How did scientists know about the sun atmosphere? We know for instance that sodium (Na) is present in the sun atmosphere. How did they detect it? Did they take a trip to the sun with their chemistry lab and analyzed the sun atmosphere? I doubt.

The answer is that each element has its “finger prints” or spectral lines.

Each element has a specific atom. Ordinarily, an atom is in the state of lowest possible energy, its ground state. In the Bohr model, the ground state corresponds to the electron being in the innermost orbit. However, an atom can absorb energy which raises it to a higher energy level (corresponding in the Bohr picture to the movement of an electron to a larger orbit). The atom is then said to be in an exited state. Generally, an atom remains exited for only a very brief time. After a short interval, it drops back down to its ground state, with the simultaneous emission of light. The atom may return to its lowest state in one jump, or it may make the transition in steps of two or more jumps, stopping at intermediate levels on the way down. With each jump, it emits a photon of the wavelength that corresponds to the energy difference between the levels at the beginning and end of that jump.

Thus it can be absorbed by those hydrogen atoms with electrons in their second orbits. Other photons will have the right energies to raise electrons from the second to the fourth, or the first to the fifth orbit, and so on. Only photons that have exactly this energy can be absorbed. All the other photons will stream past the atom untouched. Thus, the hydrogen atoms absorb light only at certain wavelengths, which are then darker in the spectrum (A) Conversely, hydrogen atoms in which electrons move from larger to smaller orbits emit light of those energies that correspond to the energy difference between permissible orbits.

Because atoms that have absorbed light and have become excited generally de-excite themselves and emit that light again, you might wonder why dark spectral lines are ever produced. In other words, why doesn’t this re-emitted light “fill in”’ the absorption lines? Some of the re-emitted light actually is received by us, but this light fills the absorption lines only to a slight extent. The reason is that the atoms re-emit light in mostly random directions, and only a small fraction of it is directed towards the observer. On the other hand, we can observe the re-emitted light as emission lines if we can view the absorbing atoms from a direction from which little or no background light is coming (B)-as we do ,for example, when we look at clouds of hot gas located in the space between the stars.