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In astronomy, magnitude is the logarithmic measure of the brightness of an object, measured in a specific wavelength or passband, usually in the visible or near-infrared spectrum. An imprecise but systemic determination of the magnitude of objects was introduced in ancient times by Hipparchus. Astronomers use two different definitions of magnitude: apparent magnitude and absolute magnitude. The apparent magnitude (m, ''or'' vmag for the visible spectrum) is the brightness of an object as it appears in the night sky from Earth, while the absolute magnitude (Mv, V ''and'' H) describes the intrinsic brightness of an object as it would appear if it were placed at a certain distance from Earth. This distance is 10 parsecs for stars and 1 astronomical unit for asteroids and planets. The size of an asteroid is typically estimated based on its absolute magnitude.〔(【引用サイトリンク】title=Glossary—Absolute magnitude (H) )〕 The brighter an object appears, the lower the value of its magnitude, with the brightest objects reaching negative values. The Sun has an apparent magnitude of −27, the full moon −13, the brightest planet Venus measures −5, and Sirius, the brightest visible star in the night sky is at −1.5. An apparent magnitude can also be assigned to man-made objects in Earth orbit. The brightest satellite flares are ranked at −9, and the International Space Station appears at a magnitude of −6. Since the scale is logarithmic, each step of one magnitude changes the brightness by a factor of about 2.512. A magnitude 4 star is exactly a hundred times brighter than a magnitude 9 star, as the difference of five magnitude steps corresponds to (2.512)5 or 100.〔(【引用サイトリンク】title=Apparent & absolute magnitude )〕 == History == The magnitude system dates back roughly 2000 years to the Greek astronomer Hipparchus (or the Alexandrian astronomer Ptolemy—references vary) who classified stars by their apparent brightness, which they saw as size (''magnitude'' means "bigness, size"〔 〕). To the unaided eye, a more prominent star such as Sirius or Arcturus appears larger than a less prominent star such as Mizar, which in turn appears larger than a truly faint star such as Alcor. The following quote from 1736 gives an excellent description of the ancient naked-eye magnitude system:
Note that the brighter the star, the smaller the magnitude: Bright "first magnitude" stars are "1st-class" stars, while stars barely visible to the naked eye are "sixth magnitude" or "6th-class". The system was a simple delineation of stellar brightness into six distinct groups but made no allowance for the variations in brightness within a group. Tycho Brahe attempted to directly measure the “bigness” of the stars in terms of angular size, which in theory meant that a star's magnitude could be determined by more than just the subjective judgment described in the above quote. He concluded that first magnitude stars measured 2 arc minutes (2’) in apparent diameter (1/30 of a degree, or 1/15 the diameter of the full moon), with second through sixth magnitude stars measuring 3/2’, 13/12’, 3/4’, 1/2’, and 1/3’, respectively.〔 〕 The development of the telescope showed that these large sizes were illusory—stars appeared much smaller through the telescope. However, early telescopes produced a spurious disk-like image of a star (known today as an Airy disk) that was larger for brighter stars and smaller for fainter ones. Astronomers from Galileo to Jaques Cassini mistook these spurious disks for the physical bodies of stars, and thus into the eighteenth century continued to think of magnitude in terms of the physical size of a star.〔 Johannes Hevelius produced a very precise table of star sizes measured telescopically, but now the measured diameters ranged from just over six ''seconds'' of arc for first magnitude down to just under 2 seconds for sixth magnitude.〔 〕〔 〕 By the time of William Herschel astronomers recognized that the telescopic disks of stars were spurious and a function of the telescope as well as the brightness of the stars, but still spoke in terms of a star's size more than its brightness.〔 Even well into the nineteenth century the magnitude system continued to be described in terms of six classes determined by apparent size, in which
However, by the mid-nineteenth century astronomers had measured the distances to stars via stellar parallax, and so understood that stars are so far away as to essentially appear as point sources of light. Following advances in understanding the diffraction of light and astronomical seeing, astronomers fully understood both that the apparent sizes of stars were spurious and how those sizes depended on the intensity of light coming from a star (this is the star's apparent brightness, which can be measured in units such as watts/cm2) so that brighter stars appeared larger. 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「Magnitude (astronomy)」の詳細全文を読む スポンサード リンク
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