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Physics LibreTexts

15.20 Galaxy Size and Luminosity

Two of the basic, observable characteristics of a galaxy are its radial size and luminosity. Measuring these characteristics, however, is not trivial and often requires also determining the galaxy's distance. It is also hard to define either the total size or total luminosity since galaxies typically appear to simply fade into the background and have no sharply defined edges. Look at any photograph or CCD image of a galaxy and you will see the light smoothly fades away until it becomes indistinguishable from the background sky level. Where this fade-away occurs can actually change with photographs or CCD images of increasing sensitivity. As you look deeper, galaxies seem to get bigger!

The conventional way to measure a diameter involves summing up the total light from the galaxy through a very large aperture so that the total light measured does not depend very much on the particular size of the aperture used. In other words the apertures is big enough that very little of the total light is missed. The diameter is then defined as the size enclosing half of the total light (the exact percentage doesn't matter as long as it is large and applied consistently from galaxy to galaxy). Galaxies vary widely in size. Dwarf ellipticals and irregulars can be as small as a few kilo parsecs across; several companions to the Milky Way are this size. The disks of spiral galaxies range in size from 10 to 50 kpc. The largest galaxies in the universe are giant elliptical galaxies with diameters up to 200 kpc. 

The luminosity of a galaxy is derived directly from its distance and apparent brightness. As with the size, measuring the apparent brightness of an extended object is not simple, because the brightness of the galaxy falls off toward the edge. Two strategies are used. The first is to measure the flux through a large enough aperture that essentially no light is missed. The other is to use the fact that the light from most galaxies falls off with distance from the center in a simple and predictable way. In this strategy, we can make a mathematical model of the light distribution and use it to calculate the total brightness and hence the luminosity. Galaxies have a wide range in luminosity from 1011 times solar luminosity for giant elliptical to 105 times solar luminosity for the puniest satellites of the Milky Way. If they are made of stars typically like the Sun, this means they contain anywhere from 100,000 up to 100 billion stars. Since most stars are low mass stars, or red dwarfs, the actual numbers are an order of magnitude higher, from a million up to a trillion stars.

The luminosity of a galaxy isn't strongly correlated with the galaxy's radial size. While large galaxies do give off more light than dwarf systems, the scatter in total luminosity as a function of size is huge. There are small galaxies with very high concentrations of stars and large galaxies that are diffuse, where the stars are distributed over a large region of space. In general, astronomers prefer to instead discuss galaxies' mass to luminosity relationships, which do correlate when galaxies of only a single type (e.g. spiral or elliptical) are considered.

Astropedia Image
Perseus Dwarf Galaxy CGW 2003 J031910.4+4129. Click here for original source URL.

Astropedia Image
M101, the Pinwheel galaxy, as seen by the Hubble Space Telescope. Click here for original source URL.