Looking at the Bohr atom and how atoms can be excited and subsequently emit photons when they de-excite
a correction: around 19:00 when discussing the different ways that the atom can de-excite, I overlooked one of the possible routes (4 to 3 to 2 to 1 in 3 steps). this would add a 6th distinct photon energy.
Also just wanted to point out that the spectrum of light you would see from a star is more complicated than I suggest in the video. This doesn't affect the key point, but I should have chosen an electrically excited gas (e.g a neon tube) as an example of something that gives a simple spectrum of bright emission lines.