A star’s spectrum explained

This image shows the spectrum of a star obtained using spectroscopy. That is the splitting of light into different wavelengths using a prism. This principle is the same as that behind the creation of rainbows, where light from the Sun is split into its different wavelengths inside rain droplets; we see these different wavelengths as different colours. Likewise, light coming from stars can be passed through a prism and split into its different wavelengths. Some of these wavelengths – such as those in the infrared and UV parts of the spectrum – are invisible to our eyes, but detectors can measure them.

Some starlight is absorbed at specific wavelengths by molecules and atoms in a star’s outer layer. This means that the spectrum of a star has some places where light is ‘missing’, these regions are called absorption lines.

This image shows the two ways spectra with absorption lines can be visualised. In the upper image we see a rainbow with several black lines – the absorption lines. However, at the locations of these lights not all light is completely blocked. If we look at the brightness of the light in its different wavelengths in the bottom part of the image, we see that at the wavelengths of the absorption lines there is still some light. This is because the molecules and atoms do not absorb all the starlight.