In the previous chapters we have dealt with galaxies and with stars inhabiting them. But what lies between the individual stars? Here, in seemingly empty space, we find the material from which new suns can coalesce. This material may be thinly distributed or collected in giant clouds, or, most remarkably, in radiating nebulae of hydrogen and other gases called HII regions. A single gas cloud can only emit radiation of low energy, in the radio window. Normally, it is not hot enough to produce more energetic, shortwave radiation. However, a cloud in which young stars have formed through condensation provides a fascinating view and is a frequent target for astronomical imaging. The shortwave, energetic radiation of these young stars excites the gas of the cloud, so that it glows. But how does a cloud “glow”? The hydrogen atoms of the cloud are ionized—stripped of their electrons by the stellar radiation; as the electrons fall back to the atomic nuclei, they emit light at well-defined and characteristic wavelengths.
KeywordsWhite Dwarf Molecular Cloud Supernova Remnant Young Star Planetary Nebula
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