ASTRON C10 Lecture Notes - Lecture 30: Vesto Slipher, Spiral Galaxy, Visible Spectrum
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4. 5% normal dark matter matter of protons and neutrons. Dark matter does not play a dominant role in planetary systems. Vesto slipher: noticed that spectra of spiral nebulae have larger redshifts. Didn"t know their distances and didn"t really know what this meant . 1929 used newly derived distances (especially via cepheid variables) On small scales (solar systems, galaxies and galaxy clusters) nothing is expanding. It"s empty space between galaxy clusters that"s expanding (superclusters also expand but more slowly than void space) Now : obtain optical spectrum, measure the redshift, compute the recession velocity (from z = v/c, use hubble"s law to compute distance ( the higher the speed, the less it"s accurate) There is a maximum distance that we can see, that"s the observable boundary of the universe but we"re fairly certain there"s more beyond that. and it gets increasingly harder with expanding space in between. Some elliptical galaxies formed from the merging of two or more spiral galaxies.
