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
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Lesson summary
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 = Ei − Ef.
- 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.
2
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.
6
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.
2
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
- λmaxT = 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
1
Gas clouds between the star and us add their own absorption lines.
2
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
1
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.
2
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.
3
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.
4
Binding energy
- The minimum energy required to separate the nucleons of a nucleus to infinity.
5
Kinetic energy
- K = ½mv2, energy of motion.
6
Fusion
- Joining of light nuclei into a heavier one.
7
Luminosity
- The total power of radiation emitted by a star.
8
Nucleus
- Small, dense, positively charged centre of the atom.
9
Reaction
- The partner force acting on the other object.
10
Parallax
- Apparent shift between objects caused by viewing from different positions
11
Radius
- Distance from the centre to the circular path.
12
White dwarf
- Small, hot, dense core left by a low-mass star.
13
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
1
Saying absorption lines are made by the star's core — they come from the cooler outer atmosphere.
2
Thinking every element has the same lines — each has a unique pattern.
3
Using Wien's law with temperature in degrees Celsius — use kelvin.
4
Mixing up redshift and blueshift — a receding source gives longer wavelengths.
5
Confusing the continuous spectrum, which has no gaps, with an emission spectrum.
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