LSST Teacher Guide


The H-R Diagram: A window to the stars

Standards

Introduction

This activity explores the idea that stars have fundamentally different properties (size, energy output, color and temperature). It introduces an important data visualization tool, the Hertzsprung-Russell (H-R) Diagram. Students will use LSST data for a star cluster to create an H-R Diagram, then use it to explore patterns in star characteristics.

Students will not have an appreciation for why the H-R Diagram is such a valuable tool unless they use it in the same way as astronomers, to find information about star properties, distances, relative ages and composition. We recommend that you consider at least one of the LSST extension activities listed at the end of this guide to extend the depth of students’ investigation and underscore the critical role of the H-R Diagram in astronomy.

Learning Outcomes

Students should develop an awareness that cool, dim main sequence stars are much more common than bright, hot blue stars.

Students should be able to use a star’s position on the H-R Diagram to infer properties about its relative temperature, brightness and size.

Students should become familiar with the range of star temperatures, brightness and sizes.

Prerequisite Concepts

  • Students should have previously been introduced to the H-R Diagram, if only as a graphic with labeled areas (Main Sequence, Giants, White Dwarfs). Alternatively, they can be provided with a labeled diagram to use as a reference while doing this investigation.
  • There is a relationship between color and temperature in stars. Red stars have the coolest temperature. As temperature increases, star colors change from red to orange, yellow, white, and finally, blue.
  • Two factors affect the energy output (luminosity) of a star: temperature and surface area (size). Stars with higher temperatures and larger surface areas have greater energy output.

Level and Time


Introductory. 1- 2 hours or class periods
.

More information about timing options for this investigation.

NGSS Disciplinary Core Ideas

*HS-ESS1.A: The study of stars’ light spectra and brightness is used to identify compositional elements of stars, their movements, and their distances from Earth.

 
“Nuclear fusion within stars… releases the energy seen as starlight… Stars go through a sequence of developmental stages—they are formed; evolve in size, mass, and brightness; and eventually burn out.” A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas , pp. 173-174.

Practices and cross-cutting concepts for this investigation may found on the NGSS assessment rubric link.

Teacher Notes


Some star cluster data sets may not reveal the classic distribution of stars shown on an H-R Diagram. A very young cluster may display few giants or no white dwarfs. A very old cluster may be missing the uppermost part of the main sequence. By comparing the H-R Diagrams of the stars clusters generated by a number of other class members, the general patterns will become evident.

Common Student Misconceptions


The H-R Diagram is not an actual representation of the physical arrangement of stars in space.
 Each point on the diagram represents one star.

Bridge to learning: The online notebook displays a color image of a star cluster alongside its H-R Diagram. Direct students to use the built in interactive tool to click on individual stars in either the image or the graph. Ask questions such as “Where are the blue stars on the H-R Diagram? (left side) Where are the blue stars in the cluster image? (randomly dispersed)

Almost all stars are white.

Bridge to learning: This preconception results from the fact that humans see all but the brightest stars as white, due to the fact that the cones (color receptors) in our eyes need a considerable amount of light in order to function. By examining both the data on the H-R Diagram and the star cluster image, students will develop an appreciation for the range of colors.

Common Student Questions

Blue main sequence stars are more difficult to form and they have relatively short lifetimes when compared to other colors.

Learn More

Color is a function of temperature. When white dwarfs initially form, they may look blue, but over time, their color will change as they cool. Does that mean there could be other colors of white dwarfs? Yes. The coolest white dwarf star detected has a surface temperature of about 4900K, making it orange in color. But another problem is that as temperature drops, so does luminosity, so seeing these faint, cool white dwarfs is not so easy.

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It’s possible for a star to have a peak blackbody curve in the violet range, but this sort of star would also produce high amounts of blue light. Since our eyes are more sensitive to blue, it would seem blue to us.

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Our Sun actually outputs more light in green wavelengths than any other color, but since it is emitting all the other colors in relatively even amounts, they all mix together to look white to our eyes. It’s only when there’s a great imbalance of color (for example, much more blue than red) that the stars appear to be blue or red, etc.

Learn More

The groups of stars represent different stages of stellar evolution. Main sequence stars are stable, hydrogen-fusing stars. A star spends about 90% of its lifetime in this phase, so that’s where most of the stars are located. Giants (dying stars) and white dwarf (dead) stars occupy other discrete areas. Another way to look at it is that there are groupings of temperatures and luminosities that naturally result from certain stellar processes, such as hydrogen-burning, helium burning or stars that have ceased fusion.

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The main sequence is a sort of “mass sequence”. Stars of various masses produce the characteristic range of temperatures and luminosities due to their rate of hydrogen-fusing. A star spends about 90% of its lifetime in this phase, so that’s where most of the stars are located.

Learn More

It’s a tradition. The original diagram published by Henry Norris Russell arranged the stars along the x-axis by spectral classes from B through M. A star of spectral class B had a blackbody spectrum that peaked in short (blue) wavelengths, while the blackbody spectrum of a class M star peaked longer (red) wavelengths. The (peak) wavelengths actually would increase from left to right, in traditional order. Later, astronomers began listing temperatures along the x-axis, and that works the opposite way. A blue star has a large temperature value but a small peak blackbody wavelength.

Instructional videos


Introduction: Stars (1 minute) depicts images of stars and clusters, and asks the questions, “How are stars different from each other, and what may cause these differences?”

Assessment: They Might Be Giants (3 minutes without pauses) shows a group of students discussing what they learned about stars and the H-R Diagram. Your students are periodically asked to vote on the correct idea and to explain why.

Options for Differentiation


This activity may be differentiated by assigning the difficulty level of the data: (easy, average or complex).

NGSS Assessment Rubric and sample questions for assessment

The above questions may be added to quizzes or the student notebook investigation.

Background


The H-R Diagram is a plot of the temperature of a star (on the x- axis) vs. its energy output (on the y-axis). In this investigation, stellar energy is listed in luminosity values. There are many variations of this plot, with additional or alternate descriptions along both axes.

Most of the stars on an H-R Diagram occupy a region from upper right to lower left, the Main Sequence. The stars above the Main Sequence (the giants and supergiants) are brighter than Main Sequence stars because of their larger size, while the stars below it (the white dwarfs) are dimmer due to their very small size.

Open Stax Astronomy textbook: The H-R Diagram

LSST Extension Investigations

Sizing up the Sun compares the Sun to other stars and develops an understanding of how the 
H-R Diagram may be used to determine the life span, mass, and size of stars.

Finding the Distance and Age of a Star Cluster explores how stars evolve using the same tools and methods as astronomers to determine distance to a star cluster and its approximate age.

Ideas for Further Study

  • Examine the spatial distribution of clusters vs. their size, age or distance
  • Compare globular to open clusters – their relative age, size, distance, or appearance
  • Study a specific sub-population of the H-R Diagram with different clusters. Examples could include planetary nebulae, AGB stars, blue stragglers, Cepheids, Young Stellar Objects, etc.
  • Construct and compare H-R Diagrams of extended clusters, or dwarf galaxies
  • Study star clusters in the Large Magellanic Cloud or Small Magellanic Cloud

NGSS assessment for the H-R Diagram: A Window to the Stars

This rubric has been designed to match the learning outcomes of this investigation. It may be used in conjunction with other assessment activities described at NGSS Assessment on the Education Hub. A customizable version of this rubric is also found there.

Scale for scoring:

  • 0 Student demonstrates no growth, application or understanding, even with major prompting and assistance.
  • 1 Student demonstrates partial or incomplete growth, application or understanding, but only with major prompting and assistance.
  • 2 Student demonstrates partial or incomplete growth, application or understanding, with only minor prompting and assistance.
  • 3 Student demonstrates competency in application or understanding.
  • 4 Student demonstrates outstanding mastery of application and understanding.

NGSS Assessment Rubric Novice 1 Point On the way 2 Points Competent 3 points Advanced 4 points
Practices
1 Developing and using models
2 Analyzing and interpreting data
3 Obtaining, evaluating, and communicating information
4 Using mathematical & computational thinking
5 Engaging in argument from evidence
Disciplinary Core Ideas
HS-ESS1.A
Crosscutting Concepts
1 Patterns
2 Scale, proportion and quantity
3 Energy and matter

Additional Questions for assessment

These questions may be used for class discussion or added to quizzes or the student notebook.


Question
10 Sketch an H-R diagram. Label the axes and major regions of your diagram. On the graph axes, draw arrows to show the direction of increasing values.
11 There are two groups of red stars on the H-R Diagram. Compare and contrast their properties.
12 Describe the characteristics of the most common type of main sequence star.
13 Select one star near each of the four corners of the graph. Label the properties of each of the four stars (Hot/cool, bright/dim small/large, blue/red).
14 How would a star’s luminosity change if its temperature decreased?
15 Summarize the characteristics of stars on the main sequence. Include a description of their trends in terms of luminosity, temperature and size.
16 What physical property of stars could explain why stars in the upper right of the H-R Diagram are brighter than stars in the lower right?
17 The white main sequence star Altair has 11 times the Sun’s luminosity and the white dwarf Sirius B only has about 3% of the Sun’s luminosity yet they have almost the same surface temperature. Explain how this is possible.
18 Stars in the main sequence are fusing hydrogen. Since they are all fusing the same element, what physical property could explain why some are hotter than others? .

Questions – Standards Correlation

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