The "classical" model of the hydrogen atom is one electron orbiting one proton.
The electrons are located at specific distances from the nucleus. At rest (n=1), the orbit of the electron has a radius of meters (Bohr classical radius). The electron can also be on larger orbits, associated with integers n = 2, 3, 4...n = ∞
The nucleus has a radius of meters. The atom at rest is 10000 times bigger than the nucleus. If the nucleus was a coin of 10 centimes, the atom would be a sports ground.
An atom can pass from a fundamental state (n=1) to an excited one, absorbing a photon. It can also come back to the fundamental state emitting light, the colour of which (wavelength) depends on the energy levels of the atom.
The transition from a level n2 to a level n1 corresponds to an emission/absorption of wavelength , such that , with
If the atom gets a sufficient amount of energy, the electron passes from the level n=1 to the level infinity. The atom loses its electron and becomes an ion. The corresponding wavelength is meters, i.e. in the UV range.
In the atmosphere of a star, the hydrogen atoms, enlightened by the star, only absorb the colours that make them pass from one level to another.