What Affects the Brightness of Stars
Lesson 2 of 9
Objective: SWBAT identify characteristics that make certain stars brighter than others.
The Why Behind Teaching This
Unit 6 teaches students about Earth's Place in the Universe. Standard 5-ESS1-1: Support an argument that differences in the apparent brightness of the sun compared to other stars is due to their relative distance from Earth, is one standard covered. Standard 5-ESS1-2: Represent data in graphical displays to reveal patterns of daily changes in length and direction of shadows, day and night, and the seasonal appearance of some stars in the night sky, is the other standard covered.
Throughout this unit, students will learn about classifying stars, patterns of stars, and the effects of rotation and revolution. We will be creating models, graphing data, tracing our shadows, and much more.
This specific lesson covers standard 5-ESS1-1 because it provides students with information about characteristics that affect the brightness of stars. Distance from Earth is one major factor that determines brightness but other factors such as size, temperature, age, etc. also affect the brightness. In this lesson students will relate stars to flashlights to help them think about each of these characteristics. They will then relate their ideas to information about real stars.
The goal of this lesson is for students to identify various characteristics of stars that could cause one to appear brighter than another.
Students will demonstrate success of this goal by correctly comparing two stars on the exit ticket given at the end of the lesson.
Preparing For The Lesson:
- Computer and overhead for showing the Youtube video
- Groups need whiteboards and markers for creating a list of things that affect brightness.
- A small flashlight and a large flashlight to use during the discussion
- A long area such as a hallway for shining the flashlights at different distances
- A copy of the What affects the brightness of stars question cards for each group
Introducing the Lesson with a Video
I begin today's lesson with a video on star size comparison. This video begins with Earth's moon and works its way through the planets, eventually ending with a variety of large stars. I chose this video because it provides students with a visual of the size of our star compared to other stars in the universe. Some of these stars are ones that were researched during the Classifying Stars Lesson. Knowing that there are much larger stars in the universe, will lead us into our discussion for the day, on why our sun is so bright compared to other stars if it is much smaller.
Our discussion over the video remains focused on the size differences. We do not go into detail about how the size impacts the brightness or any other characteristics. In the next section of the lesson groups create a list of things they believe impact the brightness of stars and I do not want any discussion that takes place here to influence their ideas.
Why Some Stars Appear Brighter Than Others
My classroom is already set up in table groups. Each group is a heterogeneous ability group made up of 3 or 4 students. I have groups set up this way to offer peer assistance during class activities and group work. I provide each group with a whiteboard and marker, and ask students to create a list of characteristics they believe affect the brightness of a star.
The three ideas that were consistent on all boards are listed in the picture of the whiteboard above. All groups included size, temperature, and distance from the sun. You can see in the video of group writing what affects brightness that some groups also included other items. One group in the video included age and another group, not in the video, included composition. We will be testing some of these ideas during the next part of the lesson.
Testing Our Ideas
I show students two flashlights, one small and one large. These will represent our stars for the following part of the lesson. I chose to use flashlights because each component that we are going to be discussing can be related back to the flashlights. A flashlight is something that students have used before and can make connections to better than stars, which they are unable to hold and manipulate.
I provide each group with the first card from the What affects the brightness of stars question cards. This card has them think about how the distance from Earth affects brightness. I begin with this card because I will be referring back to it with each of the other points considered on the other two cards.
For the first card, we consider how distance from the Earth affects brightness. I give groups a few minutes to discuss the questions and then we share out.
- If the two stars are the same distance from Earth, which would appear brighter? Groups said the larger star would appear brighter.
- If we move the larger flashlight far away and keep the small on close, which would appear brighter? Groups said the smaller one.
Because I am trying to put so much emphasis on the distance from Earth having the most effect on brightness, I want to demonstrate this part with the flashlights. Having a visual to refer back to will help the students retain this information better. I have a student hold both flashlights at the same level and shine them on a wall straight in front of her. As you can see in the video of student modeling brightness of stars with flashlights, the larger flashlight shines brightly on the wall and the small flashlight cannot even be seen.
I then take students out into the hallway so we can check their answer for the second question on the card. I have one girl hold the large flashlight way down the hall and shine it on a wall at the end. I then give the small flashlight to another student and have her walk towards the wall until her light is brighter than the large flashlight. As you can see in the video of students modeling brightness of stars in the hallway, the student with the small flashlight has to get very close to the wall for it to appear brighter.
The class returns inside and I provide each group with a copy of the second card from the set, which is on age. I allow a couple of minutes for groups to discuss the questions and then we share out.
- If the light bulb and batteries in both flashlights are new, which would appear brighter? Groups say the larger one would appear brighter.
- How would this change if the bulb or batteries in the larger were much older? Groups tell me that the small one with a new bulb and batteries would be brighter.
I relate this back to the distance from Earth by asking students, Which do you think would be brighter if the large one with an older bulb is closer to Earth and the small one with a new bulb is much further? Students tell me that they believe the large one with an older bulb would appear brighter because it is larger and because it is closer.
I provide each group with the third and final question card which has them considering how temperature affects the brightness. I allow a couple of minutes for groups to discuss the questions and then we share out.
- What causes one of the flashlights to be hotter than the other? I get many different answers for this question. Groups think it is how long the flashlight is left on, which can be true but that is not what causes the heat initially. I share a story about buying a light for a scent plug in I have. The wattage on the bulb I took out was 15W. I could not find the size I needed in 15W so I bought that size in 5W not thinking that it would make a difference. The scented wax that goes in the plugin does not melt with the new bulb...why? Students are able to tell me that the strength of the bulb is not high enough.
- What would this compare to in the stars? We discuss this together and come up with what gases are burning in the star because some gases may burn hotter than others. Some students wanted to say the bigger stars would produce more heat but I reminded them that stars of the same size are different temperatures just like light bulbs of the same size are different strengths.
- If the two stars are the same temperature, which would be brighter, Why? Groups tell me the larger one would be brighter because it is larger.
I connect this back to distance affecting brightness the most by asking students, If both stars are the same temperature, the small one is close to Earth and the large is much farther from Earth, which would appear brighter? They all tell me the small one would appear brighter because it is closer.
I summarize what we have discussed:
- There are many characteristics that affect the brightness of stars.
- If two stars are the same temperature, same age, and same distance from the Earth, the larger one will appear brighter.
- If the temperature, age, or size is different, the one closest to Earth will appear brighter, even if that is the smaller one.
The Truth Behind the Brightest Stars
To wrap up this lesson, I provide each student with a copy of the The Truth About the Brightness of Stars Chart. I show information located on Space.com about the brightness of stars. As I show the information, students fill in the chart.
As we go through them, I ask students questions to check for understanding:
- If Rigel is so much further away from Earth (1,400 lightyears), than Achernar (69 LY) and Procyon (11.4 LY), why might it appear brighter?
- The sun is a medium sized star but has the highest magnitude by alot. Why does it appear so bright to us?
I provide each student with a copy of the brightness of stars exit ticket. Students must compare the brightness of Vega and Arcturus on the exit ticket. In their responses, I am looking for students to demonstrate an understanding of what affects the brightness of stars. A Proficient answer would be something like this:
Arcturus is the brighter star. I believe it is brighter because it is larger.
Students who earn proficient would only give one correct answer but not go in depth. A student who earns exemplary would give more than one possible answer and explain their reasoning. An exemplary answer may look something like this:
Arcturus is the brighter star, but Vega is closer. Since I know that Vega is closer it must be a factor other than distance from Earth that causes Arcturus to appear brighter. Arcturus might appear brighter because it is larger than Vega, or they may be the same size but Arcturus could be a newer star and Vega older, or the composition or temperature of the two stars could be different causing one to appear brighter.
The majority of students earned PR, 4 out of 36 earned EX. There were also 6 students out of 36 that were not able to demonstrate proficiency because they were not able to identify which star was brighter or give at least one reason why it is brighter. All these students were ESE students and will need reteaching. I will probably partner then up with students who earned EX and have them reteach their peers.