Stellar Spectra, Spectral Classes and Surface Composition
A stellar spectrum combines an approximately thermal continuum with absorption and sometimes emission lines. Temperature affects excitation and ionisation, so line strengths need atmospheric modelling to reveal abundances. Spectral classes follow temperature. Line widths and wavelength shifts give different clues about atmospheric conditions, rotation and radial motion.
What a learner can do afterwards
- Explains why visible hydrogen Balmer absorption peaks near A-star temperatures through the competing effects of excitation and ionisation
- Distinguishes a continuum-based temperature estimate from abundance inference using line strengths and atmospheric temperature and pressure
- Names line-broadening mechanisms, distinguishes width from central wavelength, and interprets radial Doppler shifts under stated assumptions
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You have studied blackbody spectra: hotter bodies peak at shorter wavelengths. A star's smooth background, its continuum, is approximately thermal. Its shape helps estimate surface temperature after accounting for dust reddening and the instrument's response. Atomic line patterns identify elements. Estimating their abundances needs an atmosphere model, so temperature and composition estimates are not independent.
Absorption raises an electron between allowed energy levels; emission lowers it. A hot, optically thick source gives a continuum. Cooler gas in front can make absorption lines. Excited, thin gas without a bright background can show emission lines. Temperature changes the line pattern used for classification. The main stellar classes run O, B, A, F, G, K, M, from hottest to coolest.
Visible Balmer absorption starts from n = 2, the first excited level of neutral hydrogen. Cool atmospheres leave most neutral hydrogen in the ground state. Near A-star temperatures, enough neutral atoms occupy n = 2 for strong Balmer absorption. Hotter atmospheres ionise more hydrogen. Ionised hydrogen has no bound electron for a Balmer transition. Excitation and ionisation together explain the peak near A types.
Identify an element by matching several line wavelengths to laboratory patterns, allowing for wavelength shifts. Estimate abundance by comparing line strengths with atmosphere models that include temperature and pressure. These conditions change excitation and ionisation. A weak or missing line need not mean that the element is scarce. A red continuum alone cannot measure hydrogen abundance.
Thermal motion and turbulence produce a spread of Doppler shifts. Rotation mixes light from approaching and receding surface regions. Collisions and electric fields cause pressure broadening. These effects widen lines; the instrument adds width too. Width alone cannot identify one cause. In an ideal symmetric model, broadening leaves the centre fixed. Real profiles can be asymmetric.
Compare a line's centre with its laboratory wavelength. If only radial Doppler motion causes the shift, longer means moving away and shorter means approaching. Other shifts, including gravitational redshift, must be separated before assigning radial velocity. Width and centre can change together. Unchanged width does not prove that its individual causes are unchanged.
The observed centre is at a longer wavelength. A radial Doppler explanation means recession. This axis does not show line width.
A line-profile plot shows brightness against wavelength. Compare ideal symmetric absorption dips using the same width definition. A dip centred at 540 nm with width 2 nm becomes broader if its width rises to 4 nm while its centre stays at 540 nm. A dip of width 2 nm centred at 541 nm instead has a shifted centre without extra width. Compare the centre and width separately.
Continuum shape helps estimate temperature. Model line strengths to infer abundance. Balmer absorption peaks near A types. Compare line width and central wavelength separately.
This lesson, its questions and flashcards were written for this dataset. Astronomy textbook records were consulted for scientific facts and teaching design; their wording is not reproduced.
The readings and flashcards were written for this dataset. Astronomy textbook records informed the scientific fact checks, without reproducing their wording. Atomic spectra and Blackbody radiation are linked video resources credited to Khan Academy in the source catalogue. No video reuse licence is asserted.
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What the smooth background tells you
A stellar spectrum records how much light reaches us at different wavelengths. Its smooth background is called the continuum. Narrow features can sit above or below that background as emission or absorption lines.
The continuum resembles thermal radiation. Comparing its shape with thermal models helps estimate the temperature of the emitting layers. This extends the blackbody method you have already studied, but a real stellar atmosphere is not a perfect blackbody.
The measured shape also depends on what happens between the star and the measurement. Dust can make the light redder, and an instrument responds differently at different wavelengths. Those effects must be considered before turning continuum shape into a temperature estimate.
Atomic lines supply other constraints. Their wavelength patterns help identify elements and ionisation states. Their strengths depend on abundance and on the atmospheric conditions that make the transitions possible. Temperature estimated from the continuum can therefore help interpret the lines. These are different diagnostics of one atmosphere, not measurements that can always be treated independently.
An atom needs the right starting state
A bound electron can absorb a photon when its energy matches an allowed upward transition. The electron must already occupy the lower level involved. Knowing the energy gap does not tell us how many atoms are ready to absorb.
Visible Balmer absorption in hydrogen starts from n = 2. A neutral atom in the ground state cannot make that same Balmer transition. An ionised hydrogen atom has no bound electron at all. It must capture an electron before bound-level transitions become possible.
This starting-state requirement matters when estimating composition. A star may contain plenty of an element while showing a weak line from one particular state. Temperature and pressure influence excitation and ionisation, changing the number of suitable absorbers.
Use several lines when identifying an element. Compare their pattern with laboratory wavelengths after accounting for any spectral shift. To estimate abundance, compare strengths with an atmosphere model. A missing line is evidence for low abundance only when the conditions and measurement sensitivity would otherwise make that line detectable.
Why hydrogen lines peak within the sequence
The main stellar classes run O, B, A, F, G, K, M from high to low surface temperature. Their line patterns change because atoms occupy different excitation and ionisation states at different temperatures.
Hydrogen shows why line strength alone cannot order every star by temperature. In a cool atmosphere, most neutral hydrogen is in its ground state. Very little occupies n = 2, the starting level for visible Balmer absorption. Heating increases excitation, making more atoms available for those transitions.
At higher temperatures, increasing ionisation removes bound electrons from more hydrogen atoms. Balmer absorption therefore weakens again. Its greatest strength occurs near A-star temperatures, where enough hydrogen remains neutral and enough occupies n = 2. This does not mean A stars have the greatest hydrogen abundance.
A weak Balmer line can occur on either side of the maximum. Other spectral features help distinguish a hotter atmosphere from a cooler one. Classification uses a pattern of features rather than one line alone.
Cooler spectral types include L, T and Y. Spectral type describes atmospheric properties. Some low-mass stars and brown dwarfs have similar spectra, so a cool type alone does not always distinguish them.
Different measurements from the same spectrum
A line profile shows brightness across the wavelengths around a line. Compare its centre and width separately. An ideal symmetric line can become wider without moving its centre; real profiles can also be asymmetric.
Thermal motion, turbulence and rotation combine different Doppler shifts. Collisions and electric fields can cause pressure broadening. Instrumental response adds width too. A measured width alone cannot identify a unique physical cause.
For radial Doppler motion, a line centre at a longer wavelength than its laboratory value indicates recession. A shorter wavelength indicates approach. Other offsets, including gravitational redshift, must be considered before interpreting the measured shift as radial velocity. A star can have both shifted and broadened lines.
Pressure-sensitive features also help constrain the atmosphere. Combined with temperature and calibrated stellar models, they can help estimate luminosity class and luminosity. Temperature class alone does not determine luminosity because stars of the same temperature can have different radii.
An estimated luminosity can be compared with apparent brightness, corrected for dust, to estimate distance. This method depends on calibration and atmospheric interpretation. It is not a distance obtained from temperature class or line-centre shift alone.
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8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.