A map of the stars Teach article

Author(s): Umaimah Muffy

From notebook sketches to space-telescope data: explore how mapping starlight using the Herzsprung-Russell (H-R) diagram helps us trace the life stories of stars across the universe.

The Hertzsprung-Russell (H-R) diagram displays stellar properties such as luminosity as a function of surface temperature, revealing distinct regions like the main sequence, red giants, and white dwarfs.
©ESO

Reading the sky through data 

Some stars blaze blue, others glow red. Some shine brilliantly, dominating the night sky, while others barely flicker. If you could draw the life story of every star in the universe, what would it look like?

Astronomers answer this question with a remarkable tool: the Hertzsprung-Russell (H-R) diagram, often described as “astronomy’s map of the stars”. Developed over a century ago by Ejnar Hertzsprung and Henry Norris Russell, this simple diagram shows the luminosity of a star against their surface temperature, revealing hidden patterns that explain how stars are born, evolve, and die.[1] Learn more about the historical background of the Hertzsprung-Russell diagram in info sheet 1: historical roots.

Each point on the diagram represents a star at a particular stage in its life. As stars evolve, they move across the chart along different paths depending on their mass. For example, some stars move from the bright, hot regions of young blue stars towards cooler regions of the diagram, where they may evolve into red giants and eventually end their lives as white dwarfs.

In this activity, students will learn how to build their own H-R diagram using real stellar data – just like astronomers do. They will thereby discover how a single chart can unlock the grand story of our galaxy, from nearby stars to the discoveries made by the Gaia space mission, an ESA observatory mapping the Milky Way.[2]

Activity 1: Plotting the Hertzsprung-Russell diagram

The estimated duration for this activity is 20–25 minutes. 

H-R diagram generated in Python using Gaia Data Release 3 (DR3) dataset.
Image courtesy of the author

In activity 1, students recreate their own H-R diagram using real data from 10–20 well-known stars, such as the Sun, Sirius, Vega, Betelgeuse, and Rigel. 

The activity can be done in different ways:

  • by hand on graph paper 
  • OR in Excel or Google sheets (using scatter plots) 
  • OR using Python (or Google Colab for a dynamic digital version)

Materials

Procedure

  1. Use the following stellar data:
    1. Temperature (T or Teff): The surface temperature of a star, which determines its colour. Blue stars are hotter, red stars are cooler (x-axis).
    2. Absolute magnitude (Mᵥ): A star’s brightness if it were placed at a standard distance of 10 parsecs from Earth (y-axis).
  1. Plot each star as a point on the graph.
  2. Optionally, colour-code points by spectral class to add visual contrast.
  3. Observe the patterns that emerge: the main sequence, red giants, and white dwarfs.
  4. Ask students to answer the questions below in the activity 1 worksheet. Possible answers to these questions are included in the activity 1 answer sheet.
    1. Why do most stars fall along a single curved band – the main sequence – instead of being scattered randomly? 
    2. What might this suggest about how stars live and evolve?
H-R diagram constructed in Excel using data from the brightest stars. The data is available here.
Image courtesy of the author

Info box:

  • Temperatures are approximate and derived from standard spectral class-temperature mappings, suitable for classroom plotting.
  • Absolute magnitudes are taken from established bright-star catalogues.
  • These approximations are accurate enough to reproduce the characteristic shape of the real H-R diagram.

Activity 2: Evolutionary track of the Sun

In activity 2, students trace the evolutionary track of the Sun on the H–R diagram. The evolutionary track is the path a star follows on the H-R diagram as it ages and changes in temperature and brightness. 

Estimated time: 20–25 min

Materials

  • H-R diagram template (printed or digital from activity 1)
  • “Evolution of the Sun” diagram or reference images (see below)
  • Coloured pencils (if printed) or digital annotation tools
  • Activity 2 worksheet

Procedure

  1. While looking at the H-R diagram together, briefly recap with your students temperature, luminosity, and stellar classes.
  2. Ask students to locate and label the Sun’s position at key evolutionary stages on the H-R diagram, using the reference materials or guided hints.
    1. Protostar
    2. Zero-age main sequence
    3. Current main-sequence position
    4. Red giant phase
    5. Helium-burning/Horizontal branch (if included)
    6. White dwarf
  3. Ask them to complete the table in the activity 2 worksheet , answering the following three questions for each of these stages (answers can be found in the activity 2 answers sheet):
    • How does the Sun’s brightness change from one stage to the next?
    • How does its temperature change from one stage to the next?
    • What processes inside the Sun could explain these shifts?
  4. Let students write a short summary on how a star’s life stages determine its track on the H-R diagram.
Evolution of the Sun from the main sequence to white dwarf.
Image: Szczureq/Wikimedia Commons, CC BY-SA 4.0  

Uncovering the life stories hidden in the stars

Now that the students have plotted the stars and observed the patterns, it’s time to understand what the diagram reveals about the life of each star. The clusters and sequences that the students discovered are more than just points on a graph. They reveal fundamental properties of stars and how they evolve.

1. Understanding stellar properties

The H-R diagram plots stellar surface temperature against luminosity, which represents the total energy output of a star per unit time and reflects its intrinsic brightness. Temperature (or spectral type) is plotted on the x-axis and luminosity (or brightness compared to the Sun) is plotted on the y-axis. Despite only having these two quantities, every point on the diagram carries hidden information about the star’s colour, size, and stage of life.

Good to know:

Astronomers never measure a star’s temperature directly. Instead, they analyse how much light the star emits in different colours, also known as its spectrum. From these colour patterns, they can infer the star’s temperature, classify it, and place it on the H-R diagram.[3]

2. Mass and life stage

Each star’s position on the H-R diagram is largely determined by its mass, which dictates how it lives and dies. More massive stars live fast and die young, while smaller stars like the Sun evolve slowly and end quietly. The main sequence on the H-R diagram is like a stellar highway, showing stars of different masses in the prime of their lives. The clusters of red giants and white dwarfs represent what happens when stars leave that highway.

3. Stellar evolution

By reading the H-R diagram, the life story of stars can be followed:

  • Main sequence: the diagonal band on the H-R diagram where most stars (including the Sun) spend the majority of their lives fusing hydrogen in their cores
  • Red giants: large, cool, and luminous stars in a late stage of evolution that have exhausted their core hydrogen and expanded 
  • White dwarfs: small, dense, faint stellar remnants; left after a star like the Sun has shed its outer layers

The plotted activity demonstrates these stages visually, showing how stars move across the diagram as they age.

Evolution of a star along its lifetime depends on its initial mass.
Image courtesy of students.um.edu

Activity 3: Dive deep – mapping the galaxy with Gaia DR2

After plotting their own H-R diagrams, students have seen how patterns emerge in star brightness and colour. Now imagine doing this for over two billion stars! That’s exactly what astronomers using ESA’s Gaia mission have achieved.[2]

The Gaia mission, a European Space Agency project mapping the positions, distances, and motions of over two billion stars in the Milky Way, has produced the most detailed H-R diagram ever made. Using data from Gaia Data Release 2, astronomers plotted more than four million stars located within 5000 light-years of the Sun (Data Release 2).[4] Compared with the earlier Hipparcos data, Gaia’s diagram reveals remarkable new features: a thin main sequence, binary star tracks, red clumps of helium-burning stars, and even splits in white dwarf branches.[4,5]

By selecting stars based on their proper motion, astronomers can separate the galaxy’s main populations:

  • Thin disk: young, metal-rich stars
  • Thick disk: older, metal-poor stars
  • Halo: twin main sequences showing different chemical histories[6]
Hertzsprung-Russell diagram from the Gaia mission. 
Image taken from Ref. [5]

Gaia has even shown that our galaxy isn’t static: it ripples and wobbles, like a giant wave spreading from its centre (like a pebble dropped in water). By comparing your small H-R diagram with Gaia’s billion-star map, you can see how simple data visualization uncovers profound patterns and grows into cosmic discovery.[7] It shows how age, composition, and motion shape the Milky Way, and that stars are not randomly scattered but follow physical laws dictated by mass, energy, and time.

The following activity takes 15–20 min.

Materials 

Procedure 

Encourage students to think about the patterns they uncovered and what they reveal about the way science works. Possible answers to these questions are included in the activity 3 answers sheet.

  1. What trends do you observe between temperature and luminosity on your diagram?
  2. Which stars do not follow this trend, and what might their positions reveal about their stage of life?
  3. How do your plotted results compare with Gaia’s billion-star H-R diagram?
  4. How could limited or uncertain data change the apparent shape of the H-R diagram and what biases might this introduce?
  5. How does metallicity (element composition) influence star colour and brightness?
  6. What surprised you most about your plotted data and why?
  7. If you had to explain the H-R diagram to someone using only three sentences, what would you say?

References

[1] Introduction to the H-R diagram: https://www.cosmos.esa.int/web/cesar/the-hertzsprung-russell-diagram

[2] The Gaia mission by the European Space Agency: https://www.esa.int/Science_Exploration/Space_Science/Gaia

[3] Video on how to construct H-R diagrams: https://science.nasa.gov/asset/hubble/constructing-the-hertzsprung-russell-diagram-for-globular-star-cluster/

[4] Brown AGA et al. (2018) Gaia Data Release 2: Summary of the contents and survey properties. Astronomy & Astrophysics 616. doi: 1051/0004-6361/201833051

[5] A Gaia Hertzsprung-Russell diagram from 2018: https://www.esa.int/ESA_Multimedia/Images/2018/04/Gaia_s_Hertzsprung-Russell_diagram

[6] Hertzsprung-Russell diagrams from Gaia Data Release 2: https://www.cosmos.esa.int/web/gaia/gaiadr2_hrd

[7] The galaxy’s great wave discovered by the Gaia mission: https://www.esa.int/Science_Exploration/Space_Science/Gaia/Gaia_discovers_our_galaxy_s_great_wave

Resources

Author(s)

Umaimah Muffy, a space sciences graduate, is an aspiring writer and science communicator. She blends her love for astronomy, art, and storytelling to craft engaging science narratives that explore how art can deepen scientific understanding. She works in marketing and engagement at the Pakistan Maritime Museum, where she organises educational events to inspire curiosity and learning. She lives with two curious cats who are her devoted muses and critics.

Review

This article is a good opportunity for teachers and students to learn about stellar astrophysics through one of the most iconic relations: the Hertzsprung-Russell Diagram (H-R diagram). This simple relationship allows astrophysicists to study the life cycle of stars and to understand their physical and chemical evolution through the years, ultimately providing insight into the history of galaxies and the Universe itself.

The diagram can be used to explain and emphasize the students’ knowledge of math. How can they create a graph and interpret the information it contains? More advanced students can also use their programming skills to start learning how to create plots using programming languages. It gives students the chance to understand how a star works and how it evolves chemically. To relate to other fields, it can also be used to explain why we would prefer to build fusion reactors rather than fission ones, as this is easily understandable when you see how stars produce their energy. It is also useful for medicine and spectroscopy applications, which are useful for studying and characterising materials. This helps to reinforce concepts of light and the electromagnetic spectrum and their applications in everyday devices.

Jorge Andrés Villa Vélez, CERN, Switzerland/France

License

CC-BY
Text released under the Creative Commons CC-BY license. Images and supporting materials: please see individual descriptions.

Related articles

Teach

What are stars made of?

Find out how we know what the Sun (and stars) are made…

Physics, Chemistry, Astronomy / space

         

Understand

Fusion in the Universe: when a giant star dies…

Péter Székely from the University of Szeged, Hungary, and Örs Benedekfi from the European Fusion Development Agreement in Garching, Germany,…

Physics, Astronomy / space