Absolute Magnitude and Distance Modulus

Absolute Magnitude and Distance Modulus

           The apparent brightness of a star depends upon their intrinsic brightness or luminosities (The total amount of radiant energy emitted per second from the surface of stars) and also on their distance from the observer. Since the distance vary continuously the apparent magnitudes(m) do not provide the actual brightness or luminosities. To get actual brightness or luminosities of stars the term Absolute magnitude was introduced.

Absolute Magnitude(M) of a star is defined as the apparent magnitude of the star if the star was placed at a standard distance of 10 par sec. The absolute magnitude of a star depends on the actual or intrinsic brightness and not on the distance from the observer.

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Relation between Apparent Magnitude(m) and Absolute Magnitude(M)

              Let B­_m  be the brightness of a star with magnitude m and is at a distance d, B_M­ be the brightness of another star with magnitude M and is at a distance D

The Brightness of a star is inversely proportional to the square of the distance

Since M is the absolute magnitude and D is the standard distance 10 parsec, logD = log10 = 1

This is the fundamental relation between the Apparent magnitude(m) and the Absolute Magnitude(M). The quantity  m-M depends only on the distance d and is called Distance Modulus.

The absolute magnitude of normal stars lies between -10 and 20.