The Curriculum Problem

The 2019 revision of the NRW Gymnasium physics curriculum for Sekundarstufe I requires students to be able to describe, in broad outline, the typical stages of stellar evolution. This is new territory for many teachers — it is not a topic that would have appeared in teacher education programmes of ten or twenty years ago, and few teachers have personal experience with it from their own school or university courses.

More fundamentally: stellar evolution is a topic where the usual experimental approach does not work. You cannot compress an interstellar gas cloud in a classroom. You cannot observe a star form in real time. The timescales involved are tens of millions to billions of years; the spatial scales are measured in light-years and astronomical units. The experimental toolkit that works for optics, mechanics, and even much of electromagnetism simply does not apply.

This creates a genuine pedagogical challenge. Students have strong interest in astrophysical topics — the ROSE study documents this consistently — and stellar evolution involves physical concepts that are curriculum-relevant (gravity, pressure, energy, radiation). But the standard path from “concept” to “experiment” to “understanding” is not available in the usual form.

Two approaches are described here. One uses what students do have — smartphones and household materials — to model the physics of stellar formation through analogy. The other accepts that some physics is better learned through structured play, and designs accordingly.

DIY Experiments for Stellar Formation

The physics of star formation starts with an interstellar gas cloud and the competition between gravity and pressure. A cloud collapses when gravity wins: specifically, when the cloud is massive enough (or cold enough) that gravitational attraction overcomes the thermal pressure of the gas. This is the Jeans criterion, and it is the quantitative condition that separates clouds that will form stars from clouds that will disperse.

The qualitative version is accessible to secondary school students: a dense, cold, massive cloud is more likely to collapse than a diffuse, hot, small one. Once collapse begins, it is self-reinforcing — increasing density increases the gravitational attraction, which drives further compression, which increases the density further.

Two DIY experiments were developed to give students a physical encounter with the key concepts, using materials that can be assembled at home or in school without specialist equipment.

Experiment 1: Compression and heating. When a gas is compressed, it heats. This is directly measurable with the temperature sensor in a smartphone (or a separate Bluetooth thermometer connected to phyphox) and a simple compression apparatus — a syringe, a sealed container, or an inflation device. Students observe the temperature rise during compression and temperature drop during expansion, establishing the qualitative relationship. In the stellar formation context: the collapsing gas cloud heats as it compresses, which is why a protostar is hot long before nuclear fusion ignites.

Experiment 2: Self-reinforcing compression. A simple model of the positive feedback loop in gravitational collapse: a weighted ball in a flexible container, which compresses a small spring or air cushion. The more the ball compresses the cushion, the further it falls. Students can explore the threshold conditions under which the system reaches a stable equilibrium versus continues to compress indefinitely — a qualitative model of the Jeans criterion.

Both experiments are designed to be performed with available materials at the DIY/home level. The smartphone’s sensor integration via phyphox provides quantitative data where possible, maintaining the connection to real measurement that is a design principle across all the astro-lab experiments.

Why Stellar Evolution Is Hard to Experiment With

A methodological note worth making explicit: the shift from direct experiment to analogy experiment to board game is not a retreat from rigor. It is a recognition that different kinds of physical and conceptual content require different pedagogical approaches.

For exoplanet detection, we can build a genuine analogy: the physics of a planet blocking a star’s light and a ball blocking a lamp’s light are structurally identical. The analogy experiment produces data whose interpretation follows the same logic as the real scientific data.

For stellar evolution, the analogy is weaker. The compression of a gas syringe models one aspect of the collapse (temperature increase) but not the self-gravitating dynamics, the radiation pressure that eventually halts collapse, or the nuclear ignition that defines the transition from protostar to main sequence star. No tabletop experiment captures the whole process.

This is important to tell students: the experiment models this aspect, and not those aspects. Making the model limits explicit is part of the scientific literacy the unit is supposed to develop.

“Staub und Sterne”: A Board Game for Stellar Evolution

The board game “Staub und Sterne” (Dust and Stars), designed by Miriam Küpper and Alexander Küpper, takes a different route to the same content.

Games have been used in physics education in all phases of a lesson: as entry points (introducing a topic without immediately constraining it to a specific physical question), as vehicles for content acquisition, and as reinforcement and assessment tools. For stellar evolution specifically, the argument for a game is strong: the content involves a branching process with multiple pathways depending on a single initial parameter (mass), it is cyclic (the remnant of stellar death seeds the gas cloud that forms the next generation of stars), and it is inherently dynamic — the drama of a supernova is hard to convey through a diagram but easy to convey through play.

The target audience is years 7–8 (or year 8–9 depending on the school’s internal curriculum placement). The learning objectives:

• Describe the stages of stellar evolution as a function of mass
• Name the possible end states (white dwarf, neutron star, black hole) and the stellar paths that lead to each
• Describe stellar evolution as a cyclic process: the gas cloud produced at the end of a star’s life can, under the right conditions, seed the formation of new stars

The game “Staub und Sterne” (the name translates as “Dust and Stars”) has players navigating a star through its lifecycle, with the key branching decision determined by the star’s initial mass. A low-mass star follows one path; a high-mass star follows another. Both paths end in a stellar remnant and a dispersed gas cloud — raw material for the next cycle.

The game design incorporates the research on flow experience in learning: cooperative or competitive play, immediate feedback on decisions, the kind of engaged attention that is rare in conventional physics lessons and that the ROSE study data suggest is precisely what is missing for many students in physics classrooms.

A Note on What Experiments Cannot Reach

There is a broader point here that the exoplanet posts sidestep because the experiments for exoplanet detection are so unusually good. For most astrophysics — stellar evolution, galactic dynamics, cosmology — there is no analogy experiment that captures the full physics. The observable has been observed, the theory has been developed, but the pedagogical problem of how to give students a physical encounter with that knowledge remains genuinely difficult.

Games, simulations, interactive visualisations, and structural analogies all have a role. Each of them is a partial solution; none of them is what a well-designed experiment is. Knowing which approach fits which content, and being honest with students about the limits of the model you are using, is part of what physics teaching requires.

The experiments described in this post are a start on one small part of that problem.

The exoplanet experiments from the same project are described in the astro-lab@home, Hunting Exoplanets with Your Phone, and Fremde Welten posts.

The misconceptions students bring to stellar evolution — about the Sun, gravity, nucleosynthesis, and the language of astronomy — are documented in detail in Please Stop Saying the Sun Is on Fire, written as a companion to the September 2020 teacher training session that motivated much of this work.

References

Spicker, S. J., & Küpper, A. (submitted). Einfache DIY-Experimente zum Verständnis der Sternentstehung für den Physik- und Astronomieunterricht sowie zu Hause. Astronomie+Raumfahrt im Unterricht.

Küpper, M., & Küpper, A. (2022). Sternentwicklung spielerisch verstehen: Konzeption eines Brettspiels für den Physikunterricht der Sekundarstufe I. Presentation at AG Lehrerfortbildung, Universität zu Köln.

Elster, D. (2008). Was interessiert Jugendliche an den Naturwissenschaften? VFPC Verein zur Förderung des physikalischen und chemischen Unterrichts.

MSB NRW (2019). Kernlehrplan für die Sekundarstufe I — Gymnasium in Nordrhein-Westfalen: Physik. Ministerium für Schule und Bildung NRW.

Ward-Thompson, D., & Whitworth, A. (2011). An Introduction to Star Formation. Cambridge University Press.

Why Misconceptions Are Not Just Wrong Answers

Before the list, a clarification that matters pedagogically.

When education researchers say “misconception,” they do not mean a random error or a gap in knowledge. A misconception is a stable, self-consistent mental model that students actively use to interpret new information. It persists not because the student hasn’t heard the correct explanation but because the incorrect model handles a wide range of everyday experience reasonably well.

“Fire is a thing that makes heat and light and consumes fuel” is a perfectly adequate mental model for everything a student encounters outside a physics class. It explains candles, campfires, gas hobs, and car engines. The fact that it also leads the same student to conclude that the Sun “burns” in the chemical combustion sense is not a failure of intelligence — it is the natural extension of a model that works.

The implication, which Bransford, Brown, and Cocking put plainly in 2000: if you ignore what students already believe and simply present the correct model, “the understanding they develop can vary substantially from what the instructor intended.” The new information gets interpreted through the existing model, not in place of it. You end up with students who can repeat “the Sun fuses hydrogen” while still, in their mental model, imagining it as a very large and very hot fire.

With that said: here is the list.

The Sun Is Not a Star

This one leads because it is the most structurally interesting.

Bailey et al. (2009), in a study of students’ pre-instructional ideas about stars and star formation, document the following category of response: the Sun is a special kind of astronomical body with its own distinct properties. It is not a star. Stars are the things you see in the night sky. The Sun is different.

This is not an isolated finding. Schecker et al. (2018) document the same pattern in the German context. Students who know perfectly well that “the Sun is a star” as a stated fact will nonetheless, when asked to reason about stellar properties, implicitly exempt the Sun from those properties. Stars are far away, they are small and faint, they are cold and distant. The Sun is close, large, and bright. Ergo the Sun cannot really be a star, whatever the textbook says.

The pedagogical consequence is that teaching stellar evolution to students who hold this model requires first collapsing the Sun/star distinction — otherwise everything that follows is about something unfamiliar and distant rather than about the object eight light minutes away that we can observe in detail.

A companion misconception: all stars are smaller than the Sun. This is the inverse problem. Students who correctly classify the Sun as a star but have only seen stars as faint points of light infer that stars must be small. The Sun, which they know to be large, therefore cannot be a typical star. Betelgeuse — a red supergiant with a radius approximately 700 times the Sun’s, which if placed at the Sun’s position would extend past the asteroid belt — tends to produce strong cognitive dissonance when it is first encountered.

The Sun Is on Fire

The combustion model of stellar energy is, empirically, the most common student conception and the hardest to dislodge.

From Favia et al.’s misconception inventory, translated loosely:

• “The Sun is made of fire.”
• “Stars run on fuel: petrol or natural gas.”
• “The Sun is made of molten lava.”
• “The Sun is a heat-planet.”

Bailey et al.’s quantitative data: when asked how stars produce light, 32% of students described chemical burning. A further 28% described unspecified “chemical reactions.” Only 7% named nuclear fusion. Only 3% could both name fusion and correctly connect it to the production of light.

The combustion model is coherent and consistent. It gives you a mechanism (fuel + oxygen → heat and light), a timescale (stars eventually run out of fuel and go dark), and a product (visible light and heat). What it cannot handle is the scale: the Sun has been burning for 4.6 billion years and has approximately 5 billion years of fuel remaining. Chemical combustion at the Sun’s luminosity would exhaust any chemically plausible fuel supply in tens of thousands of years. This is the crack in the model that fusion fills — not by saying “the Sun burns differently” but by replacing the entire energy mechanism with one that operates at scales the combustion model cannot reach.

One related misconception worth noting explicitly: the Sun is hottest at its surface. This is the intuitive model — things are hot near the fire and cooler further away. The corona’s temperature of a million Kelvin, far above the photospheric 5,778 K, violates this so thoroughly that it remained an active research problem for decades (and, in some senses, still is). Students encountering coronal heating for the first time do not usually reject it, but they do find it genuinely strange in a way they cannot articulate — which is the signature of something colliding with a stable prior model.

Gravity Only Works When Things Move

The gravity misconceptions documented in the research literature are worth treating separately because they have direct consequences for understanding stellar formation — which depends entirely on gravity acting on stationary or slowly drifting gas clouds.

The relevant findings:

Gravity requires motion (Palmer, 2001). A significant proportion of students believe that gravity only acts on objects that are in motion. A stationary object is not subject to gravitational attraction. A table sitting on the floor: fine, no gravity needed. A gas cloud drifting slowly through space: also fine. A gas cloud being compressed by gravitational self-attraction: this requires gravity to act on particles that are not yet moving, which the model cannot accommodate.

Force implies movement (Gunstone & Watts, 1985). The more general version: forces produce motion, and where there is no net motion, there is no net force. The concept of force balance — two equal and opposite forces summing to zero net force, with the object not moving — is not available to students holding this model. It is hard to overstate how consequential this is for astrophysics. Almost every stable astrophysical structure — a main-sequence star, a planetary orbit, a galaxy’s rotation — is a force balance. Students without the concept cannot reason about any of them correctly.

Gravity only acts on Earth (Bar, Brosh, and Sneider, 2016). Students in the space context often reason that gravity is a property of Earth specifically. In space, things are “weightless” — and weightlessness is interpreted as the absence of gravity rather than as the experience of free fall in a gravitational field. The result: gravity cannot be the mechanism by which an interstellar gas cloud collapses, because gas clouds are in space and gravity does not work there. Asghar and Libarkin (2010) found that only one in five non-physics college students could correctly describe gravity as an attractive force between masses, using the correct vocabulary.

These are not fringe findings. They are the majority conception at the pre-instructional stage. Any unit on stellar formation that opens with “gravity compresses the gas cloud” is speaking to students who mostly do not believe that gravity can do that to a gas cloud in space.

Metals Always Existed

This misconception is my personal favourite because it requires no incorrect intuition — it requires an absence of information that most people have never had reason to acquire.

Students and adults who have not encountered stellar nucleosynthesis simply have no model for where heavy elements come from. Asked directly, a common response is that metals “always existed” — they are a feature of the universe, present from the beginning. The alternative framing: “stars create matter from nothing” — which captures the sense that something is being generated, without a mechanism.

The correct picture: the Big Bang produced primarily hydrogen and helium, with trace amounts of lithium and beryllium. Every heavier element — including all the carbon in your body, all the iron in your blood, all the oxygen in every breath — was synthesised in a stellar interior or in a supernova. The gold in a wedding ring was produced in a neutron star merger. We are, in the precise sense of the phrase, made of star stuff; but not because stars are somehow magical, because the nuclear physics of stellar interiors and violent stellar deaths is the only process in the universe that can manufacture these elements.

This has a direct implication for stellar evolution education: if students believe metals always existed, the cycle of stellar death and new star formation — in which dying stars enrich the interstellar medium with heavy elements that become part of the next generation of stars and their planets — loses most of its meaning. The cycle is interesting precisely because it explains why later-generation stars and their planets have a richer elemental composition than first-generation stars. Remove that frame and you have a sequence of events with no cumulative significance.

Some Language-Based Misconceptions (A Brief Digression)

Since I promised something about quantum leaps: the phrase “quantum leap” in everyday usage means a sudden, large, discontinuous advance. In physics, a quantum transition is the smallest possible discrete change in a system’s energy state. The electron moves from one energy level to another; the photon is emitted or absorbed; the scale of change is on the order of electron-volts. It is, emphatically, not large.

The astronomy version of this class of error:

“Light year” used as a unit of time. “That happened light years ago.” A light year is the distance light travels in one year — approximately 9.46 × 10¹² kilometres. It is a unit of distance, not time. This one is so embedded in everyday usage that correcting it usually produces mild annoyance rather than reconsideration.

“Shooting stars.” Meteors — small rocky or metallic bodies entering the atmosphere — have nothing to do with stars. They are typically the size of a grain of sand to a pebble. The visual resemblance to a moving point of light crossing the sky is where the name comes from; the resemblance to stellar physics is zero.

“Black holes suck things in.” Black holes do not have more gravity than the object that formed them at the same distance. If the Sun were replaced by a black hole of equal mass, the planets would continue on their current orbits. A black hole is only a black hole within its Schwarzschild radius; beyond that it is a gravitational field like any other. What black holes have is a point of no return — the event horizon — beyond which escape velocity exceeds the speed of light. They do not actively pull. They are very massive objects that objects can fall into.

“The dark side of the Moon.” The Moon has a far side (permanently facing away from Earth, due to tidal locking) and a near side. Both sides receive approximately equal sunlight over the lunar cycle. The far side is not permanently dark; it has a day and a night like the near side. “Dark side” persists in common usage because Pink Floyd used it as an album title and nobody wanted to call it “The Far Side of the Moon.” (Although Douglas Adams would have had something to say about that.)

Why This List Matters for Teaching

The misconceptions described above are not randomly distributed. They cluster around three areas where intuitive extrapolation from everyday experience leads systematically away from the correct physics:

1. Scale: human intuition was not built for 150 million kilometres, let alone 4.6 billion years or the 9.46 × 10¹² km in a light year. The Sun cannot be fire because fire cannot last 4.6 billion years; but “4.6 billion years” is not a number that everyday experience makes graspable.

2. Energy mechanism: combustion is the dominant frame for “things that produce heat and light.” Nuclear fusion is not part of everyday experience at any scale. The conceptual distance between them is not factual but mechanistic — it requires replacing an entire causal model.

3. Gravity: our direct experience of gravity is of a downward force, active at Earth’s surface, which keeps things from floating away. The idea of gravity as a universal mutual attraction between all masses — active in empty space, responsible for cloud collapse and galaxy formation — is a substantive generalisation that everyday experience does not motivate.

The pedagogical literature’s recommendation is not to avoid these topics but to surface the prior models explicitly before presenting the correct physics. If you ask students “where does the Sun’s energy come from?” before you teach nuclear fusion, you learn what they believe and you create the cognitive conditions for productive conceptual conflict. If you simply present the fusion model without that step, students add “fusion” to their vocabulary while retaining “fire” in their mental model.

The experiments Alexander Küpper and I have been developing through the astro-lab project — described in the stellar evolution post and the astro-lab@home post — are designed with these specific misconceptions in mind. The net-based gravity experiment addresses the “gravity doesn’t work in space” and “force requires motion” problems directly, by making gravitational attraction between all particles visible as a material structure. The pressure-temperature experiment makes the “compression heats the gas” step concrete before any mention of fusion.

These are not complete solutions to deeply held misconceptions. But they are a start at building the conceptual scaffolding that makes “and then fusion begins” something other than an assertion to be memorised and filed away without understanding.

References

Asghar, A. A., & Libarkin, J. C. (2010). Gravity, magnetism, and “down”: Non-physics college students’ conceptions of gravity. The Science Educator, 19(1), 42–55.

Bailey, J. M., Prather, E. E., Johnson, B., & Slater, T. F. (2009). College students’ preinstructional ideas about stars and star formation. Astronomy Education Review, 8(1). https://doi.org/10.3847/AER2009038

Bar, V., Brosh, Y., & Sneider, C. (2016). Weight, mass, and gravity: Threshold concepts in learning science. Science Educator, 25(1), 22–34.

Bransford, J. D., Brown, A. L., & Cocking, R. R. (Eds.) (2000). How People Learn: Brain, Mind, Experience, and School. National Academy Press.

Favia, A., Comins, N. F., & Thorpe, G. L. (2013). The elements of item response theory and its framework in analyzing introductory astronomy college student misconceptions. I. Galaxies. Astronomy Education Review.

Gunstone, R., & Watts, M. (1985). Force and motion. In R. Driver, E. Guesne, & A. Tiberghien (Eds.), Children’s Ideas in Science (pp. 85–104). Open University Press.

Palmer, D. (2001). Students’ alternative conceptions and scientifically acceptable conceptions about gravity. International Journal of Science Education, 23(7), 691–706. https://doi.org/10.1080/09500690010006527

Schecker, H., Wilhelm, T., Hopf, M., & Duit, R. (Eds.) (2018). Schülervorstellungen und Physikunterricht. Springer.

Changelog

• 2025-09-14: Updated the DOI for Bailey et al. (2009) to the correct 10.3847/AER2009038.
• 2025-09-14: Changed “would extend past Mars” to “would extend past the asteroid belt” for Betelgeuse at ~700 R☉. At ~3.26 AU, Betelgeuse’s radius exceeds Mars’s orbital distance (1.52 AU) by more than a factor of two and reaches well into the asteroid belt (2.2–3.3 AU).