Quantifying Light—luminosity And Spectral Classes
Astronomy is based upon the information we can derive by what we observe when we gaze at the stars. One of the characteristics of a star that can be determined observationally is its luminosity—the amount of light that the star emits. When combined with other information about a star such as its size or temperature, luminosity can indicate the intensity of fusion reactions taking place in the stellar core. Luminosity cannot always be determined by direct observation, however, as distance can decrease the apparent luminosity of an object. The Sun, for example, is not excessively luminous as stars go; it only appears brighter than any other stellar object because it is so close to us.
Magnitude is another way of expressing the luminosity of a star. The Greek astronomer Hipparchus developed the magnitude scale for stars, rating their brightness on a scale of 1 to 6. According to the scale, a star of first magnitude is defined as appearing 100 times as bright as a star of sixth magnitude, so the larger the magnitude, the fainter the object. As telescopes have allowed astronomers to peer deeper into the universe, the scale has expanded: Sirius, which appears to be the brightest star in the heavens, has an apparent magnitude of −1.27, while Pluto has a magnitude of 14.
Apparent magnitude, like apparent luminosity, can be deceptive. To avoid invalid comparisons, astronomers have developed the concept of absolute magnitude, which is defined as the apparent magnitude the object would have when viewed at a distance of 32.6 light years. Thus, measuring the distance to various objects is an important task in astronomy and astrophysics.
The color of light emitted by a star indicates its temperature. At the beginning of the century, astronomers began classifying stars based on color, or spectral classes. The classes are O, B, A, F, G, K, and M. O-type stars are the hottest (63,000°F [34,632°C]) and tend to appear white or blue-white, while M-type stars are the coolest (5,400°F [2,952°C]) and tend to appear red; our yellow sun, type G, falls in the middle. Another rating—L-type, for dim, cool objects below M-type—has recently been proposed for addition to the listing.
Astronomers can glean a tremendous amount of information from stellar magnitudes and glasses. Between 1911–13, Danish astronomer Ejnar Hertzsprung (1873–1967) and American astronomer Henry Norris Russell (1877–1957) independently developed what is now known as the Hertzsprung-Russell diagram that plots the magnitude and color of stars. According to the diagram, most stars fall on a slightly curving diagonal that runs from very bright, hot stars down to very cool, red stars. Most stars follow this so-called main sequence as they gradually burn out. Some stars fall off of the main sequence, for example red giants, which are relatively cool but appear bright because of their enormous size; or white dwarfs, which are bright but so small as to appear faint.
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