technique or wavelength filter. This is why objects brighter than Vega, such as Sirius (Vega magnitude of −1.46. or −1.5), have negative magnitudes. However, in the late twentieth century Vega was found to vary in brightness making it unsuitable for an absolute reference, so the reference system was modernized to not depend on any particular star's stability. This is why the modern value for Vega' magnitude is close to, but no longer exactly zero, but rather 0.03 in the V (visual) band. Current absolute reference systems include the AB magnitude system, in which the reference is a source with a constant flux density per unit frequency, and the STMAG system, in which the reference source is instead defined to have constant flux density per unit wavelength. Decibel Another logarithmic scale for intensity is the decibel. Although it is more commonly used for sound intensity, it is also used for light intensity. It is a parameter for photomultiplier tubes and similar camera optics for telescopes and microscopes. Each factor of 10 in intensity corresponds to 10 decibels. In particular, a multiplier of 100 in intensity corresponds to an increase of 20 decibels and also corresponds to a decrease in magnitude by 5. Generally, the change in decibels 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 smaller (weaker) than the reference, which might need to be compensated by an increase in the capability of the camera by as many decibels. See also AB magnitude Color–color diagram List of brightest stars Photometric-standard star UBV photometric system Notes References External links Observational astronomy Units of measurement in astronomy Logarithmic scales of measurement Concepts in astronomy la:Magnitudo (astronomia)