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J2000 Right Ascension galaxy. A map of continuum emission from an HII region is shown in Fig. 15.10. (Note: In the encounter between the electron and proton, the proton also accelerates and gives off radiation. However, the acceleration of the proton is much less than that of the electron, by the ratio of their masses. This means that the radiation given off by the protons is not very important.)

HII regions also give off spectral line radiation, called recombination line radiation. When an electron and proton recombine to form a hydrogen atom, the electron often ends up in a very high state. The electron then starts to drop down. It usually falls one level at a time. Larger jumps are also possible, but less frequent. With each jump, a photon is emitted at a frequency corresponding to the energy difference for the particular jump. (The energies are given by equation 3.6.) For very high states, the energy levels are close together and the radiation is in the radio part of the spectrum. As the electron jumps to lower states the lines pass through the infrared and into the visible. Generally, the electron can go all the way down to the ground state before the atom is re-ionized. We even see Ha emission as part of this recombination line series. This gives HII regions a red glow. (This red glow allows us to distinguish HII regions from reflection nebulae, which appear blue.)

Fig 15.10.

Radio image (made with the VLA) of free-free emission from an HII region, the Orion Nebula (for which optical images appear in Fig. l5.28).This is a higher resolution image than the single dish version in Fig. 4.25. It shows the fine scale structure in the core of the nebula.The image was made with the VLA in D (smallest) configuration at 8.4 GHz, providing 8.4 arc sec resolution.This is a nine-field (3 X 3) mosaic.The interferometer picks up less than one-half of the total flux density because it is insensitive to the extended emission. Of course, it also gives beautiful detail of the structure in the nebula. [D. Shephard, R. Maddalena, J. McMullin, NRAO/AUI/NSF]

HII regions expand with time. When an HII region first forms (Fig. 15.11), it must grow to its equilibrium radius. Even after it reaches this equilibrium size, it will continue to expand. This is because the pressure in the HII region is greater than that in the expanding cloud. The higher pressure results from the higher temperature in the HII region. Remember, the temperature in an HII region is about 104 K, while that in the surrounding cloud is less than 100 K. The densities in the HII region and surrounding cloud are similar.

As the HII region expands, it can compress the material in the surrounding cloud, possibly initiating a new wave of star formation, as illustrated in Fig. 15.12. This is one possibility that has been discussed for the triggering of star formation. The gas compressed by an expanding HII region will not automatically form stars. That is because the gas will be heated as it is compressed. If that heat is not lost, the temperature of the cloud will increase.

HII Region

Molecular Cloud

Fig 15.11.

HII region in a molecular cloud. HII regions usually form near the edge.

The pressure will increase and the gas will expand again. The re-expansion of compressed gas can only be avoided if the gas can cool as it is compressed. Radiation from molecules such as CO in the surrounding cloud can help with this cooling process.

15.6.2 Masers

We have already seen how the process of stimulated emission can lead to a multiplication - or

Compressed Gas

Expanded HII Region

Expanded HII Region

Compressed Gas

Fig 15.12.

The HII region expands, compressing gas deeper within the cloud. If this gas can cool quickly, then it can collapse to form more stars.

amplification - in the number of photons passing through a material. In the stimulated emission process, one photon strikes an atom or molecule, and two photons emerge. The two photons are in phase and are traveling in the same direction. The fact that they are in phase means that their intensities add constructively. Stimulated emission can only take place if the incoming photon has an energy corresponding to the difference between two levels in the atom or molecule, and the atom or molecule is in the upper of the two levels.

If only a few atoms or molecules are in the correct state, there will not be a significant increase in the number of photons. Suppose we designate the two states in the transition as 1 and 2. The population of the lower state is n1 and the population of the upper state is n2. The requirement for amplification is that n2 /n1 be greater than g2 /g1, where the g are the statistical weights. The situation is called a population inversion, since it is the opposite to the normal situation. Formally, it corresponds to a negative temperature in the Boltzmann equation (see Problem 15.22). This is clearly not an equilibrium situation. The population inversion in a particular pair of levels must be produced by a process, called a pump. The pump may involve both radiation and collisions. The net effect of the pump process is to put energy into the collection of atoms or molecules so that some of that energy can come out in the form of an intense, monochromatic, coherent (in phase) beam of radiation.

This was first realized in the laboratory, in the 1950s, by Charles Townes (then at Columbia University). Townes won the Nobel Prize in physics for this work. Since microwaves were being amplified in the process, the device was called a maser (Fig. 15.13), an acronym for microwave amplification by stimulated emission of radiation. Subsequently, lasers were developed for the amplification of visible light. In any laser or maser, two things are necessary: (1) a pump to provide the population inversion, and (2) sufficient path length to provide significant amplification. In interstellar space, the path length is provided by the large side of interstellar clouds. In the laboratory that path length is provided by mirrors. (Laboratory masers are used as amplifiers in some radio telescope receivers.)

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