Luminosity; magnitude

Japanese: 光度 - こうど（英語表記）luminosity; magnitude

A unit of luminosity that expresses the brightness of a celestial object. It is also called the luminosity class or magnitude. Ancient Greek astronomers classified the brightness of stars into six levels, with about 20 of the brightest stars in the sky being classified as 1st magnitude stars, and stars that are barely visible to the naked eye being classified as 6th magnitude stars. In the 19th century, precise measurements of the amount of light emitted by stars were made, and it was discovered that a 1st magnitude star is about 100 times brighter than a 6th magnitude star, so the fifth root of 100 (2.512) was defined as the ratio of 1st magnitude. This definition is used to determine the magnitude of any wavelength of light, but the luminosity perceived by the human eye is called the visual magnitude, and that measured by photography, which is more sensitive to shorter wavelengths than the naked eye, is called the photographic magnitude. The magnitude of a star is currently determined based on the internationally established magnitude of a standard star near Polaris.

Luminous intensity
luminous intensity

A quantity defined to measure the intensity of a light source. When a light source is small compared to the distance to the observation point and can be considered a point light source, the light energy emitted from it in a certain direction within a unit solid angle in a unit time is called luminous intensity. The unit is candela, which is 1/60 of the luminous intensity per ^cm2 of a black body at the melting point of platinum.

Source: Encyclopaedia Britannica Concise Encyclopedia About Encyclopaedia Britannica Concise Encyclopedia Information

Japanese:

天体の明るさを表わす単位。光級あるいは等級ともいう。古代ギリシアの天文学者は全天で最も明るい星約 20個を1等星とし，肉眼でかろうじて見える星を6等星として，星の明るさを6段階に区分した。 19世紀に星の光量の精密な測定がされるようになり，1等星の明るさが6等星の約 100倍であることがわかり 100の5乗根 (2.512) を1等級の比と定義した。光のどの波長域でもこの定義により等級が決められるが，特に人間の眼に感じる明るさによる光度を実視等級，肉眼よりも短波長に敏感な写真測定によるものを写真等級と呼ぶ。どの星を何等星とするかについては，現在北極星付近の標準星について国際的に定められた等級を基準として決定している。