Reading starlight as a spectrum — Astronomy, 11–13 years
Starlight carries more than brightness. Spread into a spectrum, it contains lines that reveal what a star is made of, how hot it is and whether it is moving towards or away from us.
Light with a fingerprint
White-looking starlight is a mixture of many colours. A spectroscope spreads those colours into a band, and dark or bright lines appear in it. Each chemical element makes a particular set of lines, like a fingerprint, because its electrons can change energy in particular amounts. Astronomers can therefore study a star without collecting a sample from it.
Why spectra matter
A telescope can show a star as a tiny point, but a point does not tell us its recipe. Astronomers needed a way to learn about distant objects using only the light that arrived here. Laboratory experiments showed that heated elements produce repeatable lines. Matching a star’s lines with those laboratory patterns turned light into evidence about chemistry, temperature and motion.
Finding hydrogen in a star
Imagine a laboratory hydrogen lamp with a bright line at 656 nanometres. First, an astronomer spreads a star’s light into a spectrum. Next, a dark line appears at the same 656-nanometre position. Finally, the match is checked against other hydrogen lines, such as 486 nanometres. Several matching lines are strong evidence that hydrogen is in the star’s outer gas.
The reasonable mistake
A reasonable mistake is to assume that a red-looking star must contain a lot of red-coloured material. Colour is not paint in space. It tells us mainly about the distribution of light and the star’s surface temperature, while its chemical elements are identified by their narrow spectral lines. A star can look red and still contain mostly hydrogen and helium.
Spectra beyond stars
Astronomers use spectra to study planets, glowing gas clouds and galaxies. A shifted line can show that an object is moving, while the strength of lines can help estimate temperature or the amount of a gas. The same basic idea is used on Earth in chemical testing and in some medical instruments: light helps identify substances that are hard to inspect directly.
Keep exploring
Other languages
Loading MyLeoNes™…