atomic energy levels

Imagine a staircase where you can stand on a step or another step, but never float in between. The electrons in an atom are a bit like that: they are only allowed to have certain specific amounts of energy, called energy levels, and nothing in between. This is one of the strangest and most important facts in all of physics, because it is the reason every kind of atom carries its own private set of 'steps'.

An electron sits near the nucleus when it has little energy (the bottom step, called the ground state) and farther out when it has more (the higher steps, the excited states). To move up a step the electron must absorb exactly the right amount of energy; to move down it must release exactly that amount. The gaps between steps are fixed for each element — hydrogen has one ladder, sodium another, iron yet another — so the energy an atom can take in or give out comes only in those exact-sized chunks.

These fixed levels are the deep reason starlight can be read at all. When an electron jumps between two levels it deals in a photon of one precise energy, and energy fixes colour. So each element stamps light with a unique pattern of colours set by its own ladder of levels. Decode the pattern and you have identified the element — across a hundred trillion kilometres of empty space, without ever touching the star.