# Absolute and apparent magnitude

1. Dec 8, 2011

### mmssm

is that both of absolute and apparent magnitude counts only visible light?
since i found some books said absolute magnitude counts luminosity instead of intensity, where luminosity includes also the non-visible frequencies.

thank you!

2. Dec 8, 2011

### cepheid

Staff Emeritus
Welcome to PF mmssm!

No, you can define a magnitude in any band (range of wavelengths). In fact, magnitude is something that only makes sense IF you specify which in which photometric band that magnitude was measured. Usually a band is defined by using a filter that admits only a certain range of wavelengths, so that that is the range of wavelengths that is measured. For instance, one traditional photometric system uses three filters that define three different bands: U, B, and V (for ultraviolet, blue, and visible), each of which is centered on a different wavelength. You then have to be specific about whether you're talking about U-band, B-band, or V-band magnitudes.

Many other photometric bands exist as well. For instance in the near to mid-infrared you have the I, J, H, and K bands. You can measure the magnitude of an object in any one of these bands.

It's true that the absolute magnitude is a measure of the intrinsic power output (luminosity) of an object, but it's NOT true that you're measuring that output over all wavelengths. Just like the apparent magnitudes that I discussed above, absolute magnitudes are always specific to a particular photometric band, and you have to specify WHICH band it was measured in when you state an absolute magnitude.

If you do measure the intensity of a source over all wavelengths, then this is called the "bolometric" intensity , and you can define a corresponding "bolometic" magnitude that is the magnitude when all wavelengths are taken into account. However, this is usually hard to measure (especially from the ground, since not all wavelengths make it through the atmosphere).