Star Colours, Aboriginal Sky Knowledge and Stellar Fusion

Two stars can look equally red and have completely different interiors. One might be a small red dwarf, while the other is a massive red supergiant. Their surface temperatures could be similar, but the nuclear reactions producing their energy could be very different.

So, if a star looks red, can you conclude that it uses the proton-proton chain rather than the CNO cycle?

No. Colour tells us about the temperature of a star’s visible outer layer. The dominant fusion pathway depends on conditions much deeper inside, particularly the core temperature, stellar mass, and stage of evolution.

Understanding that distinction brings together three interesting areas of astronomy: Aboriginal observations of star colours, the physics of stellar temperatures, and nuclear fusion.

01Aboriginal observations of star colours

The colour of stars has long been an important part of Australian First Nations astronomical knowledge. However, there is no single Aboriginal system of astronomy. Different language groups have their own observations, classifications, stories, and interpretations of the sky.

Several publicly documented accounts demonstrate particularly careful observations of stellar colour.

The Aranda and Luritja observations

A historical record made by B. G. Maegraith following observations at Hermannsburg in 1929 describes the astronomical knowledge of Aranda and Luritja people in Central Australia.

A later synthesis by Dianne Johnson, Night Skies of Aboriginal Australia, reports that the Aranda distinguished stars by colour, including red, white, blue, and yellow.

One particularly interesting example is Antares, the bright star in the constellation Scorpius. It was described as tataka indora, meaning very red.

The stars of the Hyades cluster, near Aldebaran, were also distinguished through colour. The published account describes separate groupings of red and white stars.

The same record connects Antares’ red appearance with an ancestral woman decorated in red ochre.

These observations demonstrate that colour was a meaningful characteristic used to distinguish individual stars and groups of stars. It was not simply a matter of noticing which stars looked brightest.

The historical record does not always clearly distinguish between Aranda and Luritja contributions, so individual details should be attributed cautiously.

The Gagadju observations

Gagadju elder Bill Neidjie also described the colour of stars in his accounts of traditional sky knowledge.

He associated a pink star with a King Brown snake and described its appearance changing between pink and white.

This is particularly interesting because the account draws attention not only to the apparent colour of a star, but also to changes in how it appears.

The published passage does not establish a modern astronomical identification for that particular star. It is important not to assign it a specific catalogue name without supporting evidence.

The Wailwun observations

Another documented example comes from the Wailwun people of northern New South Wales.

In his 1875 publication Kamilaroi and Other Australian Languages, William Ridley recorded information provided by an elder identified as King Rory.

The account gives the name Guembila, meaning red, for Arcturus (Alpha Boötis).

This is a particularly useful example because it connects a recorded colour description with an identifiable star.

What can we learn from these observations?

These accounts demonstrate detailed observation and classification of the night sky.

They also show that colour could carry cultural significance, connecting stars with ancestral beings, animals, and other aspects of particular knowledge systems.

There is an important distinction, though.

These records document observations and cultural interpretations. They do not suggest that traditional observers described stars using modern nuclear physics terminology or the proton-proton chain and CNO cycle.

Modern astrophysics provides another way of investigating the same objects. It allows us to connect the light arriving from a star with its physical properties, including its surface temperature and internal structure.

The two perspectives should be understood in their own contexts rather than treating one as a translation of the other.

The historical examples above are drawn from publicly available publications, including Johnson’s Night Skies of Aboriginal Australia and Ridley’s digitised work, available through AIATSIS. These are historical records, not comprehensive accounts of contemporary community knowledge.

02What does a star’s colour actually tell us?

Imagine heating a metal object.

At a relatively low temperature, it might glow a dull red. As its temperature increases, the glow becomes orange, then yellow-white.

Stars behave in a similar way.

A star produces electromagnetic radiation across a broad range of wavelengths. The distribution of this radiation depends strongly on the temperature of its visible outer layer, called the photosphere.

The photosphere is the region from which most of the visible light we observe escapes.

As the photosphere becomes hotter, the star emits proportionally more radiation at shorter wavelengths.

This produces the general relationship:

• Red and orange stars have relatively cool photospheres.
• Yellow stars have intermediate photospheric temperatures.
• White and blue-white stars have hotter photospheres.
• Blue stars have the hottest photospheres.

Wien’s displacement law

We can make this relationship quantitative using Wien’s displacement law.

\[ \lambda_{\max}T=b \]

Where:

• \(\lambda_{\max}\) is the wavelength at which the emitted intensity is greatest, measured in metres.
• \(T\) is the photospheric temperature, measured in kelvin (K).
• \(b\) is Wien’s displacement constant, approximately \(2.90\times10^{-3}\,\mathrm{m\,K}\).

Rearranging:

\[ T=\frac{2.90\times10^{-3}}{\lambda_{\max}} \]

This equation tells us something important: as the peak wavelength decreases, the temperature increases.

A red star has a longer peak wavelength than a blue star, indicating a lower photospheric temperature.

Remember that a star emits a broad spectrum, not just one wavelength. The peak wavelength is useful for estimating temperature, but it is not necessarily the exact colour that your eye sees.

For particularly hot stars, the peak can even lie in the ultraviolet region. They still appear blue-white because of the distribution of their visible light.

03Why surface temperature and core temperature are different

Here is where a common misconception appears.

Students often learn that hotter stars tend to be more massive and have hotter cores. They then assume that a red star must have a relatively cool core.

That second conclusion is not reliable.

We need to distinguish between two regions:

• The photosphere, which produces the visible light we observe.
• The core and surrounding fusion regions, where nuclear reactions release energy.

These regions can have dramatically different temperatures.

The relationship between mass and temperature

For a main-sequence star, gravity plays a major role in determining the conditions inside it.

A more massive star has greater gravitational compression. Its interior must generate sufficient pressure to balance the inward pull of gravity.

This produces higher central pressures and temperatures.

The higher core temperature increases the rate of nuclear fusion.

For main-sequence stars, there is also a general relationship between mass and photospheric temperature:

• Low-mass stars tend to have cooler surfaces and appear red or orange.
• Sun-like stars have intermediate surface temperatures.
• High-mass stars tend to have hotter surfaces and appear blue or blue-white.

This relationship is useful, but it applies to stars in the appropriate evolutionary stage.

What happens when a star becomes a red giant?

A star does not remain on the main sequence forever.

Eventually, hydrogen in its central core becomes depleted. The core can no longer maintain the same balance between gravity and the pressure produced by its energy generation.

The core contracts, while the outer layers expand.

As the outer layers expand, their surface temperature decreases. The star becomes redder.

But notice what has happened: the surface has cooled because the star has expanded. This does not mean that its entire interior has become cold.

Hydrogen fusion can continue in a shell surrounding the helium-rich core. In massive evolved stars, this hydrogen-burning shell can remain extremely hot.

Consider Antares, the bright red star in Scorpius.

Modern astronomy identifies Antares as a red supergiant. It is enormous and has a comparatively cool photosphere.

Astronomers have even used the European Southern Observatory’s Very Large Telescope Interferometer to study its extended atmosphere.

Its red appearance is consistent with its relatively low surface temperature, but that colour does not tell us the complete story of its interior.

You can read about the observations in ESO’s study of Antares.

The important distinction is:

A star’s surface temperature does not directly determine the temperature of its core or hydrogen-burning shell.

This is the key idea we need before comparing the two fusion pathways.

04The proton-proton chain and the CNO cycle

Stars generate energy through nuclear fusion.

During hydrogen fusion, lighter nuclei combine to produce helium. A small amount of mass is converted into energy.

The overall reaction can be represented as:

\[ 4\,{}^{1}_{1}\mathrm{H} \rightarrow {}^{4}_{2}\mathrm{He} +2e^+ +2\nu_e +\text{energy} \]

Four hydrogen nuclei produce one helium-4 nucleus, alongside positrons and electron neutrinos.

There are two important pathways through which this hydrogen fusion occurs:

1. The proton-proton (p-p) chain.
2. The carbon-nitrogen-oxygen (CNO) cycle.

Both produce the same overall result, but their reaction sequences and temperature sensitivities differ.

The proton-proton chain

The proton-proton chain is the dominant hydrogen-fusion process in the Sun and other relatively low-mass main-sequence stars.

It begins when two protons interact and form a deuterium nucleus.

The simplified sequence is:

First reaction: Two protons combine to form deuterium, producing a positron and an electron neutrino.

Second reaction: Deuterium combines with another proton to produce helium-3.

Third reaction: Two helium-3 nuclei combine to produce helium-4, releasing two protons.

The released protons can participate in further reactions.

The overall result is the conversion of hydrogen into helium.

The p-p chain can operate effectively at the relatively lower core temperatures found in stars such as the Sun.

It is not restricted to the Sun, though. It is also important in lower-mass main-sequence stars.

The CNO cycle

The carbon-nitrogen-oxygen cycle also converts hydrogen into helium.

However, instead of beginning with direct interactions between two protons, it involves a sequence of nuclear reactions involving carbon, nitrogen, and oxygen nuclei.

These nuclei act as catalysts.

A catalyst helps a reaction pathway occur without being consumed overall. During the CNO cycle, carbon, nitrogen, and oxygen isotopes participate in the reactions, but the original carbon nucleus is regenerated by the end of the cycle.

The overall result is still the conversion of four hydrogen nuclei into one helium-4 nucleus.

The major difference is the temperature dependence of the reaction rates.

The CNO cycle is much more sensitive to temperature than the proton-proton chain.

This means that as the core temperature increases, the CNO cycle becomes increasingly important relative to the p-p chain.

In sufficiently hot, massive main-sequence stars, the CNO cycle becomes the dominant source of energy from hydrogen fusion.

Comparing the two pathways

Both pathways can operate in the same star.

When we say that one pathway dominates, we mean that it contributes most of the energy generated by hydrogen fusion. We do not mean that the other pathway has completely stopped.

For more detail on these reactions, see OpenStax’s explanation of stellar nuclear fusion.

05When observed colour changes, but the star does not

There is one more complication worth understanding.

Sometimes a star appears to change colour over a short period.

This does not necessarily mean its photospheric temperature is changing.

Consider looking at a bright star close to the horizon. Its light has to travel through a much thicker section of Earth’s atmosphere than light from a star directly overhead.

The atmosphere contains moving pockets of air with different temperatures and densities.

These affect how incoming starlight travels towards the observer.

Atmospheric turbulence, wavelength-dependent refraction, and scattering can change the apparent brightness and colour of a star.

This is particularly noticeable when the star is low in the sky.

A connection with Meriam sky knowledge

A particularly interesting documented example comes from the Meriam people of the Torres Strait.

Meriam is a distinct Torres Strait Islander community, not an Aboriginal mainland community.

In a 2019 open-access study by Duane Hamacher, John Barsa, Segar Passi, and Alo Tapim, observations of changing stellar appearance are discussed alongside local weather knowledge.

The study’s Meriam co-authors contributed their knowledge and approved its publication.

The late Meriam elder John Barsa described blue-white stars rapidly changing towards red and orange as an indicator associated with changing winds and atmospheric conditions.

These observations were useful in anticipating weather, including approaching storms.

The researchers also examined the physical processes that can influence the apparent colour of starlight.

They discussed atmospheric turbulence, chromatic refraction, and wavelength-dependent scattering. These effects can be particularly noticeable for stars close to the horizon.

There is a useful scientific distinction here:

• Scintillation refers specifically to rapid changes in the apparent brightness of a star.
• Apparent colour changes can accompany atmospheric effects, but are not the definition of scintillation.
• Neither phenomenon requires the star’s actual photospheric temperature to change.

The research also explains that star observations can be considered alongside other environmental indicators, rather than being treated as an isolated weather measurement.

You can read the original collaborative study here: Indigenous Use of Stellar Scintillation to Predict Weather and Seasonal Change.

This example highlights why it is important to distinguish between a star’s intrinsic properties and the appearance of its light as it travels through Earth’s atmosphere.

06Questions and solutions

Question 1

A star has a peak emission wavelength of 650 nm.

Calculate its approximate photospheric temperature using Wien’s displacement law.

State whether its photosphere is hotter or cooler than the Sun’s, given that the Sun’s peak emission wavelength is approximately 500 nm.

Solution 1

The star’s photospheric temperature is approximately \(4460\,\mathrm{K}\), making it cooler than the Sun.

Using Wien’s displacement law:

\[ \lambda_{\max}T=2.90\times10^{-3}\,\mathrm{m\,K} \]

Rearranging:

\[ T=\frac{2.90\times10^{-3}}{\lambda_{\max}} \]

Convert the wavelength:

\[ 650\,\mathrm{nm}=650\times10^{-9}\,\mathrm{m} \]

Substitute:

\[ \begin{aligned} T&=\frac{2.90\times10^{-3}} {650\times10^{-9}}\\ &=4.46\times10^3\,\mathrm{K} \end{aligned} \]

Therefore:

\[ \boxed{T\approx4460\,\mathrm{K}} \]

The Sun’s peak wavelength is shorter, indicating a higher photospheric temperature.

The star’s longer peak wavelength is consistent with its redder appearance.

However, this calculation does not tell us which fusion pathway dominates inside the star.

Question 2

A historical published account of Aranda sky knowledge describes Antares as very red and records red and white groupings of stars in the Hyades.

The account also connects Antares’ redness with an ancestral woman decorated in red ochre.

A student makes the following claim:

“The observations prove that the Aranda had measured the surface temperature of Antares and understood that it was a red supergiant.”

Evaluate this claim.

Solution 2

The claim goes beyond what the historical record establishes.

The observations demonstrate detailed classification of star colours and culturally significant interpretations of celestial objects. They do not demonstrate that the observers used modern spectroscopy or stellar-evolution models.

Modern astronomy identifies Antares as a red supergiant with a comparatively cool photosphere. This classification is supported by astronomical measurements and physical models.

However, identifying a star as very red is not the same as measuring its temperature or determining its evolutionary stage.

The important distinction is between:

• What was observed: the apparent colour of Antares and its distinction from other stars.
• What was culturally understood: the star’s significance and its connection with an ancestral figure.
• What modern astrophysics establishes: Antares’ physical classification and the properties of its photosphere.

It would be inaccurate to project modern nuclear terminology onto the historical account.

The observations are valuable in their own right and should be understood within their specific cultural context.

Question 3

During an evening observation, a student notices that a bright star close to the horizon appears to alternate rapidly between blue-white and red-orange.

The star also appears to flicker in brightness.

The student concludes that the star’s surface temperature must be changing rapidly, potentially causing its fusion pathway to switch between the CNO cycle and the proton-proton chain.

Explain why this conclusion is incorrect.

Identify the atmospheric effects that could produce the observed changes in appearance.

Solution 3

The student’s conclusion is incorrect because rapid changes in apparent colour do not indicate rapid changes in the star’s actual temperature or fusion pathway.

The most likely explanation is that Earth’s atmosphere is changing the appearance of the incoming starlight.

There are several relevant effects.

1. Atmospheric turbulence

Air pockets with different densities and temperatures change the refractive index of the atmosphere.

As starlight passes through these regions, its apparent direction and intensity can fluctuate.

This produces the familiar twinkling of stars.

2. Wavelength-dependent refraction

Different wavelengths of light can be refracted by slightly different amounts.

This can produce apparent colour changes, particularly when the star is close to the horizon.

3. Atmospheric scattering

The atmosphere can scatter different wavelengths by different amounts.

This can change the apparent colour of incoming light, especially when it passes through a long atmospheric path.

None of these effects requires a change in the star’s actual photospheric temperature.

The fusion reactions inside a star cannot switch between the p-p chain and CNO cycle every few seconds in response to atmospheric conditions on Earth.

The Meriam observations discussed earlier provide an interesting example of how changing stellar appearances can be used to interpret local weather conditions.

The observed changes are real, but the physical interpretation must distinguish between the star itself and the atmosphere through which its light travels.

Question 4

Two stars, A and B, are both confirmed to be on the main sequence.

Their properties are:

(a) Calculate the approximate photospheric temperature of each star.

(b) Identify the likely dominant hydrogen-fusion pathway for each star.

(c) Explain why the photospheric temperatures alone would not provide a complete explanation of the fusion pathways.

Solution 4

Star A has a photospheric temperature of approximately \(5800\,\mathrm{K}\) and is dominated by the p-p chain.

Star B has a photospheric temperature of approximately \(20,000\,\mathrm{K}\) and is dominated by the CNO cycle.

Part (a): Calculate the temperatures

Using Wien’s displacement law:

\[ T=\frac{2.90\times10^{-3}}{\lambda_{\max}} \]

For star A:

\[ \lambda_{\max}=500\times10^{-9}\,\mathrm{m} \]

Therefore:

\[ \begin{aligned} T_A&=\frac{2.90\times10^{-3}} {500\times10^{-9}}\\ &=5800\,\mathrm{K} \end{aligned} \]

For star B:

\[ \lambda_{\max}=145\times10^{-9}\,\mathrm{m} \]

Therefore:

\[ \begin{aligned} T_B&=\frac{2.90\times10^{-3}} {145\times10^{-9}}\\ &=20,000\,\mathrm{K} \end{aligned} \]

The results are:

\[ \boxed{T_A=5800\,\mathrm{K}} \]

\[ \boxed{T_B=20,000\,\mathrm{K}} \]

Star B has a significantly hotter photosphere.

Part (b): Identify the dominant fusion pathways

Star A has a mass similar to the Sun and is on the main sequence.

Its core temperature is consistent with hydrogen fusion being dominated by the proton-proton chain.

Star B has a much greater mass.

Its stronger gravitational compression produces a substantially hotter core. At these higher core temperatures, the CNO cycle becomes the dominant hydrogen-fusion pathway.

Therefore:

• Star A: proton-proton chain.
• Star B: CNO cycle.

The p-p chain can still operate in star B, but it is not expected to provide most of its hydrogen-fusion energy.

Part (c): Explain the role of mass

The photospheric temperature describes the outer visible region of a star.

Fusion takes place much deeper inside.

Although surface temperature and stellar mass are related for main-sequence stars, the photospheric temperature is not a direct measurement of core temperature.

The additional information about mass and evolutionary stage allows us to make a more reliable prediction about the internal conditions.

This is why we consider all three properties rather than using colour alone.

Question 5

Two stars are observed to have almost identical photospheric temperatures of \(3500\,\mathrm{K}\). Both appear red.

However, their internal structures are very different.

Star L

• Mass: \(0.4M_\odot\).
• Evolutionary stage: main sequence.
• Hydrogen fusion occurs in its core.
• Core temperature: \(12\,\mathrm{MK}\).

Star G

• Mass: \(12M_\odot\).
• Evolutionary stage: red supergiant.
• Helium fusion occurs in its central core.
• Hydrogen fusion continues in a surrounding shell.
• Hydrogen-burning shell temperature: \(25\,\mathrm{MK}\).

For this question, use the simplified assumption that the p-p chain dominates hydrogen fusion at \(12\,\mathrm{MK}\), while the CNO cycle dominates at \(25\,\mathrm{MK}\).

(a) Identify the dominant hydrogen-fusion pathway in each star.

(b) Explain how both stars can have the same photospheric temperature but different dominant hydrogen-fusion pathways.

(c) A student argues that because both stars are red, they must both use the p-p chain. Explain precisely why this reasoning fails.

(d) Identify the nuclear fusion process occurring in the central core of star G.

Solution 5

Star L is dominated by the p-p chain, while star G is dominated by the CNO cycle in its hydrogen-burning shell. Star G also undergoes helium fusion in its central core.

Both stars appear red because they have similar photospheric temperatures, but their internal conditions are very different.

Part (a): Identify the hydrogen-fusion pathways

For star L:

\[ T_{\text{core}}=12\,\mathrm{MK} \]

At this temperature, the p-p chain dominates according to the assumptions supplied in the question.

Therefore, star L uses the proton-proton chain as its dominant hydrogen-fusion pathway.

For star G:

\[ T_{\text{shell}}=25\,\mathrm{MK} \]

The hydrogen-burning shell is considerably hotter.

At this temperature, the CNO cycle dominates.

Therefore, star G uses the CNO cycle as its dominant hydrogen-fusion pathway.

Part (b): Explain the difference in their interiors

The two stars have similar photospheric temperatures, but they have very different masses and evolutionary stages.

Star L is a low-mass main-sequence star.

It generates energy through hydrogen fusion in its central core. Its relatively low core temperature favours the p-p chain.

Star G is a massive red supergiant.

During its evolution, its central hydrogen supply has been depleted. Its core has changed, and hydrogen fusion now occurs in a surrounding shell.

Its outer layers have expanded substantially, reducing the photospheric temperature.

However, the hydrogen-burning shell remains hot enough for the CNO cycle to dominate.

The crucial point is that the photosphere and hydrogen-burning shell are different regions with different temperatures.

Part (c): Explain why red does not mean p-p fusion

The student’s reasoning incorrectly assumes that photospheric temperature determines the temperature of the hydrogen-burning region.

This is the exact constraint that changes the answer.

For star L, the hydrogen-burning region is its relatively cool central core.

For star G, the hydrogen-burning region is a much hotter shell surrounding a helium-rich core.

Both stars have the same photospheric temperature, but the temperatures and locations of their hydrogen-burning regions are different.

Therefore, their dominant fusion pathways are also different.

The claim that red stars must use the p-p chain is false.

Part (d): Identify the process in star G’s central core

Star G’s central core is undergoing helium fusion, rather than hydrogen fusion.

The helium-burning process is primarily the triple-alpha process, in which helium-4 nuclei combine to produce carbon-12.

This is a different nuclear reaction from both the p-p chain and the CNO cycle.

The star can therefore have helium fusion in its core while hydrogen fusion continues in the surrounding shell.

This is a more complete picture of an evolved massive star.

The next useful concept is the Hertzsprung-Russell (H-R) diagram. It combines stellar luminosity and photospheric temperature to help distinguish main-sequence stars from giants and supergiants.

Once you can identify a star’s evolutionary stage, its mass and internal structure become much more useful for predicting which fusion processes dominate. That is a far more reliable approach than trying to read a star’s nuclear reactions from its colour alone.