Science Weblecture for Unit 62
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You should recall from our discussion of atomic structure that when an electron changes to a lower energy level, it emits energy E in the form of light at a wavelength hc/ν. The exact amount of energy is determined by the energy levels available to the electron in a particular element or molecular structure, so the wavelengths of light hydrogen gives off differ from the wavelengths of light helium gives off. The emission spectra of a particular glowing hot object like a star or even a tea kettle can tell us what elements are present in it.
Electrons can also absorb energy if they are in a low energy state. Cooler gases in front of hot stars, or the atmosphere of a star itself, will absorb some of the light energy passing through it. The amounts of energy to "jump" to a higher orbit exactly equal the amounts of energy given off when the electron "falls" to a lower orbit, so a dark-line absorption spectrum against a continuous spectrum will also tell us the nature of the elements or molecules absorbing the light.
By looking at the absorption spectra of thousands of stars, we can start ot make some assumptions about the kinds of stars there are in the universe.
Read through the description of stellar classes at the Australian Telescope Education site.
Like the Periodic Table, the Hertzprung-Russell diagram packs a lot of information into a very small space. Spatial layout reveals new information and relationships beyond the absolute data presented.
Read about the HR diagram.
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