Studying the line spectra produced by hot gases and absorbed by cooler gases allows us to identify the elements in stars. When a gas is very hot, it doesn’t emit all wavelengths of light.

This causes line emission spectra to be produced ... produces photons with different energy and so the line spectra for different elements will be different. This means that line spectra can ...

The first, somewhat furtive, signal from element 63 was recorded in 1885 by Sir William Crookes, who found an anomalous red line (609 nm) in the emission spectrum of a samarium sample.