Spectral energy distribution

For other uses, see Sed (disambiguation).

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).^[1] It is used in many branches of astronomy to characterize astronomical sources. For example, in radio astronomy they are used to show the emission from synchrotron radiation, free-free emission and other emission mechanisms. In infrared astronomy, SEDs can be used to classify young stellar objects.

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

Detector for spectral energy distribution

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.^[2]

These detector properties can be divided into

• those that merely attenuate the beam, including
  1. residual atmosphere between source and detector,
  2. absorption in the detector window when present,
  3. quantum efficiency of the detecting medium,^[2]
• those that redistribute the beam in detected energy, such as
  1. fluorescent photon escape phenomena,
  2. inherent energy resolution of the detector.^[2]

See also

• Astronomical radio source
• Astronomical X-ray sources
• Background radiation
• Bremsstrahlung
• Cosmic microwave background spectral distortions
• Cyclotron radiation
• Electromagnetic radiation
• Synchrotron radiation
• Wavelength dispersive X-ray spectroscopy