Galaxies Lecture 01

• Galaxy Facts
  • Stars from the most massive component (95% of mass)
  • They sometimes have satellite galaxies
  • Dwarf galaxies are the most common
  • They come isolated or in groups
  • Groups contain tens of galaxies, clusters contain thousands.
• Star Clusters vs. Galaxies
  • Clusters are a few pc across; galaxies are larger than 1 kpc.
  • Clusters have $10^{5}$ stars; galaxies have over $10^{7}$.
  • Galaxies are always self-gravitating
  • Galaxies contain gas, dust, and cosmic rays
• Dynamics of Stars
  • The stellar mass of galaxies completely determines the dynamics.
  • Calculating the collision time of stars:

${\begin{aligned}\ell _{mfp}&={1 \over n\sigma }\\t_{col}&={1 \over n\sigma v}\\&={1 \over (.25pc^{-3})(\pi (2R_{\odot })^{2})(20{km \over s})}\\&\sim 2.5\cdot 10^{18}yrs\\\end{aligned}}\,\!$

• • Stars never collide (collision time greater than Hubble time).
• Dynamics of Gas
  • Calculating collision time of gas:

${\begin{aligned}t_{col}&={1 \over n\sigma v}\\&={1 \over (1cm^{-3})(\pi (10^{11})^{2})(1{km \over s})}\\&=500yrs\\\end{aligned}}\,\!$

• • Gas collides often on timescales of galactic dynamics
• Energy Changes
  • Gas energy changes via heating and cooling
  • Stellar energy changes via changes in gravitational potential (ex. galaxy collisions, ejection of stars from a region)