelope picture to each student: Envelope 4 Pictures. Pictures add an element of excitement to learning and they provide support for students who learn best using visual aids. 

On the front cover of the fourth envelope in student envelope books, I model how to paste the picture and write the investigative questions for today's lesson: What causes stars to be brighter? What causes some stars to appear brighter? (Example of Student Envelope).


I want to inspire interest in today's lesson and capitalize on student curiosity, so I show the following video clip that compares our sun to other stars. During the video, most students are blown away by how small our sun is in comparison to other stars in the Universe. Students excitedly ask if they can watch it again! 


40 minutes

Hertzsprung-Russel Diagram

I pass out an H-R Diagram (Hertzsprung-Russel Diagram) to each student. I purposefully leave the vertical and horizontal axises blank as I want students to take an active part in completing this diagram. To get students started, I model how to use crayons to color each of the columns blue, white (we added a touch of gray), yellow, red-orange, and red (Coloring the Diagram). 

Observing a Campfire

To build a connection between star color and temperature, I show students the following video of a campfire. I ask: What do you notice about the color of the flames? Where are the flames the hottest? Where are the flames the coolest? Students volunteer to come up to the board to point to the campfire and share their thinking: Students Sharing Thoughts. I love how the last student brings up her personal experiences with roasting marshmallows! 


Following the video, I ask: What does a campfire have to do with stars? Student responses were exciting to hear: Students Explaining how a Campfire is Like Stars. Most students make the connection between blue flames near the coals in the fire with hot stars and red flames at the tips of flames with cool stars! 

Labeling the X-Axis

I continue: Knowing how the color of a star relates to temperature, what do you think the bottom axis of our H-R diagram should be labeled? (Average Surface Temperature - degrees Celsius). We then label the blue stars "hot stars" and the red stars "cools stars."

Labeling the Y-Axis

I call students' attention to the Hertzsprung-Russel Diagram picture on the front of their envelopes (below), I ask: What do you think we should label the vertical axis? A student observes that the vertical axis is labeled "luminosity." What does luminosity mean again? (how bright a star actually is). We then label the y-axis: Luminosity (Absolute Brightness). I draw a dotted line across the diagram at the same level as our sun (Sol). I explain: All the stars below this line are dimmer than our sun. All the stars above this line are brighter our sun. 


Main Sequence Stars

At this point, we begin taking notes on Main Sequence Stars, White Dwarf Stars, Giants, and Supergiants. (I provide information on each topic from my own research.) During this time, I model and project notes on my paper (Teacher Model of Notes) as students complete the notes same notes on their own papers (Student Notes).

Beginning with Main Sequence Stars, I ask students to outline this section of the diagram with a black marker. I then explain: Main Sequence Stars represent 90% of the observed stars. Our sun, also called Sol, is included in the main sequence stars. 

Star Size

This is a great point to begin talking about star size. Referring to the Hertzsprung-Russel Diagram picture on the front of their envelopes (above), I ask: What do you notice when you look at the size of stars in the main sequence? (As the temperature increase, the star size increases.) We then label the stars above Sol as "bigger" and the stars below Sol as "smaller." These little labels along the way help students grasp the complex concepts! 

White Dwarfs

White dwarf stars are small hot stars. Scientists believe they are the leftover centers of old stars. They also believe that one day, our star will become a white dwarf star, but not for at least another 5 billion years. 

Giants & Supergiants

Giants are massive stars. They are very rare. Some giants are hot stars. What color are the hot stars? (blue) Other giants are cool stars. What color are the cool giants? (red) Regardless of their temperatures, all giants are so big that they are actually brighter than Sol. 

Supergiants are the same as giant stars, but they are much bigger and much brighter. 

Making Observations

Next, we move on to analyzing the H-R diagram further. I project and take notes on my page by rewriting student thinking: (Teacher Model of Observations). Students also take notes at this time: Student Example of the H-R Diagram Handout. Here's a list of student observations: 

  • Smaller suns (stars) in the main sequence are cooler and dimmer. 
  • All supergiants are bigger and brighter than the sun. 
  • Main sequence stars range from blue/hot stars to red/cool stars.
  • The bigger the star, the brighter it is (in the main sequence).
  • The hotter the star, the brighter (in the main sequence).


I am reminded of how much students can learn just by analyzing a diagram! 

Reflect & Apply

15 minutes

Before students reflect and apply on today's findings, I want students to explore how the size and distance of stars can affect the brightness of stars using flashlights. For this reason, I set up a quick demonstration for students to investigate how a smaller flashlight can appear to be brighter than larger flashlight. (Yesterday, students completed Flashlight Investigation #1 and today, students complete Flashlight Investigation #2). 

Flashlight Investigation Set Up

Prior to this lesson, I found Two Flashlights (One Big, One Small). I also made sure that each flashlight had new batteries as battery power can impact the brightness of flashlights and throw your entire demonstration off! 

I invite students to create the following t-chart on a lined sheet of paper: Flashlight Investigation T-Chart to encourage students to visualize the investigation using a diagram and to think about the investigation by drawing conclusions. Here's what the teacher model will look like when this investigation is complete: Flashlight Investigation #2 Teacher Model. As I model how to complete the t-chart throughout the investigative process, students also complete their own t-charts during this time: Example of Student Flashlight Investigation

Flashlight Demonstration

For the First Part of the Investigation, two student volunteers hold both flashlights at an equal distance from the white board: Different Sized Flashlights, Same Distance. I ask students to turn and talk: What do you notice? What conclusions can you draw about two stars that are the same brightness? After some time, we record a conclusion as a class (per student suggestions), "The bigger the star, the brighter it is." 

For the Second Part of the Investigation, two student volunteers hold the big flashlight further back and the smaller flashlight closer up: Different Sized Flashlights, Different Distances. Students observe that the smaller flashlight that is closer to the white board appears to be brighter than the larger flashlight that is further away from the whiteboard. Students agree to draw the following conclusion, "When a star is closer to Earth, it can appear brighter than bigger stars that are farther away."

This is an important understanding for students to grasp as it aligns with Disciplinary Core Idea, 5-ESS1-1, The Universe and its Stars: The sun is a star that appears larger and brighter than other stars because it is closer. Stars range greatly in their distance from Earth. 

Sharing Findings

Now that students have built meaning and understanding by observing, questioning, and exploring, it is important to provide students with the opportunity to apply their findings. For this reason, I invite students to use their research to answer the guiding questions, "What causes stars to be brighter? What causes some stars to appear brighter?"

I pass out a copy of Flashlight Investigation Findings (cut into half sheets) to each student and I provide the following prompt to get students started: Some stars are brighter because... 

Teacher Note: By asking students to develop explanations based upon evidence and research findings, I am supporting Science & Engineering Practice 7: Engaging in Argument from Evidence.


As students finish, many students volunteer to read their explanations aloud. This allows others more time to complete their explanations. It also provides me with the opportunity to provide feedback and encourage evidence-based arguments: Student Sharing Explanation.

Student Work

Here are a couple examples of student explanations. Most students grasped the concept that two stars can be the same brightness, but appear to be different brightnesses because of their sizes and distances from Earth. 


At the end of this lesson, students place the following items in today's envelope: