Because astronomers make so many measurements that are indirect, it is beneficial to have many different ways of measuring the same thing so that results can be compared to ensure accuracy. For this reason, astronomers have developed many different ways to measure distance to astronomical objects. Another important reason for creating different types of measures is that each of these measurements suffer limitations and are only accurate for objects within a certain distance from Earth. This is the case with parallax. Red shift combined with Hubble's relation is one of the farthest reaching measurement strategies known today and can reach beyond 100 million parsecs. Sometimes, however, it is useful to make more simple measurements that have smaller limits to determine distances to objects which are closer. Astronomers are frequently hindered by the limits of observation time and available funds. They are encouraged to be as efficient as possible and therefore it is useful to have many methods available to make the measurements they need. One common measurement for distance that does not reach as far as red shift, but is much simpler to carry out is Galactic Distance Brightness.
Brightness is a good measure for astronomers to use because of the simple way it relates to distance. Think of streetlights on dark nights: from far away it is possible to look at a street light with no discomfort. However, if you looked at that same streetlight close up, it would hurt your eyes. This is because light is related to distance in the following way:
which is called the "inverse-square relation."
This relationship means that the brightness of an object to an observer (apparent brightness) is equal to the amount of light the object emits per second (luminosity) divided by the distance the observer is from the source squared (distance2) times a constant number (4). In other words, as you move away from a light-emitting source, the more quickly it appears to grow dim. This is why the streetlight does not hurt your eyes from far away, but does from a small distance.
Astronomers have discovered that galaxies of a certain type have the same intrinsic brightness (luminosity), which is a measure of how bright the galaxy is at the source, or in a sense the "wattage" of the galaxy. Since a galaxy's "type" is based upon simple visual cues, it is possible to tell what type a galaxy is just by looking at it, almost as if light bulbs of different wattages were different shapes. Therefore, by looking at a galaxy and determining its type, we know its luminosity. Because light becomes dimmer the farther the viewer is from its source according to the inverse-square relation, we can figure out how far away the galaxy is, and roughly how far away all the other galaxies in its cluster are, simply by measuring how bright it appears to us.
Back | Next