{"id":173,"date":"2018-11-19T04:46:36","date_gmt":"2018-11-19T04:46:36","guid":{"rendered":"http:\/\/phyp06.epgpbooks.inflibnet.ac.in\/?post_type=chapter&p=173"},"modified":"2018-11-19T05:03:07","modified_gmt":"2018-11-19T05:03:07","slug":"h-r-diagram-contd","status":"publish","type":"chapter","link":"https:\/\/ebooks.inflibnet.ac.in\/phyp06\/chapter\/h-r-diagram-contd\/","title":{"rendered":"H. R. Diagram \u2013 (contd.)"},"content":{"raw":"
\r\n\r\n \r\n\r\n1.\u00a0<\/strong>\u00a0Learning Outcomes<\/strong>\r\n\r\n \r\n\r\nAfter studying this module, you should be able to\r\n
    \r\n \t
  • recognize the need to augment spectral classification with luminosity classification<\/li>\r\n \t
  • recall the names and characteristics of the various luminosity classes<\/span><\/li>\r\n \t
  • state the complete stellar classification of stars<\/span><\/li>\r\n \t
  • explain the concept of spectroscopic parallax<\/span><\/li>\r\n \t
  • understand how stars within a spectral class can be distinguished as giants and dwarfs<\/span><\/li>\r\n \t
  • explain how spectroscopic absolute magnitudes of stars are determined<\/span><\/li>\r\n \t
  • appreciate how CM diagrams of clusters are important<\/span><\/li>\r\n \t
  • Distinguish between globular cluster and open clusters<\/li>\r\n<\/ul>\r\n \r\n\r\n2. Introduction<\/strong>\r\n\r\n \r\n

    In the last module, we saw that if we plot some measure of luminosity (such as absolute magnitude) against some measure of stellar surface temperature (such as spectral class or colour) then a pattern emerges which shows that the stars are arranged in neat families. This pattern is called the H. R. Diagram.\u00a0 Important families of stars are the supergiants, giants, the Main Sequence and white dwarfs. The Main Sequence is like a band running from the top left corner to the bottom right corner. <\/span><\/p>\r\n \r\n

    The stars in the lower Main Sequence are the most abundant stars in the Galaxy, followed by white dwarfs and giants. The use of Stefan-Boltzmann law shows that\u00a0<\/span>in the plane of the H. R. Diagram the lines of constant radii are straight lines sloping downwards. The stellar radii increase upwards and so do the masses of stars. Since a star of a given spectral class can belong to any of the families, supergiants, giants, Main Sequence or white dwarfs, there is a need to supplement the stellar classification, which is in the horizontal direction, with\u00a0<\/span>another classification which runs vertically. This classification is called the luminosity classification, which we discuss in this module.<\/span><\/p>\r\n\r\n<\/div>\r\n \r\n\r\n3. Luminosity Classification<\/strong>\r\n\r\n \r\n

    If we draw a vertical line in the H. R. diagram, such as red line in Fig. 14.3, we find that a star belonging to a given spectral class may be a White Dwarf star, a Main Sequence star, a Giant star, or a Supergiant star. Therefore, to state that a star belongs to a certain spectral class, say G2 (for the Sun), is an incomplete statement. A comprehensive classification scheme must at least be two-dimensional; when we move horizontally along the H. R. diagram it must give the spectral class of a star and as we move upwards it should tell us the family to which it belongs. The vertical classification was suggested by Morgan, Keenan and Kellman, and is known as MKK classification, <\/strong>or the luminosity classification <\/strong>(Figs. 15.1 and 15.2). Incidentally, Fig. 14.3 also shows this classification.<\/p>\r\n \r\n\r\n\"\" \"\" \"\" \"\" \"\" \"\" \"\" \"\" \"\" \"\" \"\"\r\n\r\nSummary<\/strong>\r\n

      \r\n \t
    • The stars of a given spectral class may belong to any of the stellar families. This necessitates a two-dimensional classification, horizontal and vertical.<\/li>\r\n \t
    • The vertical classification, called the MKK or luminosity classification,designates the bright supergiants as belonging to class ?, less bright supergiants as ??, giants as ???, subgiants as ?V<\/em><\/strong>, main sequence as ?, subdwarfs (lying between dwarfs and the main sequence) as ??, and white dwarf stars as ???.<\/li>\r\n \t
    • By studying the spectrum of a star, it is possible to place it on the H. R. Diagram.<\/li>\r\n \t
    • Once the star has been placed on the diagram, we can infer its absolute magnitude. Knowing its apparent magnitude by direct observation, we can determine the parallax of the star. This parallax is known as Spectroscopic Parallax. This is useful if star\u2019s parallax cannot be determined by trigonometric method.<\/li>\r\n \t
    • Within a spectral class, we can distinguish between giants and dwarfs by looking at the line strengths and weaknesses of certain elements with respect to certain other elements.<\/li>\r\n \t
    • Plot of absolute magnitudes against ratios of strengths of certain lines within a spectral class helps us to determine absolute magnitudes of stars, called spectroscopic absolute magnitude.<\/li>\r\n \t
    • Astronomers prefer to use colours of stars to spectral class for getting the H. R. Diagram, because colours can be conveniently determined to greater accuracy. Such a diagram is called the colour-magnitude, or CM, diagram.<\/li>\r\n \t
    • CM diagrams of star clusters can help us to estimate their ages.<\/li>\r\n \t
    • Globular clusters are distinguished from the open clusters by being more densely packed than the open clusters.<\/li>\r\n<\/ul>\r\nKnow More<\/strong>\r\n

      The basic material is from Astronomy and Astrophysics with Elements of Cosmology, V. B. Bhatia, Narosa Publishing House, New Delhi, which has been amplified with the help of the following sources:<\/p>\r\nThe New Cosmos, A. Uns\u00f6ld and B. Baschek, Springer, New York\r\n

        \r\n \t
      • https:\/\/en.wikipedia.org\/wiki\/Stellar_classification<\/li>\r\n \t
      • http:\/\/astronomy.swin.edu.au\/cosmos\/M\/Morgan-Keenan+luminosity+class<\/li>\r\n \t
      • http:\/\/www.enchantedlearning.com\/subjects\/astronomy\/stars\/startypes.shtml<\/li>\r\n \t
      • https:\/\/ned.ipac.caltech.edu\/level5\/ASS_Atlas\/MK_contents.html<\/li>\r\n \t
      • https:\/\/www.e-education.psu.edu\/astro801\/content\/l7_p7.html<\/li>\r\n \t
      • http:\/\/rml3.com\/a20p\/mainseqfitting.htm<\/li>\r\n \t
      • https:\/\/people.highline.edu\/iglozman\/classes\/astronotes\/hr_diagram.htm<\/li>\r\n<\/ul>\r\n
        An Atlas of Representative Stellar Spectra by Yamashita et al. Tokyo University, Tokyo Press<\/div>\r\n
          \r\n \t
        • http:\/\/digitool.library.mcgill.ca\/webclient\/StreamGate?folder_id=0&dvs=1484627441207~368<\/li>\r\n \t
        • https:\/\/www.handprint.com\/ASTRO\/specclass.html<\/li>\r\n \t
        • http:\/\/hubblesite.org\/image\/1562\/news_release\/2004-20<\/li>\r\n \t
        • http:\/\/www.seasky.org\/astronomy\/astronomy-messier.html<\/li>\r\n<\/ul>\r\n
          Star\u2013Names and their Meanings, R. H. Allen, Harper, New York.<\/div>","rendered":"
          \n

           <\/p>\n

          1.\u00a0<\/strong>\u00a0Learning Outcomes<\/strong><\/p>\n

           <\/p>\n

          After studying this module, you should be able to<\/p>\n

            \n
          • recognize the need to augment spectral classification with luminosity classification<\/li>\n
          • recall the names and characteristics of the various luminosity classes<\/span><\/li>\n
          • state the complete stellar classification of stars<\/span><\/li>\n
          • explain the concept of spectroscopic parallax<\/span><\/li>\n
          • understand how stars within a spectral class can be distinguished as giants and dwarfs<\/span><\/li>\n
          • explain how spectroscopic absolute magnitudes of stars are determined<\/span><\/li>\n
          • appreciate how CM diagrams of clusters are important<\/span><\/li>\n
          • Distinguish between globular cluster and open clusters<\/li>\n<\/ul>\n

             <\/p>\n

            2. Introduction<\/strong><\/p>\n

             <\/p>\n

            In the last module, we saw that if we plot some measure of luminosity (such as absolute magnitude) against some measure of stellar surface temperature (such as spectral class or colour) then a pattern emerges which shows that the stars are arranged in neat families. This pattern is called the H. R. Diagram.\u00a0 Important families of stars are the supergiants, giants, the Main Sequence and white dwarfs. The Main Sequence is like a band running from the top left corner to the bottom right corner. <\/span><\/p>\n

             <\/p>\n

            The stars in the lower Main Sequence are the most abundant stars in the Galaxy, followed by white dwarfs and giants. The use of Stefan-Boltzmann law shows that\u00a0<\/span>in the plane of the H. R. Diagram the lines of constant radii are straight lines sloping downwards. The stellar radii increase upwards and so do the masses of stars. Since a star of a given spectral class can belong to any of the families, supergiants, giants, Main Sequence or white dwarfs, there is a need to supplement the stellar classification, which is in the horizontal direction, with\u00a0<\/span>another classification which runs vertically. This classification is called the luminosity classification, which we discuss in this module.<\/span><\/p>\n<\/div>\n

             <\/p>\n

            3. Luminosity Classification<\/strong><\/p>\n

             <\/p>\n

            If we draw a vertical line in the H. R. diagram, such as red line in Fig. 14.3, we find that a star belonging to a given spectral class may be a White Dwarf star, a Main Sequence star, a Giant star, or a Supergiant star. Therefore, to state that a star belongs to a certain spectral class, say G2 (for the Sun), is an incomplete statement. A comprehensive classification scheme must at least be two-dimensional; when we move horizontally along the H. R. diagram it must give the spectral class of a star and as we move upwards it should tell us the family to which it belongs. The vertical classification was suggested by Morgan, Keenan and Kellman, and is known as MKK classification, <\/strong>or the luminosity classification <\/strong>(Figs. 15.1 and 15.2). Incidentally, Fig. 14.3 also shows this classification.<\/p>\n

             <\/p>\n

            \"\" \"\" \"\" \"\" \"\" \"\" \"\" \"\" \"\" \"\" \"\"<\/p>\n

            Summary<\/strong><\/p>\n

              \n
            • The stars of a given spectral class may belong to any of the stellar families. This necessitates a two-dimensional classification, horizontal and vertical.<\/li>\n
            • The vertical classification, called the MKK or luminosity classification,designates the bright supergiants as belonging to class ?, less bright supergiants as ??, giants as ???, subgiants as ?V<\/em><\/strong>, main sequence as ?, subdwarfs (lying between dwarfs and the main sequence) as ??, and white dwarf stars as ???.<\/li>\n
            • By studying the spectrum of a star, it is possible to place it on the H. R. Diagram.<\/li>\n
            • Once the star has been placed on the diagram, we can infer its absolute magnitude. Knowing its apparent magnitude by direct observation, we can determine the parallax of the star. This parallax is known as Spectroscopic Parallax. This is useful if star\u2019s parallax cannot be determined by trigonometric method.<\/li>\n
            • Within a spectral class, we can distinguish between giants and dwarfs by looking at the line strengths and weaknesses of certain elements with respect to certain other elements.<\/li>\n
            • Plot of absolute magnitudes against ratios of strengths of certain lines within a spectral class helps us to determine absolute magnitudes of stars, called spectroscopic absolute magnitude.<\/li>\n
            • Astronomers prefer to use colours of stars to spectral class for getting the H. R. Diagram, because colours can be conveniently determined to greater accuracy. Such a diagram is called the colour-magnitude, or CM, diagram.<\/li>\n
            • CM diagrams of star clusters can help us to estimate their ages.<\/li>\n
            • Globular clusters are distinguished from the open clusters by being more densely packed than the open clusters.<\/li>\n<\/ul>\n

              Know More<\/strong><\/p>\n

              The basic material is from Astronomy and Astrophysics with Elements of Cosmology, V. B. Bhatia, Narosa Publishing House, New Delhi, which has been amplified with the help of the following sources:<\/p>\n

              The New Cosmos, A. Uns\u00f6ld and B. Baschek, Springer, New York<\/p>\n

                \n
              • https:\/\/en.wikipedia.org\/wiki\/Stellar_classification<\/li>\n
              • http:\/\/astronomy.swin.edu.au\/cosmos\/M\/Morgan-Keenan+luminosity+class<\/li>\n
              • http:\/\/www.enchantedlearning.com\/subjects\/astronomy\/stars\/startypes.shtml<\/li>\n
              • https:\/\/ned.ipac.caltech.edu\/level5\/ASS_Atlas\/MK_contents.html<\/li>\n
              • https:\/\/www.e-education.psu.edu\/astro801\/content\/l7_p7.html<\/li>\n
              • http:\/\/rml3.com\/a20p\/mainseqfitting.htm<\/li>\n
              • https:\/\/people.highline.edu\/iglozman\/classes\/astronotes\/hr_diagram.htm<\/li>\n<\/ul>\n
                An Atlas of Representative Stellar Spectra by Yamashita et al. Tokyo University, Tokyo Press<\/div>\n
                  \n
                • http:\/\/digitool.library.mcgill.ca\/webclient\/StreamGate?folder_id=0&dvs=1484627441207~368<\/li>\n
                • https:\/\/www.handprint.com\/ASTRO\/specclass.html<\/li>\n
                • http:\/\/hubblesite.org\/image\/1562\/news_release\/2004-20<\/li>\n
                • http:\/\/www.seasky.org\/astronomy\/astronomy-messier.html<\/li>\n<\/ul>\n
                  Star\u2013Names and their Meanings, R. H. Allen, Harper, New York.<\/div>\n","protected":false},"author":4,"menu_order":10,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":["prof-v-b-bhatia"],"pb_section_license":""},"chapter-type":[],"contributor":[58],"license":[],"class_list":["post-173","chapter","type-chapter","status-publish","hentry","contributor-prof-v-b-bhatia"],"part":3,"_links":{"self":[{"href":"https:\/\/ebooks.inflibnet.ac.in\/phyp06\/wp-json\/pressbooks\/v2\/chapters\/173","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/ebooks.inflibnet.ac.in\/phyp06\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/ebooks.inflibnet.ac.in\/phyp06\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/ebooks.inflibnet.ac.in\/phyp06\/wp-json\/wp\/v2\/users\/4"}],"version-history":[{"count":5,"href":"https:\/\/ebooks.inflibnet.ac.in\/phyp06\/wp-json\/pressbooks\/v2\/chapters\/173\/revisions"}],"predecessor-version":[{"id":190,"href":"https:\/\/ebooks.inflibnet.ac.in\/phyp06\/wp-json\/pressbooks\/v2\/chapters\/173\/revisions\/190"}],"part":[{"href":"https:\/\/ebooks.inflibnet.ac.in\/phyp06\/wp-json\/pressbooks\/v2\/parts\/3"}],"metadata":[{"href":"https:\/\/ebooks.inflibnet.ac.in\/phyp06\/wp-json\/pressbooks\/v2\/chapters\/173\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/ebooks.inflibnet.ac.in\/phyp06\/wp-json\/wp\/v2\/media?parent=173"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/ebooks.inflibnet.ac.in\/phyp06\/wp-json\/pressbooks\/v2\/chapter-type?post=173"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/ebooks.inflibnet.ac.in\/phyp06\/wp-json\/wp\/v2\/contributor?post=173"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/ebooks.inflibnet.ac.in\/phyp06\/wp-json\/wp\/v2\/license?post=173"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}