As far as my limited understanding goes, the atom does not strictly have an enclosure like a marble. It's a probabilistic space with nucleus at the centre. So here, are we looking at the nucleus itself?
You are probably thinking of the electron orbitals which describe the spatial distribution of where the electrons are located. Their characteristic length scale is thousands of times smaller than the wavelength of light, so there is absolutely no way of resolving this structure. From an optical perspective an atom simply looks like an infinitely small radiating dipole.
The nucleus itself is again tens of thousands of times smaller than the orbitals. Due to this small size and very high resonance energies, nuclei don't interact with visible light at all.
I don't think nuclei emit any light except when they're either fusing or breaking apart. The light is probably coming from a cloud of excited electrons, a certain number of which have an extremely high probability of existing near each Strontium nucleus.
