Astronomy & space · University
Flux, luminosity and stellar magnitudes · revision notes
Luminosity is emitted power; flux is received power per area. For isotropic emission with negligible absorption, F=L/(4πd²).
Key ideas and conditions
Luminosity and flux
Luminosity is emitted power; flux is received power per area. For isotropic emission with negligible absorption, F=L/(4πd²).
Magnitude
Magnitude differences use a logarithmic scale: m2−m1=−2.5 log10(F2/F1). A smaller magnitude denotes a brighter source in the same band.
Absolute magnitude
Absolute magnitude compares sources at the conventional distance of 10 pc. Bandpass and extinction assumptions must be specified.
Distance modulus
Ignoring extinction, m−M=5 log10(d/10 pc). Dust can dim and redden a source, so uncorrected brightness is not purely a distance indicator.
Explain without looking
- If two sources have the same flux but one has distance 3 times the other, find the luminosity ratio for the farther source.
- Why must a magnitude comparison specify the bandpass?
Further reading and review references
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