ist, the photons from the gas exhibit only a small number of wavelengths. Light from a thin cloud of hot hydrogen gas, for example, consists almost entirely of the wavelengths 656.3 nm, 486.1 nm, 434.0 nm, and 410.2 nm. Each type of atom emits a characteristic spectrum, which is how astronomers know which elements make up the nebulae. Spectra of this type are called line, or emission, spectra.

Synchrotron Emission. Synchrotron radiation occurs in thin, hot gases in the presence of strong magnetic fields. As the electrons are forced to accelerate in the magnetic field, photons are emitted. These conditions occur in pulsar-powered supernova remnants like the Crab Nebula. The energy distribution of photons in synchrotron emission is continuous.

The spectrum of an object influences our strategy for imaging it, particularly the choice of filters. It is straightforward to select filters to separate the continuous spectrum of a star or galaxy into three color bands, but emission spectra pose significant problems because the filters can either miss or "mis-classify" photons. In the next two sections, we examine the interesting problem of selecting filters for imaging astronomical objects.

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