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The apparent magnitude (''m'') of a celestial object is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere. The brighter an object appears, the lower its magnitude value (i.e. inverse relation). In addition, the magnitude scale is logarithmic: a difference of one in magnitude corresponds to a change in brightness by a factor of about 2.5. Generally, the visible spectrum (vmag) is used as a basis for the apparent magnitude. However, other spectra are also used (e.g. the near-infrared J-band). In the visible spectrum, Sirius is the brightest star after the Sun. In the near-infrared J-band, Betelgeuse is the brightest. The apparent magnitude of stars is measured with a bolometer. == History == The scale used to indicate magnitude originates in the Hellenistic practice of dividing stars visible to the naked eye into six ''magnitudes''. The brightest stars in the night sky were said to be of first magnitude (''m'' = 1), whereas the faintest were of sixth magnitude (''m'' = 6), the limit of human visual perception (without the aid of a telescope). Each grade of magnitude was considered twice the brightness of the following grade (a logarithmic scale), although that ratio was subjective as no photodetectors existed. This rather crude scale for the brightness of stars was popularized by Ptolemy in his ''Almagest'', and is generally believed to have originated with Hipparchus. In 1856, Norman Robert Pogson formalized the system by defining a first magnitude star as a star that is 100 times as bright as a sixth-magnitude star, thereby establishing the logarithmic scale still in use today. This implies that a star of magnitude ''m'' is 2.512 times as bright as a star of magnitude ''m+1''. This figure, the fifth root of 100 became known as ''Pogson's Ratio''.〔(Magnitudes of Thirty-six of the Minor Planets for the first day of each month of the year 1857 ), N. Pogson, MNRAS Vol. 17, p. 12 (1856)〕 The zero point of Pogson's scale was originally defined by assigning Polaris a magnitude of exactly 2. Astronomers later discovered that Polaris is slightly variable, so they switched to Vega as the standard reference star, assigning the brightness of Vega as the definition of zero magnitude at any specified wavelength. Apart from small corrections, the brightness of Vega still serves as the definition of zero magnitude for visible and near infrared wavelengths, where its spectral energy distribution (SED) closely approximates that of a black body for a temperature of 11,000 K. However, with the advent of infrared astronomy it was revealed that Vega's radiation includes an Infrared excess presumably due to a circumstellar disk consisting of dust at warm temperatures (but much cooler than the star's surface). At shorter (e.g. visible) wavelengths, there is negligible emission from dust at these temperatures. However, in order to properly extend the magnitude scale further into the infrared, this peculiarity of Vega should not affect the definition of the magnitude scale. Therefore, the magnitude scale was extrapolated to ''all'' wavelengths on the basis of the black body radiation curve for an ideal stellar surface at 11,000 K uncontaminated by circumstellar radiation. On this basis the spectral irradiance (usually expressed in janskys) for the zero magnitude point, as a function of wavelength can be computed (see ()). Small deviations are specified between systems using measurement appartuses developed independently so that data obtained by different astronomers can be properly compared; of greater practical importance is the definition of magnitude not at a single wavelength but applying to the response of standard spectral filters used in photometry over various wavelength bands. With the modern magnitude systems, brightness over a very wide range is specified according to the logarithmic definition detailed below, using this zero reference. In practice such apparent magnitudes do not exceed 30 (for detectable measurements). The brightness of Vega is exceeded by four stars in the night sky at visible wavelengths (and more at infrared wavelengths) as well as bright planets such as Venus, Mars, and Jupiter, and these must be described by ''negative'' magnitudes. For example, Sirius, the brightest star of the celestial sphere, has an apparent magnitude of −1.4 in the visible; negative magnitudes for other very bright astronomical objects can be found in the table below. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Apparent magnitude」の詳細全文を読む スポンサード リンク
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