Magnitude But astronomers usually give their photometric results in terms of magnitudes. If the filter is a blue filter, then the magnitude is denoted as B . For a yellow-green filter, close to the peak sensitivity of the eye, the magnitude is denoted as V for visual. An ultraviolet filter gives U magnitudes. Another common set of standard filters, with narrower passbands than the UBV filters, is the uvby for ultraviolet, violet, blue and yellow.
Bright objects have more negative magnitudes than faint objects.
The brightest star is Sirius, with a magnitude of -1.6. The faintest stars visible with the naked eye from a dark site are about sixth magnitude. The faintest objects visible with the Hubble Space Telescope are about 28th magnitude, which implies a flux nearly one trillion times smaller than the flux of Sirius. When astronomers measure the flux of an object at two or more wavelengths, they can take ratios of fluxes. If a star is far away, it is faint and has a large magnitude.