L20. Reading a Star’s Fingerprint

Unit 4 · Light and Telescopes

R-report

L20. Reading a Star’s Fingerprint

Nobody has ever brought back a sample of a star. Yet astronomers can tell you what a star is made of, how hot it is, and how fast it is moving. All of it comes from splitting its light apart.

Splitting light into a spectrum

Pass sunlight through a prism and it spreads into a band of colours. That band is a spectrum, and it is light sorted by wavelength. Look closely at a star’s spectrum and it is not perfectly smooth. It is crossed by narrow dark lines at very specific wavelengths. Those lines appear because atoms in the star’s outer layers absorb light — but only at particular wavelengths, determined by the structure of each element. Hydrogen absorbs one exact set of wavelengths, helium a different set, sodium another. No two elements share a pattern. That makes the lines a fingerprint. Photograph a star’s spectrum, measure where the dark lines fall, and compare them with patterns measured in laboratories on Earth, and you can identify the elements present in an object trillions of kilometres away. Helium was actually discovered this way. Unexplained lines in the Sun’s spectrum in 1868 revealed an element not yet known on Earth — hence the name, from helios, the Greek word for Sun.

Temperature, from colour and lines

A spectrum also reveals temperature, in two independent ways. First, the overall colour. Hot objects glow blue-white and cooler objects glow red — which is the opposite of the everyday colour coding on taps, but it is how thermal radiation works. - Blue stars: above roughly 20,000 °C at the surface. - White stars: around 10,000 °C. - Yellow stars like the Sun: about 5,500 °C. - Orange and red stars: 4,000 °C down to about 3,000 °C. Second, which lines appear. Different elements absorb only when they are in particular states, and temperature determines those states. Very hot stars show strong helium lines and weak hydrogen lines; cooler stars show molecules that would be torn apart at higher temperatures. Astronomers use these patterns to sort stars into spectral classes. So a single spectrum yields both an independent temperature estimate and a composition — from a point of light too small to resolve.

Motion, from shifted lines

The most far-reaching use of spectra is measuring motion. If an object moves toward you, the light waves arriving are slightly compressed, so every line in its spectrum shifts toward shorter, bluer wavelengths. If it moves away, the waves are stretched and the lines shift toward longer, redder wavelengths. This is the Doppler effect — the same reason a passing siren drops in pitch. The crucial detail is that the whole pattern shifts together while keeping its spacing. That is how astronomers know they are seeing a shift rather than different elements. Measure how far the pattern has moved and you get the speed directly. This one technique underpins an enormous amount of astronomy. - It reveals binary stars from the regular back-and-forth wobble of their lines. - It detects exoplanets by the tiny wobble a planet causes in its star. - It measures how fast galaxies rotate, which is how astronomers found evidence for dark matter. - It showed that distant galaxies are almost all redshifted — the observation that led to the expanding universe.

Putting the investigation together

Spectroscopy turns a point of light into a detailed report. The pattern of absorption lines identifies elements, the colour and the specific lines present give temperature, and any shift of the whole pattern reveals motion toward or away from us.

It is worth appreciating how much this single technique carries. Composition, temperature, and velocity for objects we can never visit, all extracted from light that has been travelling for years or millennia. Most of what the following lessons claim about stars, galaxies, and the expanding universe rests on it.