IB Physics SLTopic 5 — Fusion & StarsPaper 1 & 2Reading a star’s light~8 min read
Stellar Spectra
We can’t fly to a star and scoop up a sample, yet we know exactly what stars are made of. The secret is in their light. Split a star’s light through a prism and it carries a hidden barcode — a pattern of dark lines that names every element in the star’s atmosphere. Here’s how that barcode works, and how the colour of a star tells us its temperature.
📘 What you need to know
There are three types of spectra: continuous, emission line, and absorption line
A continuous spectrum (all wavelengths, unbroken) comes from a hot, dense source — like the core of a star
An emission line spectrum (bright coloured lines on black) comes from a hot, low-pressure gas
An absorption line spectrum (dark lines on a continuous background) forms when light passes through a cool, low-pressure gas
Each element makes a unique pattern of lines — a fingerprint that identifies it
Stars show an absorption spectrum: continuous light from the core, with dark lines added by the cooler outer atmosphere — this reveals the star’s chemical composition
A star’s colour depends on its surface temperature: hotter stars peak at shorter wavelengths (blue), cooler stars at longer wavelengths (red) — this is Wien’s law
The Three Types of Spectra
When light is spread out by wavelength, it forms a spectrum. Depending on the source and what the light has passed through, you get one of three distinct patterns.
The same element’s dark absorption lines fall at exactly the same wavelengths as its bright emission lines.
Continuous spectrum
A continuous spectrum contains every wavelength, blending into an unbroken rainbow. It’s produced by a hot, dense source — such as the core of a star — where atoms are packed so tightly that they emit light of all frequencies.
Emission line spectrum
When an electron drops from a higher energy level to a lower one, it emits a photon of a specific wavelength. A hot, low-pressure gas produces a set of these bright coloured lines on a black background — one line for each allowed electron transition.
Absorption line spectrum
Now shine white light through a cool, low-pressure gas. The gas atoms absorb photons at exactly the wavelengths they would otherwise emit, lifting electrons to excited states. Those wavelengths go missing, leaving dark lines in an otherwise continuous spectrum. Crucially, the dark absorption lines sit at the same wavelengths as that element’s bright emission lines.
hot, dense source
→ gives →
continuous spectrum
→ through a cool gas →
absorption spectrum
How We Read a Star’s Composition
Here’s the clever part. Deep inside a star, the hot dense core emits a continuous spectrum. On its way out, that light passes through the star’s cooler outer atmosphere. The gases there absorb their own characteristic wavelengths, so the light that finally reaches us is an absorption line spectrum — continuous, but with dark gaps.
Because each element produces a unique pattern of lines, those dark gaps act as fingerprints. Match the pattern to known elements and you can identify exactly what a star’s atmosphere contains — even one billions of kilometres away.
🧭 From starlight to chemical composition
The core emits a continuous spectrum — it’s hot and dense
Light crosses the cooler atmosphere — gases absorb their characteristic wavelengths
Dark lines appear at those missing wavelengths — an absorption spectrum reaches us
Match the line pattern to the emission lines of known elements — each match names an element present
Think of it like a supermarket barcode. Every element has its own unique set of stripes, and no two are alike. When a star’s spectrum shows the hydrogen “barcode” of dark lines, you know hydrogen is up there — no spaceship required. This is how we discovered the Sun is mostly hydrogen and helium.
Colour and Temperature
A star’s spectrum spreads across a range of wavelengths, and one wavelength carries more intensity than any other — the peak wavelength. Stars behave almost exactly like ideal black-body radiators, so the peak wavelength is set by the star’s surface temperature through Wien’s law.
Wien’s law (qualitative)
hotter star → shorter peak wavelength → bluer
The rule is short peak wavelength ↔ high temperature. So the hottest stars peak in the blue and look white or blue, while cooler stars peak toward longer wavelengths and look red or yellow. A star’s colour is therefore a direct read-out of how hot its surface is.
A hotter star peaks at a shorter (bluer) wavelength; as temperature falls, the peak slides toward longer (redder) wavelengths.
Quick recap: continuous (hot dense), emission (hot low-pressure gas), absorption (light through a cool gas); stars show absorption lines that fingerprint their elements, and their colour reveals surface temperature via Wien’s law.
WE 1
Explain why a star produces an absorption line spectrum rather than a continuous or emission line spectrum.
The source
the hot, dense core of the star emits a continuous spectrum (all wavelengths)
The atmosphere
this light passes through the cooler, low-pressure gas of the star’s outer atmosphere
the gas atoms absorb photons at their characteristic wavelengths
The result
those wavelengths are removed, leaving dark lines
→ a continuous spectrum crossed by dark absorption lines
WE 2
Two stars are observed. Star A appears blue-white and Star B appears orange-red. (a) State which star is hotter and justify your answer. (b) Explain how the absorption spectrum of Star A could be used to identify an element in its atmosphere.
Part (a)
a blue-white star peaks at a shorter wavelength than an orange-red one
by Wien’s law, shorter peak wavelength means a higher temperature
→ Star A (blue-white) is hotterPart (b)
each element absorbs at a unique set of wavelengths (its fingerprint)
compare the dark lines in Star A’s spectrum with known emission-line patterns
if the pattern matches, that element is present
→ the matching line pattern identifies the element
💡 Top tips
Match source to spectrum: hot + dense → continuous; hot low-pressure gas → emission; cool gas in front of a continuous source → absorption
Emission and absorption lines line up: the same element’s bright and dark lines fall at identical wavelengths
Lines are fingerprints: a unique pattern per element is what lets us identify composition; mention the pattern, not just “lines”
Colour = temperature: blue is hot, red is cool. Link it to Wien’s law and the peak wavelength
⚠ Common mistakes
Saying stars show an emission spectrum — their atmospheres aren’t hot enough; we see absorption lines
Describing absorption lines as bright — they’re dark gaps in a continuous spectrum
Getting the colour–temperature link backwards — blue is hotter, red is cooler
Forgetting to mention the unique pattern when explaining how elements are identified
Up next: Stellar Parallax — we switch from what stars are made of to how far away they are, using the tiny apparent wobble of nearby stars as the Earth moves around the Sun.
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