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Elements in the stars

Radiation and the atom · level 3, lesson 5 of 6

A question to start

Nobody has ever been to the Sun, or brought back a piece of it. So how do we know what it is made of?

The idea

When an element is heated until it glows, it gives out light of its own particular colours. Spread that light out with a prism and you see a pattern of bright lines, different for every element, like a fingerprint.

Sunlight spread out finely has dark lines in it. Most of them match the fingerprint of an element in the Sun's outer layers. Matching them shows that the Sun is mostly hydrogen and helium, with small amounts of many other elements, such as iron, sodium and calcium.

Helium was found this way. During a solar eclipse in 1868, watched from India, astronomers saw a yellow line in the light from the Sun's edge that matched no known element. It was named helium, from the Greek word for the Sun. It was not found on Earth until 1895.

sunlight: dark linesglowing sodium: bright lines in the same place

Worked examples

1. Sodium. Sodium glows with two close yellow lines. The same two lines appear dark in sunlight, so there is sodium in the Sun.

2. A new line. The yellow line seen in 1868 matched no element on Earth, so it had to be a new one: helium.

3. Far stars. The same method tells us what stars billions of kilometres away are made of.

Try it

  1. What does each element give out when it glows?
  2. What do the dark lines in sunlight tell us?
  3. What are the two main elements in the Sun?
  4. Where does the name helium come from?
Answers
  1. Light of its own set of colours, a pattern of lines.
  2. Which elements are in the Sun's outer layers.
  3. Hydrogen and helium.
  4. The Greek word for the Sun.

Common mistake

"We need a sample to know what something is made of." Light carries the fingerprints of the elements, so we can read what a star is made of from its light alone.

Where it leads