Principles of the use of stellar spectral classes
1. Stellar spectra
- The magnitude of a star and its colour have proved useful for learning about the luminosity and temperature of the star.
- We can also learn about a star by observing the spectrum of the light that it emits.
- Figure 1 shows a spectrum of the light emitted from the Sun, when it is viewed through a diffraction grating.
- The continuous spectrum of light is crossed by dark absorption lines, called Fraunhofer lines. Such absorption lines are produced when light passes through the cooler gases in the outer atmosphere of the Sun.
Figure 1 Energy spectrum- Figure 2 helps to explain how the process works. The diagram shows an energy level diagram for a hydrogen atom.
Figure 2 Energy level diagram - When a photon has an energy exactly equal to the energy difference between levels 1 and 2, [math]E_2 – E_1 [/math] , the photon can be absorbed by an electron in energy level 1, which is promoted to level 2.
- Similarly, if there is an electron in level 2, it can move to level 3 if it absorbs a photon of energy [math]E_2 – E_1 [/math].
- When light is absorbed in this way, the intensity of these wavelengths is reduced, so black lines appear across the spectrum.
- Each element or compound has a unique set of energy levels. These energy levels lead to a unique absorption spectrum.
- Therefore, it is possible to see which elements are present in a star’s atmosphere by analysing the absorption lines in its spectrum.
2. Spectral classes:
- When the spectra of a large number of stars were studied, it was realised that stars could be divided into a number of spectral classes. These classes were based on which elements were most prominent in the spectra of stars-and these elements varied considerably from star to star.
- Originally it was thought that the observation of prominent elements related closely to the chemical composition of the star.
- However, although there are differences in stellar chemical composition, the most important factor in spectral classes is the star’s temperature.
- The reason why temperature is very important in determining the spectral class of a star is as follows.
- For a particular absorption line to be observed, there must be atoms present with an electron in the correct energy level.
- Hydrogen is the most abundant element in all stars. It is therefore no surprise that we see hydrogen absorption lines in stellar spectra, but we see different patterns of absorption at different temperatures.
- When a hydrogen atom is relatively cold, its one electron will lie in its ground state, n=1, nearest the nucleus.
- Therefore, this electron can be excited to the n=2 level by a photon of the correct energy.
- Such photons lie in the ultraviolet part of the spectrum, so we do not see these lines when a star is viewed in visible light.
- These ultraviolet lines are more visible in a star with a surface temperature of about 8000 K than in a star with a lower temperature of, for example, 5000 K, because the hotter star emits more ultraviolet light.
- However, hydrogen lines are not the most prominent lines seen in cooler stars, because other elements absorb more light than hydrogen.
- Table 1 Lists the various spectral classes of stars, with their prominent absorption lines.
- At higher temperatures, some electrons in atoms move into higher states.
- At temperatures between about 7500 K and 25000 K, hydrogen has a significant number of atoms with electrons in the n= 2 state.
- These temperatures correspond to the A and B spectral types.
- In these stars, we see prominent hydrogen absorption lines in the visible part of the spectrum.
- The electron in the n = 2 level is able to absorb photons to lift it to the n=3, n=4, n=5 levels and so on.
- This series of lines is called the Balmer series, after the scientist who discovered them.