The light that leaves the cloud right shows absorption lines in the spectrum at discrete frequencies. According to quantum mechanics, an atom , element or molecule can absorb photons with energies equal to the difference between two energy states. Photons with specific energies will be absorbed by an atom, ion or molecule if this energy is equal to the difference between the energy levels.
However, when the radiation passes through a gas, some of the electrons in the atoms and molecules of the gas absorb some of the energy passing through. The particular wavelengths of energy absorbed are unique to the type of atom or molecule. The radiation emerging from the gas cloud will thus be missing those specific wavelengths, producing a spectrum with dark absorption lines. The atoms or molecules in the gas then re-emit energy at those same wavelengths.
If we can observe this re-emitted energy with little or no back lighting for example, when we look at clouds of gas in the space between the stars , we will see bright emission lines against a dark background. The emission lines are at the exact frequencies of the absorption lines for a given gas. Kirchhoff's Law of Spectral Analysis. The same phenomena are at work in the non-visible portions of the spectrum, including the radio range.
As the radiation passes through a gas, certain wavelengths are absorbed. Those same wavelengths appear in emission when the gas is observed at an angle with respect to the radiation source.
Why do atoms absorb only electromagnetic energy of a particular wavelength? And why do they emit only energy of these same wavelengths? Some of the reemitted light is actually returned to the beam of white light you see, but this fills in the absorption lines only to a slight extent. The reason is that the atoms in the gas reemit light in all directions , and only a small fraction of the reemitted light is in the direction of the original beam toward you.
In a star, much of the reemitted light actually goes in directions leading back into the star, which does observers outside the star no good whatsoever. Figure 3 summarizes the different kinds of spectra we have discussed.
An incandescent lightbulb produces a continuous spectrum. When that continuous spectrum is viewed through a thinner cloud of gas, an absorption line spectrum can be seen superimposed on the continuous spectrum. If we look only at a cloud of excited gas atoms with no continuous source seen behind it , we see that the excited atoms give off an emission line spectrum.
Figure 3: Three Kinds of Spectra. When we see a lightbulb or other source of continuous radiation, all the colors are present.
When the excited cloud is seen without the continuous source behind it, its atoms produce emission lines. We can learn which types of atoms are in the gas cloud from the pattern of absorption or emission lines. Atoms in a hot gas are moving at high speeds and continually colliding with one another and with any loose electrons. They can be excited electrons moving to a higher level and de-excited electrons moving to a lower level by these collisions as well as by absorbing and emitting light.
The speed of atoms in a gas depends on the temperature. When the temperature is higher, so are the speed and energy of the collisions. The hotter the gas, therefore, the more likely that electrons will occupy the outermost orbits, which correspond to the highest energy levels. This means that the level where electrons start their upward jumps in a gas can serve as an indicator of how hot that gas is.
In this way, the absorption lines in a spectrum give astronomers information about the temperature of the regions where the lines originate. We have described how certain discrete amounts of energy can be absorbed by an atom, raising it to an excited state and moving one of its electrons farther from its nucleus. If enough energy is absorbed, the electron can be completely removed from the atom—this is called ionization. The atom is then said to be ionized.
The minimum amount of energy required to remove one electron from an atom in its ground state is called its ionization energy. Still-greater amounts of energy must be absorbed by the now-ionized atom called an ion to remove an additional electron deeper in the structure of the atom. Successively greater energies are needed to remove the third, fourth, fifth—and so on—electrons from the atom. If enough energy is available, an atom can become completely ionized, losing all of its electrons.
A hydrogen atom, having only one electron to lose, can be ionized only once; a helium atom can be ionized twice; and an oxygen atom up to eight times. When we examine regions of the cosmos where there is a great deal of energetic radiation, such as the neighborhoods where hot young stars have recently formed, we see a lot of ionization going on. An atom that has become positively ionized has lost a negative charge—the missing electron—and thus is left with a net positive charge. It therefore exerts a strong attraction on any free electron.
Eventually, one or more electrons will be captured and the atom will become neutral or ionized to one less degree again. During the electron-capture process, the atom emits one or more photons. Which photons are emitted depends on whether the electron is captured at once to the lowest energy level of the atom or stops at one or more intermediate levels on its way to the lowest available level.
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