Theme E — Nuclear and Quantum Physics

E.5.6 Emission and Absorption Spectra in Stars — IB Physics HL Revision Notes & Topical Questions

5 min read 9 parts Lesson 43 of 45 1 quiz

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E.5.6 Emission and Absorption Spectra in Stars — IB PHYSICS SL notes: Explain how line spectra reveal the composition, temperature and motion of stars.SpectrumAppearanceSourceContinuousAll wavelengths, an uninterrupted band of colourHot, dense sources such as the cores of starsEm…

Dense matter has atoms so close that they interact
In a low-pressure gas the energy levels are well-defined and quantised
An electron moving from a higher to a lower level emits a photon
Absorption raises an electron to a higher level
Absorbed wavelengths match the emission lines of the same element
Each element has its own energy levels

Part 1 of 7Three types of spectrum

Spectrum	Appearance	Source
Continuous	All wavelengths, an uninterrupted band of colour	Hot, dense sources such as the cores of stars
Emission line	Discrete coloured lines on a black background	Hot, low-pressure gas
Absorption line	Dark lines on a continuous spectrum	White light passing through a cool, low-pressure gas

Dense matter has atoms so close that they interact

Dense matter has atoms so close that they interact giving a spread of energy states and photons of all frequencies.

In a low-pressure gas the energy levels are well-defined and quantised

In a low-pressure gas the energy levels are well-defined and quantised so only photons matching the gaps between levels appear.

Part 2 of 7Origin of the lines

An electron moving from a higher to a lower level emits a photon

An electron moving from a higher to a lower level emits a photon hf = E_i − E_f.
Each transition gives one wavelength, so one line of the spectrum.

Absorption raises an electron to a higher level

Absorption raises an electron to a higher level only photons with exactly the energy gap are removed.

Absorbed wavelengths match the emission lines of the same element

Absorbed wavelengths match the emission lines of the same element absorption spectrum = emission lines removed from a continuous spectrum.

Each element has its own energy levels

Each element has its own energy levels so its pattern of lines is a fingerprint.

Hydrogen example

Hydrogen example the n = 3 to n = 2 transition gives 1.89 eV, the red line at 656 nm.

Part 3 of 7Composition of stars

1Photons from the core form a continuous spectrum

Photons from the core form a continuous spectrum they pass through the gas of the outer atmosphere.

Gas atoms absorb photons at particular frequencies and re-emit them in random directions.

3Atmospheres of stars are not hot enough for an emission line spectrum

Atmospheres of stars are not hot enough for an emission line spectrum so stars show an absorption line spectrum.

4Dark lines are absorption (Fraunhofer) lines

Dark lines are absorption (Fraunhofer) lines they are caused by the cooler outer atmosphere, not the core.

5Matching the lines to laboratory spectra identifies the elements

Matching the lines to laboratory spectra identifies the elements even in extremely distant stars.
The Sun is mostly hydrogen and helium, with dark lines at the hydrogen wavelengths 486 nm and 656 nm and the helium wavelength 587.7 nm.
Helium was discovered in the Sun's spectrum before it was found on Earth.

Similar spacing of lines indicates a similar chemical composition between stars.

Part 4 of 7Temperature and spectral class

1Surface temperature decides which atoms are excited or ionised

Surface temperature decides which atoms are excited or ionised so it changes the strengths of the lines.
Hydrogen Balmer lines are strongest near 10 000 K (class A).
Hot O and B stars show ionised helium;
- the Sun (G) shows calcium and metals;
- cool M stars show molecules such as titanium oxide.

Spectral classes O, B, A, F, G, K, M run from hot to cool.

3Stars are close to black-body radiators

Stars are close to black-body radiators colour relates to temperature through Wien's law.
Shorter peak wavelength means higher temperature — hot stars are white or blue.
Longer peak wavelength means lower temperature — cool stars are red or yellow.

4Wien's displacement law

λ_maxT = 2.90 × 10^-3 m K, so the Sun at 5800 K peaks near 500 nm.

Part 5 of 7Motion and the Doppler shift

Motion and the Doppler shift

Relative motion shifts all spectral lines of a star.

A receding star is redshifted — longer wavelengths.
An approaching star is blueshifted — shorter wavelengths.

For speeds much less than c

Δλ/λ ≈ v/c.

It gives radial velocities and reveals orbiting stars in binary systems.

Redshift of galaxies shows the expansion of the universe.

Rotation broadens lines

Rotation broadens lines one side approaches while the other recedes.

Part 6 of 7Other spectra

Gas clouds between the star and us add their own absorption lines.

An exoplanet's atmosphere adds lines as the planet passes in front of its star.

3Emission nebulae give emission line spectra

Emission nebulae give emission line spectra ultraviolet light from nearby hot stars excites the gas.
The red colour of many nebulae is the H-alpha line at 656 nm.

4From one spectrum

From one spectrum temperature, composition, radial velocity, rotation and magnetic field can be read.

Part 7 of 7More from the specification

Main Sequence Stars and Thermonuclear Fusion

During the main sequence phase of stellar evolution, inward gravitational collapse is balanced by outward thermal radiation pressure generated by nuclear fusion reactions converting hydrogen nuclei into helium.

Stellar Luminosity and Spectral Classification

Stellar luminosity represents the total radiative energy released per second;
- dark lines in a star's absorption spectrum identify chemical elements present in its outer photosphere.

Astronomical Distance Measurement via Parallax

Stellar parallax measures the apparent angular shift of nearby stars against distant background stars as Earth orbits the Sun, providing a geometric baseline for determining stellar distances.

Binding energy

The minimum energy required to separate the nucleons of a nucleus to infinity.

Kinetic energy

K = ½mv², energy of motion.

Fusion

Joining of light nuclei into a heavier one.

Luminosity

The total power of radiation emitted by a star.

Nucleus

Small, dense, positively charged centre of the atom.

Reaction

The partner force acting on the other object.

Parallax

Apparent shift between objects caused by viewing from different positions

Radius

Distance from the centre to the circular path.

White dwarf

Small, hot, dense core left by a low-mass star.

Stellar parallax

Apparent shift of a nearby star against distant stars as Earth orbits the Sun.

Key terms

Term	Meaning
Continuous spectrum	Light of all wavelengths in a range, from a hot dense source.
Emission spectrum	Bright lines from a hot, low-pressure gas.
Absorption spectrum	Dark lines where a cooler gas absorbs light from a continuous source.
Fraunhofer lines	Dark absorption lines in the solar spectrum.
Wien's law	λ_max T = 2.90 × 10^-3 m K.
Doppler shift	Change of observed wavelength due to relative motion.
Spectral class	Classification O, B, A, F, G, K, M by temperature.

Avoid these mistakes

Saying absorption lines are made by the star's core — they come from the cooler outer atmosphere.

Thinking every element has the same lines — each has a unique pattern.

Using Wien's law with temperature in degrees Celsius — use kelvin.

Mixing up redshift and blueshift — a receding source gives longer wavelengths.

Confusing the continuous spectrum, which has no gaps, with an emission spectrum.

Last updated: 18 October 2026