Stars spectral classification

The evidence on which this theory of stellar evolution is based comes not only from known nuclear reactions and the relativistic equivalence of mass and energy, but also from the spectroscopic analysis of the light reaching us from the stars. This leads to the spectral classification of stars, which is the cornerstone of modem experimental astrophysics. The spectroscopic analysis of starlight reveals much information about the... 

We have been making survey observations of faint cool carbon stars using the Kiso 105-cm Schmidt telescope. Kodak IN and 103aF plates are respectively taken behind the 4-degree objective prism (700 Amm-1 at Ha) for the detection and for the spectral classification. F-band plates are utilized to obtain the position and... 

The space distribution in the Cassiopeia region is estimated on the basis of the spectral classification and the absolute magnitudes determined by Mikami (1975). It is shown that the carbon stars are distributed over the galactic plane without strong concentration onto the (Perseus) arm. The number ratio of C4 - C5, C6 -C7, and C8 - C9 stars is nearly 1 0.4 < 0.1. 

The results are summarized in Figure 1. The different symbols denote the spectral classification of the individual stars, whereas four groups are distinguished WNE-A, WNE-B, WNL and WC stars. The size of the symbols indicates the mass-loss rates. The uncertainties of the results are estimated to be 0.1 dex in T, 0.4 dex in M and 0.5 dex in L. For the 11 stars in common with the sample of Abbott et al. (1986) we find our mass-loss rates to be compatible with their radio flux if the correct ionization equilibrium in the radio emitting region is applied (Schmutz and Hamann, 1986). The model calculations show that for all but the WN2 and WN3 stars helium recombines to He before the ions enter the radio-emitting region. 

Summary. Beginning with a historical account of the spectral classification, its refinement through additional criteria is presented. The line strengths and ratios used in two dimensional classifications of each spectral class are described. A parallel classification scheme for metal-poor stars and the standards used for classification are presented. The extension of spectral classification beyond M to L and T and spectroscopic classification criteria relevant to these classes are described. Contemporary methods of classifications based upon different automated approaches are introduced. 

Keywords Spectral classification, luminosity classes, metal-poor stars, M-L-T spectral classes... 

The above mentioned system also known as Yerkes Spectral Classification. Within the system, six luminosity classes are defined on the basis of standard stars over the observed luminosity range. 

It is very important to envisage an approach that would give quick, reliable spectral classifications (or stellar parameters) for stars falling in all regions of HR diagram. The pipeline procedures are being developed for the future ambitious missions such as GAIA and PAN-STARS. 

Figure 17.2 shows the relative abundance of the elements of the universe and of the earth. The abundances are approximate, as a consequence of die difficulties in their assessment and limitations of experimental techniques. The abundances in the universe (based on spectral measurements on stars and interstellar matter) are used as a refinement of data obtained for the solar system. Stellar light is divided in spectral classes depending on the surface temperature of the star, see Fig. 17.1. The various classes (Harvard Spectral Classification) show lines of the elements as listed below in approximately decreasing intensity ...

A point in the star-forming part of the Orion nebula that emits infrared radiation (but no visible radiation, probably because it is scattered by the dense dust of the nebula). It is thought to be a young near main-sequence star of type B spectral classification, and one of the youngest stars so far observed. 

Spectral classification A system of classifying stars by their temperature. The range of spectral classifications goes as O, B, A, F, G, K, M, and recently added L. The hottest stars are O stars and the coolest L. Each temperature is also generally divided into subclasses, which depend on the luminosity of the star. 

Objects that radiate mainly at infrared wavelengths may do so because of their low temperature by astronomical standards. Objects that fall in this category start with stars of spectral classifications of K or cooler extending down the newly designated spectral classification of L. The sub-stellar classification ofBrown Dwarf links stars generating energy by nucleosynthesis to planets such as the giant and terrestrial planets in our own solar system. The planets such as the earth radiate like blackbodies at their surface... 

The spectral features observed by astronomers have led to the classification of stars into seven broad classes outlined in Table 4.1, together with their surface temperatures. The highest-temperature class, class O, contains may ionised atoms in the spectrum whereas the older stars in class M have a much lower temperature and many more elements present in the spectrum of the star. Observation of a large number of the stars has lead to extensive stellar catalogues, recently extended by the increased sensitivity of the Hubble Space Telescope. Making sense of this vast quantity of information is difficult but in the early 19th century two astronomers... 

Photoelectric or CCD photometry through colour filters is widely used for quantitative classification , i.e. to measure major spectral features with low wavelength resolution but rapidly and precisely to obtain major properties of large numbers of stars, such as... 

Bright carbon stars in the Cassiopeia region are classified into the C classification system (Yamashita 1972, 1975) using 103aF plates (AA4500 - 6800 A). Six criteria are extracted from spectral tracings of standard stars. Fifty nine... 

Draper originally devised a classification system in which stars were placed into lettered groups A, B, C, D, and so on. Over time, that system was changed and refined. Today, only seven color groups, or spectral classes, remain (from hottest to coolest) O, B, A, F, G, K, and M. While the variety of stars is such that more complex classification schemes have been developed to accurately describe them all, the chart on page 52 provides basic information on each of the seven spectral types. 

The ANN has been used in very large number of stellar applications. Vieira and Ponz [24] have used ANN on low-resolution IUE spectra and have determined SpT with an accuracy of 1.1 subclass. Bailer-Jones and Irwin [19] used ANN to classify spectra from Michigan Spectral Survey with an accuracy of 1.09 SpT. Prieto and co-workers [25] used ANN in their search of metal-poor stars. Snider and co-workers [26] used ANN for the three dimensional classification of metal-poor stars. 

Each class is divided into 10 subclasses denoted by the digits 0 to 9. Thus the sun is described as a yellow dwarf of spectral class G2. Further categorization will assign a star to a luminosity class, indicate the presence of emission lines, etc. Other classification letters are also used, e.g. DA for white dwarf stars, or W forWolf-Rayet stars. 

Spectroscopy is the key to unlocking the information in starlight. Stellar spectra show a variety of absorption lines which allow a rapid classification of stars in a spectral sequence. This sequence reflects the variations in physical conditions (density, temperature, pressure, size, luminosity) between different stars. The strength of stellar absorption lines relative to the continuum can also be used in a simple way to determine the abundances of the elements in the stellar photosphere and thereby to probe the chemical evolution of the galaxy. Further, the precise wavelength position of spectral lines is a measure of the dynamics of stars and this has been used in recent years to establish the presence of a massive black hole in the centre of our galaxy and the presence of planets around other stars than the Sun. 

All stellar spectra show absorption lines due to a variety of species. For the Sun, these were first discovered by Joseph von Fraunhofer in the early 1800s. A sample of stellar spectra is shown in Figure 1. The patterns in these lines allow stellar spectra to be grouped in a classification scheme. Depending on the spectral characteristics, stars are designated by a letter from the sequence O, B, A, F, G, K and M. This spectral sequence is summarized in Table 1. Since temperature controls ionization and excitation of the... 

In addition to this spectral class, stars are also characterized by a luminosity parameter. This luminosity classification is made on the basis of the width of spectral lines. Table 2 summarizes this classification. The width of spectral lines increases as the gas pressure increases. This so-called pressure broadening is due to the perturbation of atomic energy levels by other, nearby species. The physically largest stars have the lowest surface densities and pressures. Lines from these stars are therefore broader than from smaller stars (Figure 1 and Table 2). This difference in size, which results in a difference in stellar luminosity, has led to the naming scheme from supergiants to dwarfs. 

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