IB Physics HLTopic 5 — Fusion & StarsPaper 1 & 2reading a star’s light~16 min read
Stellar Spectra
We can never visit a star, yet we know what they’re made of and how hot they are — all from their light. Split a star’s light into a spectrum and it becomes a barcode of information. Dark lines reveal the elements in the star, and the colour reveals its temperature. This is how we know the Sun is mostly hydrogen and helium, and how we take the temperature of a star trillions of kilometres away. Here’s how to read that barcode.
📚 What you need to know
There are three types of spectra: continuous, emission line, and absorption line
Continuous spectra come from hot, dense sources (like a star’s core)
Emission line spectra (coloured lines on black) come from hot, low-pressure gas
Absorption line spectra (dark lines on a continuous spectrum) come from cool gas in front of a hot source
Each element makes a unique pattern of spectral lines — a fingerprint
Stars show an absorption spectrum, revealing their chemical composition
A star’s colour depends on its temperature (Wien’s law): hot = blue, cool = red
The three types of spectra
When you pass light through a prism or diffraction grating, you get one of three kinds of spectrum, depending on the source:
An absorption spectrum is a continuous rainbow with dark lines missing — exactly where an emission spectrum of the same element would have bright lines.
Continuous spectrum
A continuous spectrum is an unbroken rainbow containing every wavelength. It’s produced by hot, dense sources — like the tightly packed core of a star, where atoms are so close together that they emit light of all frequencies.
Emission line spectrum
An emission spectrum is a set of bright coloured lines on a black background. It’s produced by a hot, low-pressure gas. When an electron drops from a higher energy level to a lower one, it emits a photon of a specific wavelength — giving one bright line. Each transition gives a different line.
Absorption line spectrum
An absorption spectrum is a continuous rainbow crossed by dark lines. It appears when white light from a hot source passes through a cool, low-pressure gas. The gas atoms absorb photons at exactly the wavelengths they would otherwise emit, leaving dark gaps where that light is missing.
The neat symmetry to remember: an element’s absorption lines fall at exactly the same wavelengths as its emission lines. Emission = the gas gives out those photons (bright lines). Absorption = the gas takes in those same photons (dark lines). Same element, same wavelengths, opposite appearance. That’s the whole trick to matching spectra.
What stars show, and why
Stars produce an absorption line spectrum. Here’s why: the hot, dense core makes a continuous spectrum, but on its way out, that light passes through the star’s cooler outer atmosphere. The gas atoms there absorb specific wavelengths, leaving dark absorption lines in the spectrum.
Because each element absorbs a unique pattern of wavelengths — its “fingerprint” — those dark lines tell us exactly which elements are present. This is how we know the Sun is mostly hydrogen and helium, and we can do it even for stars unimaginably far away.
Hot core continuous spectrum
light passes through atmosphere
Cool gas absorbs certain wavelengths
dark lines appear
Absorption spectrum
Colour and temperature
A star’s colour tells us its temperature. Stars behave almost like perfect black-body radiators, so the wavelength at which they’re brightest (the peak wavelength) depends only on temperature, following Wien’s law:
Hotter stars peak at shorter wavelengths → they look white or blue.
Cooler stars peak at longer wavelengths → they look red or yellow.
Hot stars are blue-white (short peak wavelength); cool stars are red (long peak wavelength). Colour is a direct read-out of surface temperature.
WE 1
Explain why stars produce an absorption line spectrum rather than an emission line spectrum.
Step 1 — the core
The hot, dense core emits a continuous spectrum (all wavelengths).
Step 2 — the atmosphere
This light passes through the star’s cooler outer gas.
Step 3 — absorption
The gas absorbs specific wavelengths, leaving dark lines in the continuous spectrum.
Continuous spectrum minus absorbed lines = absorption spectrumThe atmosphere is too cool to produce a bright emission spectrum of its own; instead it removes wavelengths, giving dark absorption lines.
WE 2
Star A appears blue-white and Star B appears red. Compare their surface temperatures and peak wavelengths.
Step 1 — colour to temperature
Blue-white = hotter; red = cooler.
Step 2 — temperature to peak wavelength (Wien)
Hotter → shorter peak wavelength; cooler → longer peak wavelength.
A is hotter with a shorter peak; B is cooler with a longer peakColour, temperature and peak wavelength all link together through Wien’s law. Blue means hot and short-wavelength; red means cool and long-wavelength.
Bright lines on black? Emission → hot, low-pressure gas.
Dark lines on a rainbow? Absorption → cool gas in front of hot source.
Match the lines to an element’s fingerprint pattern.
Colour → temperature via Wien (blue hot, red cool).
💡 Top tips
Emission and absorption lines of an element sit at the same wavelengths.
Stars show absorption spectra (cool atmosphere over hot core).
Each element has a unique line pattern — a fingerprint.
Colour reveals temperature: blue = hot, red = cool.
Absorption spectrum = continuous spectrum with dark lines removed.
⚠ Common mistakes
Saying stars show emission spectra — they show absorption
Thinking blue stars are cool — blue is hottest
Confusing emission (bright lines) with absorption (dark lines)
Forgetting the dark lines are a fingerprint of the elements present
Mixing up which source gives continuous vs line spectra
Quick recap: Three spectra: continuous (hot dense source), emission (bright lines, hot thin gas), absorption (dark lines, cool gas over hot source). Stars show absorption spectra, whose dark lines are element fingerprints revealing composition. A star’s colour gives its temperature via Wien: blue = hot (short peak), red = cool (long peak).
A star’s spectrum gives us its temperature and composition — but to know how far away it is, we need a completely different trick, one based on simple geometry and the Earth’s motion around the Sun. Next page: Stellar Parallax.
Spectra and stellar temperatures unclear?
Book a free meeting and we’ll drill the three spectrum types, why stars show absorption lines, and the colour–temperature link.
