Line spectra outside the visible spectrum
Uv- and X-ray emission lines
Absorption and emission spectra also occur in the UV and X-ray parts of the spectrum. That was only discovered since astronomers can launch telescopes into space to observe these parts of the spectrum. These absorption spectra come from the outer atmosphere of the star, the corona and the region between photosphere and corona, where the temperatures are much higher than at the photosphere of the star.
Thus electrons can be exited much more and consequently their energy jumps can be larger, emitting or absorbing photons with much higher energy (shorter wavelengths). As an example the typical photosphere temperature of our Sun is 6000 K while the corona temperature can go up to several million degrees K. So with observations from space we now can also study the stellar corona from line spectra.
Radio emission lines
Since the early 1950’s radio astronomers observing the Radio part of the spectrum with their radio telescopes, predicted spectral lines in the radio spectrum. Because the wavelengths here are much longer, the energy that causes such spectral absorption or emission must be much smaller than that occurring with electrons that change their energy state.
But another process inside the Hydrogen atom, the so-called spin-flip transition releases a much smaller amount of energy and causes a line at the 21 cm wavelength. This hydrogen line was predicted in the 1940’s by the Dutch astronomers Oort and Van de Hulst and has indeed been found with radio telescopes. These observations have been extremely helpful for the study of the Milky Way and other galaxies.
The advantage of radio waves is that they penetrate dust lanes that obstruct us from seeing large parts of our galaxy in the visible spectrum. Therefore this radio technique based on the 21 cm line has been vital for our understanding of the structure and behaviour of our Milky Way and other galaxies.
This can be used to detect molecules that consist of several atoms. Now many compounds have been found, especially in inter stellar space inside galaxies, and we now know that such matter is very diverse and common in the Universe. The combination of spectral research in different wavelength regions from infrared to microwave continues to be of great importance for understanding the structure of galaxies and the makeup of inter stellar space.