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General Galaxy Characteristics

Three properties are generally used to characterize galaxies. These are outlined below.
Galaxy Color

When we refer to galaxy color we are generally using it as a proxy for the stellar population. Early type galaxies like ellipticals do not contain cool gas or dust. As a result they do not form stars and are dominated by an older, redder stellar population. Spirals, on the other hand, do form stars and therefore have a younger, bluer stellar population mixed in with their underlying old disk population. This effect is not a large one, however, and most spirals are not appreciably bluer than elliptical galaxies.

Besides stellar content there are other properties that affect the color of a galaxy. One is the presence of dust. Dust will scatter short wavelength light while letting the longer red wavelengths through. Therefore, extremely dusty galaxies will appear red regardless of their underlying stellar population (this is the same process that causes the Sun to be redder at sunrise and sunset than at midday). Indeed, such dusty galaxies will often be undergoing extremely vigorous star formation, but their thick dust content blocks a direct view of their young, bright stars. Instead we may see that they have inordinate emission in the infrared, the result of the massive stars warming the dust and causing it to glow in the IR.

Sometimes a galaxy can be bluer than we expect given its morphology or stellar content. This will be the case for galaxies with extremely old stellar populations. Generally we think of old stars as being red, but if the stars are extremely old they would have formed before the galaxy had produced many metals (in astronomy, "metals" are all elements heavier than hydrogen and helium, and all those heavier than boron are formed inside stars). They will therefore be metal-deficient. The preponderance of atomic transitions for many of these elements is in the UV part of the spectrum. In stars with many metals the absorption in this region somewhat blocks emission there; much of the flux that would normally be emitted in the UV must come out at longer wavelengths, making these stars redder than they would otherwise be. On the other hand, low-metal stars are able to emit more light in the blue and UV parts of the spectrum, which somewhat diminishes their emission at longer wavelengths. While this is a small effect, it can subtly change the color of the stars in a galaxy, making it slightly bluer would be expected (actually, slightly less red is a better way to think about it).

So color is not always a simple thing. Generally a red galaxy will contain older, cooler stars. But in some cases there is more to it, either because of the presence of dust in the galaxy or because of low metal content.

Galaxy Size

Sizes of galaxies are determined by measuring their angular extent on the sky and determining their distance.

s = 206265 ø d

Here s is the linear size, ø is the angular size in arc seconds, and d is the distance to the object. The units of s will be the same as the units of d. Of course, there are two difficulties with the practical application of this equation: (1) the precise determination of the angular size of a "fuzzy" object like a galaxy is difficult to make, and (2) the precise determination of the distance is also difficult.

The following results are typical measured values for galaxy diameters.

dE dwarf ellipticals 3 kpc
S spirals 15 kpc - 20 kpc
E ellipticals 60 kpc
cD giant ellipticals 2 mpc

Here 1 kpc is a kiloparsec, and 1 mpc is a megaparsec. A parsec is approximately 3.2 light years.


Luminosities or absolute magnitudes can be determined for a galaxy by measuring its apparent magnitude and combining that with its distance. Here is it assumed that the flux of the galaxy diminishes as the inverse of the square of its distance from us (strictly, this is true only for point sources, but the methods used to measure galactic distances generally employ such sources). Just as for size, apparent magnitude determinations are difficult for galaxies since it is difficult 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 @ 12:04pm

Science Mission Directorate Universe Division

Responsible SSU Personnel:

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

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