How Atomic Spectra Tell Us What Stars Are Made Of

In 1835, the French philosopher Auguste Comte gave an example of knowledge that humans could never obtain: the chemical composition of the stars. We could study their shapes and movements, he argued, but never what they were made of. Within about 25 years, he was proven spectacularly wrong. The key was the atomic spectrum: every element absorbs and emits light at its own unique set of wavelengths, a fingerprint that can be read from light that has travelled across billions of kilometres, or billions of light years. This article explains how.

The principle: fingerprints in light

Electrons in atoms can only have certain energies. When they move between energy levels, they absorb or emit photons of exactly the matching energy, and therefore exactly the matching wavelengths. See photons and energy levels.

• A hot, thin gas gives an emission spectrum: bright lines at specific wavelengths.
• A cooler gas in front of a hot source gives an absorption spectrum: dark lines in a continuous rainbow, at the same wavelengths.

Because every element’s energy levels are different, its lines form a unique pattern. See emission vs absorption spectra.

The dark lines in sunlight

In 1814, the German optician Joseph von Fraunhofer built precise spectroscopes and found that the Sun’s spectrum is crossed by hundreds of fine dark lines. He mapped and labelled the strongest ones (A, B, C, D…) but couldn’t explain them.

Around 1859, Robert Bunsen and Gustav Kirchhoff, working in Heidelberg, found that the Fraunhofer D lines in the yellow exactly match the bright yellow lines emitted by sodium in a flame. They realised the dark lines were produced by sodium atoms in the Sun’s cooler outer layers absorbing light from the hotter layers beneath. Soon they identified iron, calcium, magnesium and other elements in the Sun. Comte’s “impossible” knowledge had arrived. See flame test colours.

An element found in the Sun first: helium

During a total solar eclipse in 1868, astronomers studying the Sun’s glowing outer atmosphere recorded a bright yellow emission line at about 587.6 nm, close to but not matching sodium’s D lines. The English astronomer Norman Lockyer concluded it came from an unknown element, which he named helium, after helios, the Greek word for the Sun.

Helium wasn’t found on Earth until 1895, when William Ramsay detected the same line in gas released from a uranium mineral. It remains the only element discovered in space before it was found on Earth. See noble gases.

What stars are really made of

For decades, astronomers assumed stars had roughly the same composition as Earth, because their spectra showed strong lines of iron, calcium and other metals.

In 1925, the British-American astronomer Cecilia Payne (later Payne-Gaposchkin), in her doctoral thesis, applied new ideas from atomic physics about how temperature affects ionisation and excitation. She showed that the strength of a spectral line depends not only on how much of an element is present, but on what fraction of its atoms are in the right state to absorb at that wavelength.

Her conclusion was astonishing: stars are made overwhelmingly of hydrogen and helium, with only tiny amounts of heavier elements. Hydrogen’s lines looked relatively weak simply because, at stellar surface temperatures, few hydrogen atoms are in the right excited state to absorb visible light. Her result was initially doubted, but was confirmed within a few years. It’s now known that the universe’s ordinary matter is about 74% hydrogen and 24% helium by mass. See most abundant elements in the universe.

Reading a star’s temperature

A star’s spectrum reveals its surface temperature:

1. Colour: hotter stars emit more blue light; cooler stars appear redder.
2. Which lines appear: at different temperatures, different elements and ions produce strong lines.

Astronomers classify stars into spectral classes, traditionally O, B, A, F, G, K, M, from hottest to coolest:

Class	Approximate surface temperature	Typical colour	Notable lines
O	above 30,000 K	blue	ionised helium
B	10,000–30,000 K	blue-white	neutral helium, hydrogen
A	7,500–10,000 K	white	very strong hydrogen (Balmer) lines
F	6,000–7,500 K	yellow-white	hydrogen, ionised calcium
G	5,200–6,000 K	yellow (the Sun)	ionised calcium, iron and other metals
K	3,700–5,200 K	orange	neutral metals
M	below 3,700 K	red	molecules such as titanium oxide

This classification, developed largely by Annie Jump Cannon and colleagues at Harvard, who classified hundreds of thousands of stars, underpinned Payne’s work.

Hydrogen’s Balmer lines are strongest in A stars, where the temperature is just right for many hydrogen atoms to have electrons in the n = 2 level, ready to absorb Balmer-series photons. See the hydrogen emission spectrum.

Measuring motion: redshift and blueshift

If a light source moves away from us, its spectral lines shift to longer wavelengths (redshift); if it moves towards us, they shift to shorter wavelengths (blueshift). This is the Doppler effect for light.

Because atomic lines have precisely known wavelengths, the shift can be measured accurately:

• Stars orbiting each other show lines that shift back and forth, revealing binary star systems and their masses.
• Exoplanets can be detected by the tiny back-and-forth wobble they cause in their star’s spectral lines.
• Distant galaxies show redshifts that increase with distance. In the 1920s, Edwin Hubble and others used this to show that the universe is expanding.

Exoplanet atmospheres

When a planet passes in front of its star, some starlight passes through the planet’s atmosphere. Atoms and molecules in the atmosphere absorb specific wavelengths. By comparing the spectrum during and outside the transit, astronomers detect gases such as water vapour, sodium, carbon dioxide and methane in the atmospheres of planets orbiting other stars. Space telescopes such as the James Webb Space Telescope have made this a routine technique.

Chemistry between the stars

Spectroscopy also reveals the chemistry of the thin gas and dust between the stars:

• Radio astronomy detects the 21 cm line of hydrogen atoms, mapping hydrogen across the Milky Way.
• Molecular lines at radio and infrared wavelengths have revealed more than 200 molecules in space, from water and ammonia to alcohols, sugars (such as glycolaldehyde) and even buckminsterfullerene (C₆₀). See allotropes of carbon.
• Glowing nebulae show emission lines of hydrogen, oxygen and nitrogen, giving them their red, green and pink colours in images.

Where the elements came from

Spectra show that older stars contain fewer heavy elements than younger ones like the Sun. This fits the picture that heavy elements are made in stars and scattered by stellar winds, supernovae and neutron-star collisions, gradually enriching the gas from which new stars form. The iron in your blood and the calcium in your bones were made in stars. See how new elements are made.

Key takeaways

• Every element absorbs and emits light at its own characteristic wavelengths, so spectra reveal composition from afar.
• Bunsen and Kirchhoff identified the dark Fraunhofer lines in sunlight as absorption by elements such as sodium.
• Helium was discovered in the Sun’s spectrum in 1868, decades before it was found on Earth.
• Cecilia Payne showed in 1925 that stars are mostly hydrogen and helium; spectral classes reveal star temperatures.
• Line shifts reveal motion, from binary stars and exoplanets to the expansion of the universe.