Magnitude (astronomy)
In astronomy, magnitude is a measure of the brightness of an object, usually in a defined passband.
An imprecise but systematic determination of the magnitude of objects was introduced in ancient times by Hipparchus.
A more complex definition of absolute magnitude is used for planets and small Solar System bodies, based on its brightness at one astronomical unit from the observer and the Sun.
The Sun has an apparent magnitude of −27 and Sirius, the brightest visible star in the night sky, −1.46.
[1] The Greek astronomer Hipparchus produced a catalogue which noted the apparent brightness of stars in the second century BCE.
In the second century CE the Alexandrian astronomer Ptolemy classified stars on a six-point scale, and originated the term magnitude.
In 1736, the mathematician John Keill described the ancient naked-eye magnitude system in this way: The fixed Stars appear to be of different Bignesses, not because they really are so, but because they are not all equally distant from us.
[note 1] Those that are nearest will excel in Lustre and Bigness; the more remote Stars will give a fainter Light, and appear smaller to the Eye.
[5] Even into the early nineteenth century, the magnitude system continued to be described in terms of six classes determined by apparent size.
This is the modern magnitude system, which measures the brightness, not the apparent size, of stars.
Two of the main types of magnitudes distinguished by astronomers are: The difference between these concepts can be seen by comparing two stars.
Under the modern logarithmic magnitude scale, two objects, one of which is used as a reference or baseline, whose flux (i.e., brightness, a measure of power per unit area) in units such as watts per square metre (W m−2) are F1 and Fref, will have magnitudes m1 and mref related by Astronomers use the term "flux" for what is often called "intensity" in physics, in order to avoid confusion with the specific intensity.
Bolometric magnitudes are formally defined based on stellar luminosity in watts, and are normalised to be approximately equal to MV for yellow stars.
Absolute magnitudes for Solar System objects are frequently quoted based on a distance of 1 AU.
It is a parameter for photomultiplier tubes and similar camera optics for telescopes and microscopes.
Generally, the change in level is related to a change in magnitude by For example, an object that is 1 magnitude larger (fainter) than a reference would produce a signal that is 4 dB smaller (weaker) than the reference, which might need to be compensated by an increase in the capability of the camera by as many decibels.
