--- title: "Stellar Spectra, Spectral Classes and Surface Composition" description: "A star's spectrum is a continuum crossed by absorption lines, and which lines appear depends more on the surface temperature than on what the star is made of. Sorting stars by those lines gives the sp" 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: 2026-09-12 --- > **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 star's spectrum is a continuum crossed by absorption lines, and which lines appear depends more on the surface temperature than on what the star is made of. Sorting stars by those lines gives the spectral sequence, and the shape of each line carries rotation, pressure and motion along the line of sight. 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 hydrogen lines are strongest in intermediate stars rather than the hottest ones - Reads a temperature from the continuum and a composition from the line strengths, and says why the two are separate measurements - Names what broadens a spectral line and what shifts it, and distinguishes the two effects on a plotted profile ## Lesson: Reading starlight line by line A spectrum is a rainbow crossed by lines, and you read it by asking two questions in order. First the continuum: its overall colour and peak record the temperature of the glowing layers. Then the lines: which appear and how strong they are, corrected for temperature, record the atoms and ions present. Continuum shape gives temperature and line strengths give composition as separate measurements. Lines form when electrons jump between energy levels, absorbing photons on the way up and emitting them on the way down. A hot dense gas makes a smooth continuum. A cool gas in front of it carves dark absorption lines. A thin hot gas seen alone shines in bright emission lines. Stars sort into classes O, B, A, F, G, K, M from hottest to coolest. Hydrogen lines peak in A stars, not the hottest ones. In O stars the hydrogen is ionised, with no electron left to absorb. In cool stars most hydrogen sits in the ground state that the visible lines cannot reach. Only at middle temperatures is the first excited level well populated, so Balmer absorption is strongest there. The letters track ionisation balance, not hydrogen abundance. **Tip.** Each line profile carries motion imprints you must not confuse. Broadening smears the line symmetrically through thermal motion, pressure, turbulence, or rotation. A Doppler shift moves the whole line red or blue with the star motion along your line of sight. Width says how the gas churns and shift says where it is going. **Recap.** The continuum gives temperature, the lines give composition, hydrogen peaks in the middle classes, and width means churn while shift means motion. ## 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)