|
|
|
|
|
Distance ModulusThe distance modulus is the difference between the absolute magnitude and apparent magnitude of an astronomical object. It is derived from the definition of magnitude as the logarithm of the ratio of observed brightnesses of astronomical objects: m1 - m2 = 2.5 log10(I2/I1) The brightness of a light source is related to its distance by an inverse square law - a source twice as far away appears one quarter as bright. Therefore, for objects of equal absolute brightness, I1/I2 can be replaced by (d2/d1)². Absolute magnitude is defined as the apparent magnitude of an object when seen at a distance of 10 parsecs, and so the magnitude equation can be written as: Mapp - Mabs = 2.5 log10(d/10pc)² Rearranging the logarithms gives Mapp - Mabs = -5 + 5 log10d Then, given a distance modulus, the distance in parsecs is given by d = 100.2(m - M + 5) Distance moduli are most commonly used when expressing the distance to other galaxies. For example, the Large Magellanic Cloud is at a distance modulus of 18.5, the Andromeda Galaxy's distance modulus is 24.5, and the Virgo Cluster has a DM of 31.7. In the case of the LMC, this means that the supernova SN1987A, with a peak apparent magnitude of 2.8, had an absolute magnitude of -15.7 - fairly low by supernova standards.
|
 |
|
| Copyright 2005-2009 OnPedia.com. All Rights Reserved |
|
|