Astrophysics (Index)About


(a logarithmic measure of brightness)

Astronomical magnitude is an indicator of the brightness of a star (or other astronomical body) based upon differentiation of brightness by eye, which has been formalized as a logarithmic scale with lower numbers brighter, essentially -2.5 times the log 10 of the flux density. Some magnitudes (as viewed from Earth):

Relative magnitude of two objects:

m1-m2 = -2.5 log10 I1/I2

These are traditionally scaled so Vega has a magnitude of zero, though more recent methods take a specific flux density measure as zero. The above examples are apparent magnitudes, which is the magnitude as viewed from Earth, which is the typical meaning of magnitude if not qualified or in context. Absolute magnitude is defined as the apparent magnitude a star would have if it were 10 parsecs from Earth (the Sun's is 4.83). Bolometric magnitude is specifically a magnitude including all EMR wavelengths, i.e., including radio, infrared, etc. Magnitudes of stars are often cited for specific passbands or differences between passbands (color indices). With no such qualifications or context, it is best to assume magnitude means visual magnitude, an estimation of what you would see that uses the V band.

Magnitudes are cited for stars and other point sources, and also for extended sources, in which case they can be cited for the radiative flux from the source (e.g., as is cited above for the Sun, all the light from the object reaching us), or the source's surface brightness (of all or a portion of it), often used in describing galaxies.

Further reading:

Referenced by pages:
absolute magnitude (M)
AB system
Astrographic Catalog (AC)
AD Leonis (AD Leo)
APM Galaxy Survey (APMUKS)
apparent magnitude (m)
All Sky Automated Survey for SuperNovae (ASAS-SN)
astronomical catalog
astronomical survey
Bonner Durchmusterung Catalog (BD)
Beta Centauri
bolometric correction
bolometric magnitude (Mbol)
color index
cataclysmic variable star (CV)
exposure time calculator (ETC)
etendue (AΩ)
FU Orionis star (FUor)
galaxy main sequence
gas fraction estimation
globular cluster luminosity function (GCLF)
Guide Star Catalog (GSC)
Holmberg radius (RH)
I band
Kapteyn universe
K band
K correction
Lalande 21185
Lick indices
limiting magnitude
main sequence fitting
Morphological Catalogue of Galaxies (MCG)
nova (N)
north polar sequence (NPS)
R band
redshift-magnitude relation
Ross 154
Ross 248
Scholz's Star
supernova (SN)
spectral line designation
spectroscopic parallax
SuperCOSMOS Sky Survey (SSS)
surface brightness (SB)
synthetic photometry
Transiting Exoplanet Survey Satellite (TESS)
transiting planet
transit method
UBV photometric system
Vega system
XZ Catalog of Zodiacal Stars (XZ)