Stellar Magnitudes
Of principle importance to general astronomical observation is the observable brightness of the stars. Magnitude is the unit used in astronomy to describe a star's brightness. Although stellar magnitude in the visible spectrum dictates which stars can be observed under particular visible light conditions— variable due to time of observation, moon phase, atmospheric conditions, and the amount of light pollution present—magnitude also describes the relative amount of electromagnetic radiation observable in other regions of the electromagnetic spectrum (e.g., the x ray region of the spectrum).
Stars emit different amounts of radiation in different regions of the spectrum, so a star's "brightness" or magnitude 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.
The Greek astronomer Hipparchus devised the first magnitudes in the second centuryB.C.He classified stars according to how bright they looked to the eye: the brightest stars he called "1st class" stars, the next brightest "2nd class," and so on down to "6th class." In this way, all the stars visible to the ancient Greeks were neatly classified into six categories.
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 detect, V is analogous to Hipparchus' classes. For example, Hipparchus listed Aldebaran, the brightest star in the constellation Taurus, as a 1st class star and modern astronomers measure Aldebaran's V at 0.85. Astronomers often refer to a star's visual brightness as its apparent magnitude, a description of how bright the star appears to the eye (or the telescope).
Hipparchus' scheme defined from the outset one of the quirks of magnitudes: they list magnitude inversely. The fainter the star, the larger the number describing its magnitude. Therefore, the Sun, the brightest object in the sky, has an apparent magnitude of −26.75, while Sirius, the brightest star in the sky other than the Sun and visible on cold winter nights in the constellation Canis Major, has an apparent magnitude of −1.45. The faintest star you can see without optical aid is about +5 or +6 in a very dark sky with little light pollution. The faintest objects visible to the most powerful telescopes on Earth have an apparent magnitude of about +30.
More revealing than apparent magnitude is absolute magnitude, the apparent magnitude a star would have if it were ten parsecs from the Earth (a parsec is a unit of distance equal to 12 trillion mi [19.3 trillion km]). This is important because apparent magnitude can be deceiving. You know that a penlight is not as bright as a streetlight, but if you hold the penlight near your eye, it will appear brighter than a streetlight six blocks away. That's why V is called apparent magnitude: it is only how bright the star appears to be. For example, the Sun is a fainter star than Sirius—Sirius emits far more energy than the Sun does—yet the Sun appears brighter because it is so much closer. The Sun has an absolute magnitude of +4.8, while Sirius is +1.4.
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 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 with V = 6.00. One magnitude is then a factor of about 2.512 in brightness.
The Sun (V = −26.75) has an apparent visual brightness 25 magnitudes greater than Sirius. The difference in apparent brightness between the Sun and the faintest object humans have ever observed (using the Hubble Space Telescope) is more than 56 magnitudes.
In the 140 years since Pogson created the modern magnitudes, astronomers have developed many different brightness systems. In 1953, H. L. Johnson created the UBV system of brightness measurements. B is the star's brightness in magnitudes measured in the blue part of the spectrum, while U is the brightness in the ultraviolet spectral region. There are many other brightness measurement systems in use.
Accurate measurement of stellar brightness is important because subtracting the brightness in one part of the spectrum from the brightness in another part reveals important information about the star. For many stars the quantity B-V gives a good approximation of the star's temperature. It was established in 1978 that the quantity V-R, where R is the brightness in the red part of the spectrum, can be used to estimate a star's radius.
Cosmology; Quantum Electrodynamics (Qed); Seeing
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