FIGURE 4.3. Radiograph of the Orion Nebula, the Orion A radio source. This image was produced by filtering out the large-scale structure to show only a tangled web of filaments. Investigator: F. Yusef-Zadeh. (Very Large Array image courtesy of NRAO/AUI.)
of radio source called an emission nebula. The gas in the nebula has been heated to incandescence by ultraviolet radiation from young stars within it and this hot gas, at a temperature of 8,000 K, radiates light, heat, and radio waves by the thermal emission process.
Radio astronomers generally refer to emission nebulae as HII regions, where the symbol "HII" refers to ionized hydrogen, The symbol HI is used to represent cold or neutral hydrogen atoms, each of which consists of a single electron orbiting a proton. Hydrogen clouds exist everywhere in interstellar space (Chapter 6). It is within such clouds that stars arc spawned, When hot stars begin to shine, their highly energetic ultraviolet radiation streams outward and when a UV photon strikes a hydrogen atom it can kick the electron hard enough to escape the powerful grip of the proton. The hydrogen atom is ionized and its constituent proton and electron now wander about freely. A large cloud made of a mix of protons and electrons forms an HII region that emits thermal radiation.
In the interstellar gas clouds in which stars are born, the larger, hotter stars literally eat their way through their surrounding gas cocoons, converting what had been cold matter into hot gases, which radiate their own energy. Within large emission nebulae many small, compact HII regions can be formed, each associated with a newly born star. The difference between a supernova remnant and an HII region is revealed by measurements of their spectra and polarization. The HII region has a thermal spectrum and its radio emission is unpolarized, while the radiation from supernovae is nonthermal and polarized.
Other galactic nebulae that are radio sources include planetary nebulae, which represent far less violent affairs than supernovae. The supernova is the death of a massive star, while the planetary nebula signifies the death of a smaller, more normal star, The central star of the planetary nebula merely shrugs off its outer layers as it ages. What is left of the inner parts of the star may collapse to become a white dwarf and in due course will fade from view.
The existence of emission nebulae, supernovae, and planetary nebulae are constant reminders that we live in an extraordinarily dynamic universe in which everything changes and the cycles of birth, life, and death are all about us. At some time, less than 5 billion years hence, the sun will swell to become a red giant and then shed its outer layers to create a planetary nebula that will swallow the earth. Just like everything in the universe, the sun has a finite lifetime. Tens of thousands of years later astronomers on alien planets may watch its planetary nebula and study the radiation in order to better understand the nature of stellar evolution. Those beings will never know who, or what, orbited the original star.
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