--- title: "Stellar Spectra, Spectral Classes and Surface Composition" description: "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" canonical: https://lightmysky.com/learn/science/stellar-spectra-spectral-classes-and-surface-composition-mt_4FBh5G7Gdc source: https://lightmysky.com/learn/science/stellar-spectra-spectral-classes-and-surface-composition-mt_4FBh5G7Gdc.md retrieved: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # 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. Subject: Science · Area: Astronomy & Astrophysics · Ages 20 to 21 Page: https://lightmysky.com/learn/science/stellar-spectra-spectral-classes-and-surface-composition-mt_4FBh5G7Gdc ## Ready when they can - 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 ## Lesson: Reading starlight line by line 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. *(drawing: Temperature decreases from O to M. This ordering does not rank hydrogen abundance.)* 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. *(drawing: These boxes compare temperature ranges. They are not energy levels, and their spacing does not measure temperature.)* 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. **Example.** 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. **Recap.** 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.* ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Blackbody Radiation and the Quantum of Energy](https://lightmysky.com/learn/science/blackbody-radiation-and-the-quantum-of-energy-mt_B2Bk2IAOa9) - [Energy Levels and Line Spectra](https://lightmysky.com/learn/science/energy-levels-and-line-spectra-mt_RlMCSDxE5q) ## Opens up - [The Sun as a Star: Photosphere, Corona and the Activity Cycle](https://lightmysky.com/learn/science/the-sun-as-a-star-photosphere-corona-and-the-activity-cycle-mt_AMIkiU7wRz) - [The Interstellar Medium and How Stars Form](https://lightmysky.com/learn/science/the-interstellar-medium-and-how-stars-form-mt_i2HyLf_Emy)