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Galaxy Luminosity Classes

In the same sense that there are intrinsically bright and intrinsically faint stars, there are also bright and faint galaxies of the same Hubble type. Luminosity appears to be correlated with the degree to which the spiral arms are developed. Since luminosity is related to the number of stars present in a galaxy, the more luminous galaxies are also the more massive. Luminosity classes similar to stellar luminosity classes have been adopted. The designations use the Roman numerals I, II, III, IV, and V to denote galaxies of decreasing luminosity.

The classification fundamentally depends upon the strength, thickness, contrast, and prominence of the arms. The more prominent the arms, the higher the luminosity class. While luminosity class for galaxies does appear to be correlated with the mass of the galaxy for a particular location along the spiral sequence, the original hope that the luminosity class could be used to indicate intrinsic luminosity or absolute magnitude has not been verified. Indeed, the luminosity class appears not to be at all correlated with the absolute magnitude of a galaxy. A better indicator seems to be the so-called Tully-Fisher relation, which is a correlation between the mean rotational velocity (to the fourth power) of a spiral galaxy and its absolute magnitude. A similar relation (Faber-Jackon relation) holds for elliptical galaxies if the velocity dispersion is used instead of the rotational velocity (which is only meaningful for disk galaxies).


Luminosities or absolute magnitudes may be determined for a galaxy by measuring an apparent magnitude and determining the distance. Magnitude determinations are difficult for galaxies since it is hard to define the precise location of the "edge" for a nebulous object. In addition, corrections must be applied because of the attenuation by dust in our own galaxy, the attenuation due to dust in the external galaxy, the orientation of the galaxy in space (edge-on or face-on), and the K-correction due to the redshift of the galaxy.

The following results are believed to be typical absolute magnitudes.


Notice that the absolute magnitude for a dE galaxy is only one magnitude brighter than the absolute magnitude for the brightest single supergiant stars in our own galaxy and in the LMC. Just for reference, the absolute magnitude for the sun (a G2 main sequence star) is about +5, while the absolute magnitude for a star like Sirius (an A0 main sequence star) is about +1. A typical red giant star might have an absolute magnitude of about 0 (zero).

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This page was last modified on Saturday 31st January 2009 @ 11:15am

Science Mission Directorate Universe Division

Responsible SSU Personnel:

Dr. Kevin McLin (mclin at universe dot sonoma dot edu)

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