Stellar Magnitudes - How Bright It Looks: Apparent Magnitude, How Bright It Really Is: Absolute Magnitude, The Nature Of The Magnitude Scale
spectrum brightness star astronomy
Magnitude is the unit used in astronomy to describe a star's brightness in a particular portion of the electromagnetic spectrum. Stars emit different amounts of radiation in different regions of the spectrum, so a star's brightness will differ from one part of the spectrum to the next. An important field of research in modern astronomy is the accurate measurement of stellar brightness in magnitudes in different parts of the spectrum.
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Modern astronomers still use Hipparchus' categories, though in considerably refined form. With modern instruments astronomers measure a quantity called V, the star's brightness in the visual portion of the spectrum. Since visual light is what our eyes most readily detect, V is analogous to Hipparchus' classes. For example, Hipparchus listed Aldebaran, the brightest star in the…
In 1856, the British scientist N. R. Pogson noticed that Hipparchus' 6th class stars were roughly 100 times fainter than his 1st class stars. Pogson did the sensible thing: he redefined the stars' V brightness so that a difference of five magnitudes was exactly a factor of 100 in brightness. This meant that a star with V = 1.00 appeared to be precisely 100 times brighter than a star …
In the 140 years since Pogson created the modern magnitudes, astronomers have developed many different brightness systems, and the most popular are less than 50 years old. For example, in 1953, H. L. Johnson created the UBV system of brightness measurements. We've already met V, which is measured over that portion of the electromagnetic spectrum to which our eyes are most sensitive. B is th…
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