Split a star's light through a prism and dark lines appear — each element leaves its own fingerprint. Without ever visiting, astronomers read a star's temperature and ingredients from its light alone, turning stars into physics labs.
A balance of gravity and fusion
A star is a self-regulating sphere held in hydrostatic equilibrium: gravity pulls its gas inward while pressure from hot, fusing plasma pushes outward. In the core, hydrogen nuclei fuse into helium, converting a sliver of mass into energy via E = mc². This energy, working its way out over millennia, is what makes a star shine — and what holds it up against its own weight.
Split a star's light and dark absorption lines appear where atoms in its atmosphere swallow specific wavelengths. Their pattern reveals composition; the overall color and peak wavelength reveal temperature (Wien's law). We know what stars are made of without ever visiting one.
Plotting stars by temperature and luminosity yields the Hertzsprung–Russell diagram, where most stars fall along a main sequence. A star's mass sets where it lands and how it will live and die — massive stars burn hot, bright, and briefly.
- You are given three stellar spectra: one peaks in blue, one in yellow-green, one in red.
- Use Wien's law (hotter means shorter peak wavelength) to rank the three by temperature.
- Match each to a spectral class: O/B (hot, blue), G (Sun-like, yellow), M (cool, red).
- Predict which of the three is likely most massive and shortest-lived, and justify it.
What you should see: You inferred temperature, class, and rough lifetime from color alone — the core method of observational stellar astrophysics.