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									                             THE GALAXY
                                                                    Composite infrared
                                                                    colour image of
                                                                    Galactic Centre
                                                                    region taken at
                                                                    1.25, 2.2 and 3.5
                                                                    microns with

    • GALAXY: A conglomeration of stars, gas + dust
    • Topics:
          –   Binary stars
          –   Star clusters
          –   Stellar evolution: an overview
          –   Variable stars and their use as distance indicators
          –   Mass loss from stars
          –   Binary stars with compact components
          –   The interstellar medium
          –   Structure and rotation of the Galaxy
AS 1001                                                                       The Galaxy
                        Two main themes:
    • An overview of the structure of the Milky Way
      Galaxy (MWG)
          – how the various components are inter-related and mutually
          – leading to hints about the formation and evolution of galaxies.
    • An understanding of the distance scale in the
          – starting from stars with accurately known parallaxes
          – using properties of star clusters, binary stars, variable stars to
            build a picture of the MWG + distances to other galaxies.

AS 1001                                                                   The Galaxy
           Analysing radiation from stars
                             Apparent brightnesses
                             Astrophysical theory

   Radial velocities                                 Calibration of types via:
   Proper motions                                    nearby stars, stars in
   Classification of stars                           clusters, binary stars
    into different types                             for : masses, sizes,
                                                     intrinsic brightnesses,
                                                     temperatures, chemical
                   Space motions
                   Chemical composition
                    or ‘population’ group
                   3-D distribution of stars
                    of different types
AS 1001                                                               The Galaxy
                 Star clusters and
            colour-magnitude diagrams
• CLUSTERS: Congregations of
  stars with stronger mutual
  gravitational attraction than in
  general stellar field.
• Range from loose associations
  (~ 100 stars) to open clusters (n
  x 103 stars) and globular
  clusters (n x 105 stars).

                                                Globular cluster: 47 Tuc

  Open cluster: The Pleiades (D. Malin/UKSTU)
AS 1001                                                           The Galaxy
           The importance of star clusters
    • Stars in a cluster are all at ~ same distance
          – e.g. Pleiades: diameter 10 pc, distance 126 pc
          – h Persei: diameter 10 pc, distance 2200 pc
          – Hence, accurate distances to these objects.
    • Stars in a cluster had a common origin
          – formed about the same time
          – from same pre-stellar gas, i.e. same chemical composition.
    • Hence direct tests of theoretical models for
      luminosity (L) and temperature (T) as function of
      stellar mass (M) and stellar ages (t).

AS 1001                                                                  The Galaxy
                  Distances to clusters - 1
    • Parallaxes and moving clusters

                                                          Convergent point

    • Need space motions of individual stars
    • Proper motions + radial velocities + direction to
      convergent point give distance to cluster.
          – e.g. the Hyades star cluster at d = 40 pc.
    • Measure apparent magnitudes (V) and colour
      indices (e.g. B-V) of moving cluster stars
          – Hence obtain absolute magnitudes MV (using V-MV=5 log d - 5 )
    • Hence get calibration of MV vs. (B-V, spectral type),
AS 1001                                                              The Galaxy
                            Moving clusters
    • group of stars with same space velocity
                                                      θ   Vt


    • measure radial velocity
          – Vr = V cos θ
    • measure angle to convergent point
          – V and then Vt
          – Vt = V sin θ
    • measure proper motions µ in arcsec/year
          – Vt = 4.74 µ d
    • gives distance                 d=       tanθ
          – V in km/s, d in pc
AS 1001                                                        The Galaxy
                  Distances to clusters - 2
    • Colour-magnitude diagrams
    • Now better, more accurate than moving-cluster
    • Many ordinary F-type stars in solar neighbourhood
      (d < 20 to 25 pc) with accurate parallaxes (P = 1/d).
    • V, (B-V), etc values easily measured to accuracies
      < 1%.
    • Also known that interstellar space in solar
      neighbourhood is free of dust
          – hence no scattering, or extinction of starlight between these
            local F stars and us.

AS 1001                                                                The Galaxy
                     Main-sequence fitting
    • Calibration:                            2.8
          – via V - AV - MV = 5 log (d/10)
            with AV = 0, and                 MV
          – d = 1/P
          – can determine accurate MV vs.
            (B-V)0 etc relationships for      4.7
            main-sequence F stars --                 0.3                          0.6
            empirical!                                              (B-V)0

    • Application:                            MV             Move cluster C-M diagram
          – Observe a cluster of stars to                           until the two main
            determine V, (B-V) etc.                                          sequences
                                            (V-AV)                              overlap.
          – Correct for interstellar
          – Compare colour-magnitude                  Hence get
            diagram for cluster [(B-V)0 vs.           distance modulus
            (V-AV)] with (B-V)0 vs. MV                (V - AV - MV)
            diagram for solar-
            neighbourhood stars.                                    (B-V)0

AS 1001                                                                       The Galaxy
     Interstellar reddening and extinction
    • Effect of dust in interstellar space is to scatter
          – blue light is scattered more easily then red light:
                          I scat ∝ λ
          – Thus a star seen through dust will appear fainter than it would if
            there were no dust along the line of sight.
          – A star’s colour will also be affected, because of the λ–1
            dependence, so that a star appears to be redder (or less blue)
            than its intrinsic colour.

AS 1001                                                                  The Galaxy

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