Astrophysics (index)about

spectral energy distribution

(function or plot of brightness at each wavelength in the spectrum)

Electromagnetic radiation's spectral energy distribution (SED) is a function, i.e., plot of brightness or flux density versus the frequency or wavelength (the choice of which produce different results: see below). When the SED of an astronomical object is measured from Earth, the effects of Earth atmosphere are relevant. Capturing a wide range of wavelengths requires multiple instruments, which causes data-calibration challenges. But some limited bandwidths can provide signatures for specific types sources.

The alternate term, spectral power distribution (SPD) is used in some other (non-astronomy) fields.

Plotting by wavelength versus plotting by frequency has consequences: the peak wavelength differs in the two cases if each is plotted linearly, a consequence of plotting density functions of values that have a reciprocal relationship. Taking black-body spectrum as an example, plotting wavelengths by millimeter (0mm, 1mm, 2mm, 3mm, ...) crowds all the infrared, visible light, ultraviolet, etc., between 0 and 1 and spreads radio over a wide range. Plotting frequency by 100GHz (0GHz, 100GHz, 200GHz, 300GHz, ..) crowds the radio into 0-300GHz and spreads the others over the rest. The CMB SED's peak is cited at various wavelengths and frequencies, but all are valid for the spectrum representing a temperature of 2.72548 K, depending upon how it is plotted:

plotted by peak frequency peak wavelength peak photon energy
linear wavelength 282 GHz 1.063 mm 1.168 × 10-3 eV
linear frequency 160.23 GHz 1.871 mm 6.26 × 10-4 eV
log of either 222.6 GHz 1.347 mm 9.2 × 10-4 eV

Plots against the log of either produce an equivalent plot: 1mm≡300GHz, 10mm≡30GHz, 100mm≡3GHz, etc.

SED fitting (aka spectral energy distribution fitting) is a method of classifying distant galaxies, to help determine their age and degree of star formation. A challenge is that a young galaxy with dust has a SED across ultraviolet, visible light, and near infrared as an old galaxy. A fitting method is Markov chain Monte Carlo (MCMC), using random walks around a parameter-space to determine the degree of unique fit between an SED and a known type of object. GalMC is an example of software that takes this approach.


Referenced by:
angular power spectrum
Compton reflection
chemically peculiar star (CP star)
fine structure
galaxy SED
Herschel Orion Protostar Survey (HOPS)
K correction
linearly variable filter (LVF)
Rayleigh-Jeans law
radio galaxy (RG)
Spitzer Extended Deep Survey (SEDS)
sky subtraction
spectral line energy distribution (SLED)
spectral power distribution (SPD)
spectral class
spectral correlator
spectral line
spectral signature
stellar model atmosphere
Sunyaev-Zel'dovich effect (SZ effect)
Thomson optical depth
transit spectroscopy
vegetation red edge (VRE)
Wien's displacement law