Physics
Classification by Luminosity
Classification by Luminosity is a system used to categorize stars based on their brightness. It is divided into seven classes, from supergiants to dwarfs, with each class further divided into subclasses. This classification is useful in understanding the evolution and characteristics of stars.
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4 Key excerpts on "Classification by Luminosity"
eBook - PDFAstronomy Methods
A Physical Approach to Astronomical Observations
- Hale Bradt(Author)
- 2003(Publication Date)
- Cambridge University Press(Publisher)
Secular and statistical parallax extend this to longer baselines and greater distances. Standard candles , objects of known luminosity, permit distance determinations to greater and greater distances as one moves out the distance ladder . Distances to open clusters are obtained with the convergence method . Spectroscopic classification of stars makes them standard candles as does temporal variability of Cepheid variables and RR Lyrae stars. Spectral line broadening in galaxies calibrates their luminosities so they 253 254 9 Properties and distances of celestial objects become standard candles also. Other distance indicators are luminosity functions , surface brightness fluctuations , and supernovae of Type 1a. The goal of many distance determinations is to refine the Hubble law . One can use this law to find the approximate distance of a distant object from the redshift of its spectrum. 9.1 Introduction The distance to a celestial object is often crucial to determining its size or luminosity. Unlike the angular position of a star or galaxy on the celestial sphere, distances often can be obtained only indirectly. A significant fraction of astronomical history has been dedicated to the determination of distances. Even now, new techniques to determine the distances of the most distant galaxies are an important aspect of cosmological studies. The values and ranges of the distances, sizes, masses and luminosities of celestial objects tend to be on the large side; this is astrophysics, not atomic physics. It is important to have a feeling for these quantities, but this can be difficult in view of the large values. It is similar to the difficulty in gaining an intuitive feeling for a national debt. It is helpful to concentrate on the nearest power of 10 (the order of magnitude ) rather than the exact number. For example, you might memorize that there are 10 1 persons in the class, 10 6 in greater Boston, and 10 9 on the planet earth.
eBook - PDF- Kristine M. Larsen(Author)
- 2007(Publication Date)
- Greenwood(Publisher)
18 Cosmology 101 THE CLASSIFICATION OF STARS Although many people are under the impression that all stars are white, a careful survey of the night sky will convince them that this is really not the case. Some stars appear yellowish, like our sun, while others exhibit varying intensities of blue, orange, or red. As we have already seen, the color of stars is determined by their surface temper- ature, through Wien’s law. Stars also differ in size and true brightness (luminosity or absolute magnitude), with these properties related to the temperature through the Stefan–Boltzmann law. Astronomers use these properties to classify stars into various groupings, and then utilize this information to study their lives and deaths. It bears repeating that most of what astronomers learn about stars is achieved by studying their spectra. This includes composition and mo- tion. In this vein, Annie Jump Cannon lauded the universe’s “Patient light! Shining down on humanity these countless centuries until man became clever enough to wrest from its vibrations the secrets so closely concealed” (1941, 56). It is therefore natural to ask if stars exhibit differ- ent types of spectra and if so, what inherent properties are responsible for these differences. The answer was first noted by Fraunhofer himself around 1814. Not only did he study the spectral lines of the sun, but he found that some bright stars had similar spectra, while others had spectra with differing appearances. The first significant classification sys- tem was done by Jesuit priest and astronomer Father Angelo Secchi in the 1860s. His equipment was crude by modern standards, and he was limited to making drawings at the eyepiece (as Fraunhofer had done), but he was still able to divide stellar spectra into five basic classes. Type I stars, including Sirius, were bluish-white in color (and therefore hot- ter than the sun).
eBook - PDF- Jerry C. Whitaker(Author)
- 2018(Publication Date)
- CRC Press(Publisher)
In the scotopic range of intensities, the luminosity function is somewhat different from that of the photopic range. The two curves are compared in Figure 5.6 . Values are Figure 5.5 The photopic luminosity function. ( After [2].) Figure 5.6 Scotopic luminosity function (trace A ) as compared with photopic luminosity function (trace B ). ( After [2].) © 2001 by CRC PRESS LLC listed in Table 5.2 . While the two curves are similar in shape, there is a shift for the scotopic curve of about 40 nm to the shorter wavelengths. 5.2.4 Luminance Brightness is a term used to describe one of the characteristics of appearance of a source of radiant flux or of an object from which radiant flux is being reflected or transmitted. Brightness specifications of two or more sources of radiant flux should be indicative of their actual relative appearances. These appearances will greatly de-pend upon the viewing conditions, including the state of adaptation of the observer’s eye. Luminance, as previously indicated, is a psychophysical analog of brightness. It is subject to physical determination, independent of particular viewing and adaptation conditions. Because it is an analog of brightness, however, it is defined to relate as closely as possible to brightness. The best established measure of the relative brightnesses of different spectral stim-uli is the luminosity function. In evaluating the luminance of a source of radiant flux consisting of many wavelengths of light, the amounts of radiant flux at the different wavelengths are weighted by the luminosity function. This converts radiant flux to lu-minous flux. As used in photometry, the term luminance applies only to extended sources of light, not to point sources. For a given amount (and quality) of radiant flux reaching the eye, brightness will vary inversely with the effective area of the source. Luminance is described in terms of luminous flux per unit projected area of the source.
eBook - PDFCosmic Discovery
The Search, Scope, and Heritage of Astronomy
- Martin Harwit(Author)
- 2019(Publication Date)
- Cambridge University Press(Publisher)
The surface brightness of the galaxy—its brightness divided by the solid angle it covers in the sky—also is an intrinsic characteristic independent of distance. Purely angular measurements differ in being extrinsic. Without knowing the distance of the galaxy the angular dimensions provide only partial information. They can tell us whether the galaxy is round or whether it is long and filamen- tary. Similarly intensity observations yield restricted information. If a galaxy’s brightness changes as it did when the supernova of 1885 exploded in the Andromeda Nebula, we can tell the ratio of maximum to minimum emission; but the absolute brightness of the source—its intrinsic luminosity—eludes us unless we know its distance. The Class I description is restricted solely to directly observed traits, mainly because these traits provide the most generally available information about any observed source when no distance measure is available. For most astronomical sources we do, however, have a fairly good measure of distance. We can there- fore sharpen our description somewhat by adopting the Class II depiction, which replaces angular coordinates by actual spatial size and apparent bright- ness by the intrinsic luminosity of a source. We are then closer to an accurate description of the most important source traits, having removed those charac- teristics that are introduced by the chance location of the observer—the type of trait that makes the planet Jupiter look brighter than the star Betelgeuse, when actually that star is 5 × 10 12 times more luminous, but lies ten million times further away. Each Class I phenomenon can be identified by a set of phase space filters through which detectable energy is received. Each filter is assigned a designating code consisting of four numbers and a letter, in accordance with the parameters shown in table 4.1. The first number represents the wavelength band in which the phenomenon is primarily observed.
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