Caption: The spectrum of the K-type star 2MASS J19554455+4754531. The colour of the line between 400 nm and 700 nm roughly corresponds to the colour the human eye would see light of that wavelength. Below 400 nm and above 700 nm, where the human eye can see little to no light, the lines are coloured blue and red respectively. The black lines show spectral absorption lines caused by atoms and ions of different elements in the star’s atmosphere. These atoms and ions absorb at specific wavelengths, causing sharp, dark lines in the spectra. How strong these lines are depends on the temperature of the star’s atmosphere. Two stars made from the same mix of elements could have spectra with vastly different sets of lines in their spectra if they have different temperatures in their atmospheres. The spectra of K-type stars are dominated by metal atoms such as iron, sodium and calcium atoms. There are so many lines from metal atoms, far too many to mark individually, that the spectrum has a choppy, ragged appearance. The lines of hydrogen atoms and calcium ions are much weaker than in the hotter G-type stars.
Credit: IAU OAE/SDSS/Niall Deacon