Classifying Stars (Christian Butcher)

Title: Star Light, Star Bright: Classifying Stars

Principle(s) Investigated: The Sun & Stars: Using Evidence to Argue for a Star Life-cycle

Standards:

• DCI: ESS1.A: The Universe and its Stars
  • The Sun is but one of a vast number of stars in the Milky Way galaxy, which is one of a vast number of galaxies in the universe.
  • Stars’ radiation of visible light and other forms of energy can be measured and studied to develop explanations about the formation, age, and composition of the universe. Stars go through a sequence of developmental stages—they are formed; evolve in size, mass, and brightness; and eventually burn out. Material from earlier stars that explode as supernovas is recycled to form younger stars and their planetary systems. The Sun is a medium-sized star about halfway through its predicted life span of about 10 billion years.
• Science and Engineering Principles:
  • Developing and Using Models: construct a scientific explanation based on valid and reliable evidence obtained from sources
• Crosscutting Concepts::
  • Patterns: patterns can be use to identify
  • Scientific Knowledge Assumes an Order and Consistency in Natural Systems
• Common Core:
  • SL.8.5 -- Integrate multimedia and visual displays into presentations to clarify information, strengthen claims and evidence, and add interest.
  • 6.RP.A.1 -- Understand the concept of a ratio and use ratio language to describe a ratio relationship between quantities.
  • 7.RP.A.2 -- Recognize and represent proportional relationships between quantities.

Materials:

• Hubble star field photo (https://www.spacetelescope.org/images/potw1624a/)
• Star Database (synthesized from Hygdatav3 Database available at http://www.astronexus.com/hyg)
• Additional Reading to learn more about the star field photograph: "The Stars at Night Are Big and Bright … Deep in the Heart of the Milky Way."
• I poached some elements from a few different resources:
  • Sun as a Star (NASA)
  • Graphing the Stars
  • An Interactive and Engaging Solution to the Hertzsprung-Russell Diagram
• 2 light-bulbs (same lumenosity) on plug fixtures with extension cords, and 1 dimmer switch.
• Bunsen Burner
• Infrared Camera (or Thermometer)

Procedure: Give a detailed explanation of the procedure and include diagrams if possible.

NOTE: The total of this teaching event will occur over two days with my students, but I'm going to skip to the juicy parts (plus it'll go a bit quicker with just 6 "students").

PREPARATION

My students know to pick up iPads as soon as they enter class. Whatever device your students use (if they do) have them log on to a collective QUICKWRITE like this one to respond during the lesson. (If your school doesn’t have the technology, you might have students write responses on a sheet of paper and check for completion, to make sure students are engaging with the lesson.)

ENGAGE

My students are coming out of an introduction to the Sun--and they understand the Sun is a star--so I’m going to start this lesson with a fundamental question:

“Are all of the stars just like the Sun?”

I poll my students (through the quickwrite) to engage them in the question. Now I’m going to engage them in observation by showing them a picture of the night sky and seeing if observation changes their opinions:

This is a generic image of the night sky. Notice the “teapot” of the sagittarius constellation outlined in the stars. It’s difficult to tell how different the stars are from this. There are some differences in brightness, but they all look pretty similar. So what happens if we take a closer look. I show my students this picture from the Hubble Space Telescope:

This is actually a “zoom in” on a portion of the sagittarius constellation, and it should be immediately apparent that stars come in a wide variety. After a short verbal reaction, I ask my students to respond on the quickwrite:

“What are some ways we could classify these stars? What qualities could we use to separate them?”

Most students point out the stars have a wide variety of brightnesses and colors. Now I ask them if they can explain all this variety. I get lots of shrugs, but that’s okay. The question itself leads me into the next phase of the lesson.

EXPLORE & EXPLAIN

I explain to students there are a number of ways to account for the variations we see in stars. First let’s examine brightness--I say to my students:

“Stars in our galaxy are spread out over a huge area. (Remember the Galaxy Song, “100,000 light years side to side”.) So it’s reasonable to conclude that some stars are close to us, and some are very far away. Could that affect how bright they appear?”

Note: With more advanced students I might go into parallax and how we measure the distances of stars, but for my class I find it’s reasonable to establish stars can be at various distances from Earth and move on.

Take out the two light-bulbs (one on a dimmer) to demonstrate how distance changes the apparent brightness of a light source. I grab one student volunteer to hold one of the twin lights in a distant corner of the room while I hold the other at a closer distance. Both lights are turned on and room lights are shut off. I ask students to pick which is brighter and respond in the quickwrite.

When responses are in, I have the distant student join me. Students should quickly see they are the same brightness. I ask the students to explain what they saw by responding to the question, “How did distance affect the brightness of the light?”

So distance explains some of the difference in brightness--we call that apparent versus absolute magnitude--but not all. With twin light-bulbs together, I lower the dimmer on one. I ask the students to answer which light is giving off more energy. A quick discussion should get every student to recognize that a brighter lighter gives off more energy. I ask them to explain the relationship between brightness and energy, “How does the release of energy affect the brightness of the light?”

Now I ask students to answer whether this difference in brightness (the dimmed vs normal at the same distance) would be apparent magnitude or absolute magnitude? There might be some confusion, but students should come around to the correct answer.

Students should now understand how apparent brightness is a product of the distance and energy, but absolute magnitude is only determined by energy release. To assess this I bring back up the Hubble photo and ask students to compare stars. First, I ask if we are comparing apparent or absolute magnitude. (Most should realize we are comparing apparent magnitude.) Then I pick two stars and have them decide which has more apparent magnitude.

Now I pick two different stars and tell the students to imagine they are the exact same distance from Earth. “If they are the same distance, are we comparing apparent or absolute magnitude?” When they realize we are comparing absolute magnitude, then I can ask them to decide which has greater absolute magnitude. (Hopefully they can do this. Otherwise, repeat as necessary.)

As a formative assessment, I ask “What is the only factor that affects absolute magnitude?” (They should answer energy. If not, review the material.) Have your students practice with this concept. Once you’re confident they have it down, move on to the next category:

COLOR.

To re-engage the students, I bring the Hubble photo up and ask them if they notice a pattern between color and brightness. (They should notice blue stars tend to be brighter, and red stars dim.) I ask them to predict what color might mean for a star. Review some of the answers, but you don’t need to reveal the correct one yet.

Now I bring out the bunsen burner. I ignite the burner on full blast and shut off room lights. I ask the students to share observations as I go through the demonstration. At full blast, the flame is blue, but when I turn it down the flame turns yellowish-orange. I ask the students to decide which is giving off more energy. (They should realize the blue.) I follow up by asking if they think the flames are the same temperature. (Probably will get a mixed response.) Now I use a thermal camera to show the temperature change of the flame. Again I ask students if both flames are the same temperature. (They better say no.) Then I ask which is hotter, blue or yellow. Once this is cleared up, I show them the Hubble picture. I point out two stars of different colors and ask them which is hotter. I repeat this several times as a formative assessment before moving on.

EXTEND/ELABORATE

Now that we understand that stars can be classified based on color (temperature) and brightness (magnitude), we can ask, “What might explain these differences?” Verbally ask your students if they can make an educated guess for why the stars are different.

I tell my students that an effective way to discover an underlying cause is to lay out all the information and see if there’s a pattern. If there’s no pattern, then it’s probably all random--or at least an explanation is hard to see. But if we see a pattern… There must be something that’s causing that pattern.

I tell my students that about a hundred years ago scientists got pretty good at measuring the distance of stars, and with that they were able to determine the absolute magnitude. I instruct my students to work with a table partner: one person will open this star data file on an iPad. Together they will evaluate a set block of stars and plot them on a graph based on absolute magnitude and color index. (I used a physical graph with my students, but for the sake of simplicity you’ll just have this powerpoint graph.)

The complete graph should look something like this:

(Note the y-axis is flipped from its traditional layout in an H-R diagram, so that negative is actually the greatest magnitude. You can reverse your axis is you want to avoid confusion, or you can reveal the flip later.)

Give your students a chance to examine the graph and ask them to identify any patterns. (You may need to review how a random plot compares to one with a pattern.) I ask them on the quickwrite, “Do you see any patterns?” I choose a student who replied “no” to justify their answer. Then I ask for a volunteer who said “yes” to explain what patterns they see. I ask students to describe a relationship they see between color and magnitude. I continue to guide students to identify a few patterns in the data and to conclude what that explains. Once I feel confident the students recognize the pattern, I drop the bomb on them:

The overlay an H-R Diagram, and explain how it was developed over a hundred years ago by scientists doing the exact same thing they’re doing. The HR Diagram is one of the most significant revelations in modern astronomy. It helped scientists to divide stars into categories, and eventually make predictions about the life cycle of a star.

I ask students to use the H-R Diagram to predict an explanation for the lifecycle of stars. I remind them that very hot and bright stars are in the top left, and cool dim stars in the bottom right.

After reviewing some responses, I introduce this quote:

"The flame that burns twice as bright burns half as long." Lau Tzu

I ask students replace the word flame with star see if this affects their prediction. I explain the meaning of the quote, by using the following images of a campfire to show how it begins very hot and bright, but ends relatively cool and dim.

I tell student to rewrite their prediction, and after I give them a minute to respond, I ask for a volunteer to share their explanation. I may ask other students to give feedback on the explanation. Hopefully the student is in the right direction, but regardless I use the moment to explain the scientific explanation for the life cycle of stars:

Main sequence stars star out their life hot and bright. They have lots of good fuel (hydrogen) and so they give off lots of energy. Gradually a star runs through its fuel--just like a campfire--moving through the sequence until it’s just dim embers. (I tell my class this is the cycle for the main sequence stars like our Sun, but the really big stars have a slightly different end of life, and we'll talk about that later.)

EVALUATE

As a formative assessment, I point out the Sun on the H-R diagram and ask students the students respond in an exit ticket (I do this through a google form) to the following questions:

1. How does the Sun’s absolute magnitude compare to a blue giant?
2. Which is giving off more energy, the Sun or the blue giant?
3. Which is hotter, the Sun or the blue giant?
4. Does the Sun have more or less apparent magnitude than the blue giant?
5. Predict how far the Sun is through its life.

Student prior knowledge: Students will already have a grasp of gravity and be aware the Sun is a star and that stars get their fuel by reacting atoms (mostly hydrogen) in their cores.

Questions & Answers: Give three thought-provoking questions and provide detailed answers.

1. Are all the stars just like the Sun?
   • Once we accept our Sun is a star, it's easy to imagine all of the stars in the sky are just like the Sun, but when we look closely we can see stars appear in a variety of brightnesses and colors. Using other knowledge (spectroscopy, luminosity) and deeper observation we can start to understand what the characteristics of stars and classify them.
2. What explains the differences we see in stars?
   • When studying stars, we focus on two characteristics: color and brightness. Color is an indicator of temperature. Brightness--when we correct for distance--is an indicator of the energy a star gives off. When we plot stars on a graph, we can see that most stars tend to be bright and hot, or cool and dim, while a small number of stars are both cool and bright or hot and dim.
3. Do stars last forever?
   • If we think of stars like a fire--technically there's no combustion like a fire, but there is a fuel being consumed--we can recognize that nothing can burn fuel. At some point the fuel will run out. This tells us that at some point a star is going to run out of fuel and die. But just like a human being goes through a complex life cycle from birth to death, a star's life is complicated and goes through distinct phases. We can see these phases by analyzing an H-R Diagram.

Applications to Everyday Life: Explain (don't just list) three instances where this principle can be used to explain other phenomenon.

Stars fill the night sky, and a nearby star powers the Earth, making life (and civilization) possible; but it can be easy to imagine the Sun and stars are unchanging because they exist on a timescale beyond human comprehension. In the case of this lesson, we start off investigating the phenomenon that upon close inspection the stars look very different, and we end revealing a whole new phenomenon: the stars go through a natural cycle from birth to death. We can apply this other principle in a number of ways:

1. Organism go through life cycles from birth to death. While the life cycle of living things is fundamentally different from a star, we can see in both a familiar pattern in nature wherein nothing lasts forever.
2. The life cycle of a star is in many ways similar to the life cycle of a fire. Both consume finite fuel, and after ignition the fire grows hotter and more intense until it’s consuming fuel at a maximum rate. Eventually there isn’t enough fuel and the fire loses intensity until it finally dwindles and dies.
3. In the life cycle of a star we begin to understand something else significant, all matter in the universe heavier than hydrogen (and a little helium) was produced in stars. As people we can look around and appreciate the abundant varieties of chemicals, and find our connection to the Universe through the stars that made our atoms.

Photographs: Include photos and diagrams that illustrate the how the investigation is performed.

IMPORTANT NOTE:

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