It is the intricate intertwining of light and matter (atoms) that yields a plethora of knowledge and ideas about not only stars but the universe as well.
The Continuous Spectrum
We can produce a continuous spectrum of visual light using any object that behaves like a blackbody.
If we just observe the light produced by a hot low density gas we see a spectrum that consists of a series of bright emission lines on a dark background. These lines are characteristic to the discrete chemical composition of the gas.
The Interaction of Light and Matter
At the beginning of the 20th century, scientists were perplexed by the failure of classical physics to explain the characteristics of atomic spectra.
Why did atoms of a given element only emit certain lines?
Why did those same atoms absorb only those wavelengths that they emitted?
Why did atoms of a given element only emit certain lines?
Why did those same atoms absorb only those wavelengths that they emitted?
What is going on to produce these spectra?
We now have an idea of the atom's structure and energy levels as well as the various types of spectra, so now we can discuss how the atoms produce these patterns or "light fingerprints."
The electron in a higher energy state moves down to a lower energy state and produces light with energy that corresponds exactly to the specific energy difference between the two levels.
The spectra of this atom before and after the energy level transition. Notice that the one color corresponds to one specific unique transition within the atom.
The spectra of this atom before and after the energy level transition. Notice that the one color corresponds to one specific unique transition within the atom.
Absorption Spectra Creation
The electron in a lower energy state moves up to a higher energy state and absorbs light with energy that corresponds exactly to the specific energy difference between the two levels.
Absorption: We just talked about this above with the upward transitions of electrons. The atom can absorb light energy or photon. Only a photon with exactly the right amount of energy can move the electron from one level to another. If the photon has too much or too little energy the atom cannot absorb it.
Collisional Excitation – (bang!): An atom can become excited by a collision with another atom. If two (or more) atoms collide, one or both may have electrons knocked (physically) into a higher energy state. This happens very commonly in a hot gas where atoms move very rapidly and collide vigorously because of the hot temperature.
De-Excitation
Electron moves down? – De-excitation. The technical term for the emission of a photon of light with energy and wavelength that corresponds to the energy difference between the energy levels the electron moved downward between.
What happens? Light is given off!! A “light fingerprint” is born.
Gotcha! Stellar Spectra
Give an astronomer a small trickle of light and the riddle will be solved. Science has decoded the Rosetta Stone of stellar spectra. The spectrum of a star tells us a great deal about such things as temperature, motion, composition and much more. A stellar spectra is like a "mug-shot, fingerprint, and DNA test" rolled up into one science.
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