What is Absolute Magnitude

Absolute magnitude is actually the measure of a celestial object's intrinsic brightness. It is the hypothetical obvious magnitude of an object at a typical luminosity distance of precisely 10.0 parsecs or about 32.6 light years from the observer, supposing no astronomical extinction of starlight. This enables the true energy output of astronomical objects to be compared without regard to their variable ranges. As with all astronomical magnitudes, the absolute magnitude could be specified for various wavelength intervals; for stars the most often quoted absolute magnitude is actually the absolute visual magnitude that is the absolute magnitude in the visual (V) band of the UBV system. Also commonly employed is the absolute bolometric magnitude, which is the total luminosity expressed in magnitude units; it takes into account energy radiated at all wavelengths, whether observed or not.

What is Absolute Magnitude


The absolute magnitude uses the identical conventions as the visual magnitude: brighter objects have smaller magnitudes and five magnitudes matches exactly to a factor of 100, therefore a factor of 100.4 (˜2.512) ratio of brightness matches to a difference of 1.0 in magnitude. The Milky Way, for instance, has an absolute magnitude around -20.5, so a quasar having an absolute magnitude of -25.5 is 100 times brighter compared to our galaxy. If this particular quasar and our galaxy could be seen side by side at the identical distance, the quasar would be five magnitudes (or 100 times) brighter compared to our galaxy. In the same way, Canopus has an absolute visual magnitude around -5.5, while Ross 248 has an absolute visual magnitude of +14.8, for a difference of slightly greater than 20 magnitudes, so if the two stars were at the identical distance, Canopus would be seen as about 20 magnitudes brighter; stated another way, Canopus emits slightly more than 100 million (108) times more visual power compared to Ross 248.

Absolute Magnitude of Star

The absolute magnitude of a star, M is actually the magnitude the star might have if it was positioned at a distance of 10 parsecs through Earth. By considering stars at a set distance, astronomers can compare the true (intrinsic) brightnesses of various stars. The word absolute magnitude usually means absolute visual magnitude, Mv of the star, although the word ‘visual’ really restricts the measurement of the brightness to the wavelength range among 4,000 and 7,000 Angstroms.

To transform the observed brightness of a star (the apparent magnitude, m) to an absolute magnitude, we have to know the distance, d, to the star. On the other hand, if we know the distance and the apparent magnitude of a star, we can determine its absolute magnitude. Both calculations are made using: m=M5log_d/10) 

with m=M referred to as the distance modulus and d measured in parsecs.