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    MATERIAL FROM CLASS 25: Wednesday December 6

    Other galaxies; groups and clusters of galaxies; using Hubble's law to map the cosmos

    Properties of normal galaxies: approximately Table 18.1 of Zeilik.
    We would require M/L=700 to produce enough gravity to halt cosmic expansion.

    spiral irregular elliptical
    diameter (light years) 105 104 105 to 106
    luminosity (Lsun) 1010 109 1011
    mass (Msun) 1011 109 1012 to 1013
    mass/light (Msun/Lsun) 10 1 10 - 100
    color blue disk, red bulge blue red
    most light from young blue stars young blue stars red giants
    cool gas, can make stars 5% to 10% of stars up to 100% of stars tiny
    hot gas, above 106K tiny tiny up to 20% of stars
    spin fast slow slow/none
    central density medium low high

    Hubble's classification of galaxies

    from 'The Realm of the Nebulae', Yale, 1936.

    [the cosmic web: 2dF survey]

    Real galaxies illustrating Hubble's scheme:
    [the cosmic web: 2dF survey]

    Using Hubble's law to map the cosmos

    Distribution of galaxies in space, from their redshifts. Notice how the denser regions form a 'cosmic web'.
    Galaxies appear to thin out at larger distances; but this is an illusion, and happens simply because we cannot see the fainter ones.
    [the cosmic web: 2dF survey]