Usage
Distance moduli are most commonly used when expressing the distance to other galaxies in the relatively nearby universe. For example, the Large Magellanic Cloud is at a distance modulus of 18.5, the Andromeda Galaxy's distance modulus is 24.4, and the galaxy NGC 4548 in the Virgo Cluster has a DM of 31.0. 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, which is low by supernova standards.
Using distance moduli makes computing magnitudes easy. As for instance, a solar type star (M=5) in Andromeda galaxy (DM=24.4) would have an apparent magnitude (m) of 5 + 24.4 = 29.4, so it would be barely visible in the HST, which has a limiting magnitude of about 30 . This calculation can be done quickly in one's head. Since it is apparent magnitudes which are actually measured at a telescope, this way of looking at things serves to highlight the fact that many discussions about distances in astronomy are really discussions about the putative or derived absolute magnitudes of the distant objects being observed.
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