Galaxies Lecture 13

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Now we’re going to back up a little bit and talk some more about the components of galaxies. We start with stellar evolution.

  • Main Sequence
    • HHe via pp chain or CNO cycle
    • Convection mixes H and He.
    • He is heavier and sinks to center
    • Steep T gradients need convection to get heat out; smaller gradients use radiative transport.
  • HR diagram
    • He flash only happens in low mass stars when He core becomes degenerate before He can ignite.
    • Pre-MS branch (Hiyashi strip) varies L and R to maintain constant T.
    • , so higher metallicity means dimmer stars.
    • Tip of the Red Giant Branch (RGB) changes little with stellar type, so it can indicate an approximate distance.
  • Initial Metallicities
    • Set via Big Bang nucleosynthesis theory.
    • Observationally, a star hot enought ot excite He will also create He.
    • Using young, low-mass stars, we have measured , and (bolometric magnitude = integrated over frequency) for fixed z, T (metallicity temperature).
    • We can measure Y in the ISM via radio recombination lines: in H and .
  • MS lifetimes
    • , where is a fudge factor.
    • takes .
    • takes . This is independent of mass because stars only burn up to the Chandrasekhar limit.
    • Horizontal Branch (He burning) takes 0.1 Gyr.
  • Instability Strip
    • (opacity) instability sets a vertical strip of instability in HR diagram, which covers a wider range of temperatures at higher luminosities.
    • Normally, as temperature increases, opacity decreases, allowing the extra heat out.
    • Near the ionization limit, opacity increases with temperature, causing heating, which builds up pressure and expands the star.
    • The period of this expansion is roughly .
    • , where is a constant.
    • The fundamental mode of expansion is lowest mode: the whole star expands. However, there are harmonics to radial expansion, and interior zero points, so that only the outer layers of the star expand.
    • Solar Oscillations are not large scale, and are sensitive to conditions inside the star.
    • Cepheids have both fundamental and overtone oscillations. The period-luminosity rlation is tight in K-band, but wider at B, V.
  • Nuclear Physics
    • There are more neutrons in heavier elements
    • is the most stable, but can get heavier elements with fusion as well.
    • makes it easiest to get an even mass-number.
    • neutron capture in high-mass stars give other elements.
      • S process (slow) absorbs and then -decays into the next element up
      • R process (rapid) absorbs lots of and the -decays.
    • the number of elements depends on initial metallicity, neutron cross-section, and radioactivity.
    • Heavy elements are dispersed into the ISM via stellar winds, PNe, SN (Type II, also Ib, Ic) core collapse, and SN Type Ia
  • Population Evolution
    • Can use known ages of stars to study population ages and histories.
    • Age and metallicity can be indicators of the historical star formation rate.
    • In the Local Group:
      • dSp, dE have no evidence of recent star formation; old populations
      • Carina has 3 episodes of star formation; how was the gas replentished?
      • Irr have on-going star formation and young populations; how is gas maintained when SN winds eject it?