Spectral energy distribution

A spectral energy distribution (SED) is a plot of energy versus frequency or wavelength of light (not to be confused with a 'spectrum' of flux density vs frequency or wavelength).

In infrared astronomy, SEDs can be used to classify young stellar objects.

The count rates observed from a given astronomical radiation source have no simple relationship to the flux from that source, such as might be incident at the top of the Earth's atmosphere.

[2] This lack of a simple relationship is due in no small part to the complex properties of radiation detectors.

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The SED of M51 (upper right) obtained by combining data at many different wavelengths, e.g. UV, visible, and infrared (left)