15-02 Shapes of Galaxies

  • A galaxy gallery





     
    They are generally divided into 3 major classes:
    Elliptical, Sprial and Irregular.
  • Hubble's tuning fork diagram


    'Early type' (ellipticals) vs 'Late type' (spirals and irregulars)
     
  • Elliptical galaxies
     
    Giant ellipticals

    [Two giant ellipticals near the central part of the Virgo cluster, 15 Mpc away; M84 and M86 span 5 to 7 arcmin.]


    Dwarf ellipticals

    [Leo I, a dwarf elliptical of size 1 kpc, 180 kpc away]

    Note: giant ellipticals are rare, while dwarf ellipticals are quite common.
     
  • Spiral galaxies
     
    Normal spirals: Sa, Sb, Sc


    Barred spirals: SBa, SBb, SBc


    Note: towards 'Type a', a larger central bulge, more tightly wound spiral arms, less gas and dust.
     
  • Irregular galaxies




    [The Large Magellanic Cloud (LMC), 49 kpc away, 19 kpc across; to the left is a huge HII region called Tarantula Nebula or 30 Doradus, which is about 5 million solar masses, 250pc large.]


    [The Small Magellanic Cloud (SMC), 61 kpc away, 8 kpc across; to the right is a galactic globular cluster, 47 Tucanae)

    Irregulars often have more gas and dust, with active star formation.
    They are further divided as Irr I (with hint of some organized structure, OB stars, HII regions, such as LMC and SMC) and
    Irr II (asymmetric, distorted, such as M82).
     

    [Subaru telescope and Chandra X-ray Observatory images of M82, an amorphous (Irr II) galaxy]

  • Some properties of galaxies
    Elliptical Spiral Irregular
    Mass (M⊙) 105 - 1013 109 - 4 x 1011 108 - 3 x 1010
    Luminosity (L⊙) 3 x 105 - 1011 108 - 2 x 1010 107 - 109
    Diameter (kpc) 1 - 200 5 - 250 1 - 10
    Stellar populations Population II
    and old Population I    		 Disk: young Population I
    Central bulge and halo:
    Population II and old Population I    		 Mostly Population I
    Percentage of observed galaxies 20% 77% 3%
    Note: Many dwarf ellipticals are too dim to observe. Properties of Galaxies