The brightness of a celestial object as measured by the observer. Sirius, the brightest star, has an apparent magnitude of −1.46, and the faintest stars visible to the naked eye under the most favourable conditions have magnitudes of about +6.5. Stars of magnitude +26 are regularly measured at modern professional observatories, and as faint as +30 with the Hubble Space Telescope. If m carries no subscript it is assumed to be the visual magnitude. The notation m _bol denotes the apparent bolometric magnitude.