Strontium has an atomic radius of 215 picometers, and people cannot see things whose size is smaller than the wavelength of light reflected back at you from "an object." The shortest wavelength of light humans can see is much larger than this atom (around 400 nanometers).
This atom in this picture is always giving off light, but as I've explained above, there's no way you could ever resolve that light with the human eye. It would be just like when you go outside at night and look up at the stars. When you do this, you can see a single point of light because it is so far away that its angular size is essentially zero. So how does something smaller than the wavelength of visible light, emit light that we can see?
Well, what's going on with this clever photography is that the photons its emitting have been excited. A little science here: electrons relax by emitting photons. We usually get lots of photons from across the surface of an object with appreciable size. But in the case of the Strontium atom, we're getting a bunch of photons from just one source (a single point) making it something that we could see with the naked eye.
I expect Pat Dilloway to say, "Neat" in the comments. But it is neat. At least, I think so.