Spectra & Composition

Section 1 of 8

Spectrum as Data

By the end of this reading, you will be able to:

Explain
Kirchhoff’s three laws and predict whether a source produces a continuous, emission, or absorption spectrum
Interpret
the Bohr model to explain why spectral lines occur at characteristic wavelengths
Use
spectral line wavelengths to determine which elements are present in a star’s atmosphere
Classify
stars by spectral type (OBAFGKM) and explain why this sequence is a temperature sequence, not a composition sequence
Apply
the Doppler shift formula quantitatively to find radial velocities from line shifts
Distinguish
spectral type (temperature), luminosity class (surface gravity), and metallicity (heavy-element abundance)
Synthesize
line positions, strengths, widths, and shifts to infer stellar properties
Connect
spectral absorption to the greenhouse effect and calculate a planet’s equilibrium temperature

Atoms are nature’s barcode readers — each element absorbs and emits light at a unique set of wavelengths determined by its electron energy levels. By measuring where spectral lines appear and how they shift, we read a star’s composition and velocity from across the galaxy. And the same physics that decodes starlight explains why adding CO₂ to an atmosphere warms a planet.

Spectrum

The intensity of light as a function of wavelength, obtained by dispersing light through a prism or diffraction grating. A spectrum is quantitative data: its continuum slope, emission peaks, and absorption lines each encode physical conditions of the source.

The Big Idea

A star’s light is an autobiography — if you know how to read it.

In Lectures 1 and 2, you learned to measure a star’s distance, luminosity, temperature, and radius from its brightness and color. But color is blunt: it tells you the overall temperature. Today you gain a scalpel. By spreading starlight into a spectrum — resolving individual wavelengths — you can identify which atoms are in the star’s atmosphere, measure the star’s velocity toward or away from you, and refine the temperature measurement to a precision that color alone cannot achieve.

What you’ll gain: after this reading, every dark line in a stellar spectrum will tell you a story: which element absorbed the photon, what temperature put that element in the right quantum state, and how fast the star is moving. Three independent pieces of information, all from the same observation. And the same physics will explain why Venus is 460°C while Mars is −60°C.