As these photons can re emitted in any direction an external observer will detect light at these wavelengths. When they de-excite they emit photons of specific frequency and wavelength. If this cloud can be excited by a nearby source of energy such as hot, young stars or an active galactic nucleus then the electrons in atoms of the gas cloud can get excited. Stellar spectra typically look like this.Įmission spectrum: A third possibility occurs if an observer is not looking directly at a hot black body source but instead at a diffuse cloud of gas that is not a black body. This means that the resultant spectrum will show dark absorption lines or a decrease in intensity as shown in the dips in the absorption spectrum top right in the diagram above. The net effect of this is that the intensity of light at the wavelength of that photon will be less in the direction of an observer. The direction of this re-emission however is random so the chances of it travelling in the same path as the original incident photon is very small. Eventually the electron will de-excite and jump down to a lower energy level, emitting a new photon of specific frequency. Photons of specific frequency can be absorbed by electrons in the diffuse outer layer of gas, causing the electron to change energy levels. The photons emitted from the core cover all frequencies (and energies). If we were able to view the light from this source directly without any intervening matter then the resultant spectrum would appear to be a continuum as shown bottom left in the Figure 1 above.Ībsorption spectrum: Most stars are surrounded by outer layers of gas that are less dense than the core. Figure 1: How continuous, emission and absorption spectra can be produced from same source.Ĭontinuum spectrum: In this diagram, a dense hot object such as the core of a star acts like a black body radiator.
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