Magnitude (astronomy)

Magnitude (astronomy)

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It is the luminosity of a star as it appears to us that we observe it from Earth.

The first astronomer who subdivided the stars according to their magnitude, creating an appropriate scale of measurements, was the Greek Hipparchus of Nicea. In the classification of Hipparchus, a magnitude or size 1 was attributed to the brightest stars; to the weakest visible to the naked eye, magnitude 6.

With the invention of the Photometer, a measuring instrument used to determine the amount of light emitted by a star, it has been seen that a star of magnitude 1 is 100 times brighter than one of magnitude 6. This means that, wanting to give a precise scale to the classification of Hipparchus (which was empirical, since it was based on estimates made with the naked eye) each magnitude differs from the previous one or the successive one by a factor of 2.5.

The scale of magnitudes created by Hipparchus has been maintained until today with some essential modifications. It has obviously extended to all stars not visible to the naked eye: those stars that have magnitudes greater than 6 and that, at the time of Hipparchus, were not known because telescopes did not exist.

Therefore, from 6 onwards (the weakest star today visible with the most powerful ground telescopes is of magnitude 24) the magnitudes indicate ever weaker objects.

On the other hand, in relation to the brightest stars it has been seen that Hipparchus did not act very subtly, regrouping under the magnitude 1 stars that instead are much brighter. Therefore it has been thought to create a magnitude 0 and then the negative magnitudes -2, -3, etc. In this case the increasing negative numbers indicate celestial bodies always brighter (the coefficient of luminosity between one magnitude and another is obviously always the same, that is, 2.5).

This system of evaluation of the luminosity of a star is also called apparent magnitude, because it is conditioned to our position. It would suffice to place ourselves in another star to see all the reciprocal relations of luminosity change, since the distances between our observation point and the observed sources would vary.

To know the amount of energy emitted by a star, astronomers use absolute magnitude, which can be calculated by knowing the physical characteristics of the star. With the apparent and absolute magnitude known, astronomers can also determine the distance of a star from Earth with a good approximation.

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Magnification (or increase)Maksutov (telescope)